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One Chinese name for the Great Wall is the "Ten-Thousand-Li Long Wall" (). As in Greek, the number "ten thousand" is used figuratively in Chinese to mean any "immeasurable" value and this title has never provided a literal distance of 10,000 li (). The actual length of the modern Great Wall is around 42,000 li (), over 4 times the name's proverbially "immeasurable" length.
The Chinese proverb appearing in chapter 64 of the Tao Te Ching and commonly rendered as "A journey of a thousand miles begins with a single step" in fact refers to a thousand li: 千里之行，始於足下 (Qiānlǐzhīxíng, shǐyúzúxià).
The greatest horses of Chinese history including Red Hare and Hualiu (驊騮) are all referred to as "thousand-li horses" (, qiānlǐmǎ), since they could supposedly travel a thousand li () in a single day.
Li is sometimes used in location names, for example: Wulipu (Chinese: 五里铺镇), Hubei; Ankang Wulipu Airport (Chinese: 安康五里铺机场), Shaanxi. Sanlitun () is an area in Beijing.
Ri in Japan and Korea
The present day Korean ri (리, 里) and Japanese ri (里) are units of measurements that can be traced back to the Chinese li (里).
Although the Chinese unit was unofficially used in Japan since the Zhou dynasty, the countries officially adopted the measurement used by the Tang dynasty (618–907 AD). The ri of an earlier era in Japan was thus true to Chinese length, corresponding to six chō ( 500–600 m), but later evolved to denote the distance that a person carrying a load would aim to cover on mountain roads in one hour. Thus, there had been various ri of 36, 40, and 48 chō. In the Edo period, the Tokugawa shogunate defined 1 ri as 36 chō, allowing other variants, and the Japanese government adopted this last definition in 1891. The Japanese ri was, at that time, fixed to the metric system, ≈ 3.93 kilometres or about 2.44 miles. Therefore, one must be careful about the correspondence between chō and ri. See Kujūkuri Beach (99-ri beach) for a case. | Li (unit) | Wikipedia | 497 | 1495421 | https://en.wikipedia.org/wiki/Li%20%28unit%29 | Physical sciences | East Asian | Basics and measurement |
In South Korea, the ri currently in use is a unit taken from the Han dynasty (206 BC–220 AD) li. It has a value of approximately 392.72 meters, or one tenth of the ri. The Aegukga, the national anthem of South Korea, and the Aegukka, the national anthem of North Korea, both mention 3,000 ri, which roughly corresponds to 1,200 km, the approximate longitudinal span of the Korean peninsula.
In North Korea the Chollima Movement, a campaign aimed at improving labour productivity along the lines of the earlier Soviet Stakhanovite movement, gets its name from the word "chollima" which refers to a thousand-ri horse (chŏn + ri + ma in North Korean Romanization). | Li (unit) | Wikipedia | 158 | 1495421 | https://en.wikipedia.org/wiki/Li%20%28unit%29 | Physical sciences | East Asian | Basics and measurement |
Picea rubens, commonly known as red spruce, is a species of spruce native to eastern North America, ranging from eastern Quebec and Nova Scotia, west to the Adirondack Mountains and south through New England along the Appalachians to western North Carolina and eastern Tennessee. This species is also known as yellow spruce, West Virginia spruce, eastern spruce, and he-balsam. Red spruce is the provincial tree of Nova Scotia.
Description
Red spruce is a perennial, shade-tolerant, late successional coniferous tree that under optimal conditions grows to tall with a trunk diameter of about , though exceptional specimens can reach tall and in diameter. It has a narrow conical crown. The leaves are needle-like, yellow-green, long, four-sided, curved, with a sharp point, and extend from all sides of the twig. The bark is gray-brown on the surface and red-brown on the inside, thin, and scaly. The wood is light, soft, has narrow rings, and has a slight red tinge. The cones are cylindrical, long, with a glossy red-brown color and stiff scales. The cones hang down from branches.
Habitat
Red spruce grows at a slow to moderate rate, lives for 250 to 450+ years, and is very shade-tolerant when young. It is often found in pure stands or forests mixed with eastern white pine, balsam fir, or black spruce. Along with Fraser fir, red spruce is one of two primary tree types in the southern Appalachian spruce-fir forest, a distinct ecosystem found only in the highest elevations of the southern Appalachian Mountains. Its habitat is moist but well-drained sandy loam, often at high altitudes. Red spruce can be easily damaged by windthrow and acid rain.
Notable red spruce forests in West Virginia can be seen at Gaudineer Scenic Area, Canaan Valley, Roaring Plains West Wilderness, Dolly Sods Wilderness, and Spruce Mountain, all sites of former extensive red spruce forest.
Related species
It is closely related to black spruce, and hybrids between the two are frequent where their ranges meet. Genetic data suggests that the red spruce peripatrically speciated from the black spruce during glaciation in the Pleistocene. | Picea rubens | Wikipedia | 448 | 1495725 | https://en.wikipedia.org/wiki/Picea%20rubens | Biology and health sciences | Pinaceae | Plants |
Uses
Red spruce is used for Christmas trees and is an important wood used in making paper pulp. It is also an excellent tonewood and is used in many higher-end acoustic guitars and violins, as well as sound boards. The sap can be used to make spruce gum. Leafy red spruce twigs are boiled with sugar and flavoring to make spruce beer or spruce pudding. It can be used as construction lumber and is good for millwork and for crates.
Damaging factors
Like most trees, red spruce is subject to insect parasitism. Their insect enemy is the spruce budworm, although it is a bigger problem for white spruce and balsam fir. Other issues that have been damaging red spruce have included the increase in acid rain and current climate change.
One of the consequences of acid rain deposition is the decrease of soil exchangeable calcium and increase of aluminum. This is because acid precipitation disrupts cation and nutrition cycling in forest ecosystems. Components of acid rain such as H+, NO3−, and SO42- limit the uptake of calcium by trees and can increase aluminum availability.
Calcium concentration is important for red spruce for physiological processes such as dark respiration and cold tolerance, as well as disease resistance, signal transduction, membrane and cell wall synthesis and function, and regulation of stomata. Conversely, dissolved aluminum can be toxic or can interfere with root uptake of calcium and other nutrients. At the ecosystem and community levels, Calcium availability is associated with community composition, mature tree growth, and ecosystem productivity. One study testing the effects of added aluminum to soil, found that P. rubens mortality rate increased under these conditions.
During the 1980s, increased acid deposition contributed to a loss of high-elevation red spruce trees caused by leached calcium and thus decreased freezing tolerance. Additionally, the structure of the spruce needle enhances the capture of water and particles, which has been shown to add to soil acidification, nutrient leaching, and forest decline. However, more recently, reductions in acid deposition have contributed to red spruce resurgence in some mountain areas in the northeastern United States. This increase in red spruce growth has been associated with an increase in rainfall pH, which reduces bulk acidic deposition. This suggests that policies aiming to reduce atmospheric pollution in this area have been effective, although other species sensitive to soil acidification, such as sugar maple, are still continuing to decline. | Picea rubens | Wikipedia | 478 | 1495725 | https://en.wikipedia.org/wiki/Picea%20rubens | Biology and health sciences | Pinaceae | Plants |
Conservation
The red spruce has low genetic diversity as well as a narrow ecological niche, meaning the tree is easily susceptible to changes within its environment. The Central Appalachian Spruce Restoration Initiative seeks to unite diverse partners with the goal of restoring historic red spruce ecosystems across the high-elevation landscapes of central Appalachians. The partners that make up this diverse group are Appalachian Mountain Joint Venture, Appalachian Regional Reforestation Initiative, Canaan Valley National Wildlife Refuge, Natural Resources Conservation Service, The Mountain Institute, The Nature Conservancy, Trout Unlimited, U.S. Forest Service Northern Research Station, U.S. Forest Service Monongahela National Forest, West Virginia Division of Natural Resources, West Virginia Division of Forestry, West Virginia Highlands Conservancy, West Virginia State Parks, and West Virginia University.
Prior to the late 19th century, of red spruce were in West Virginia. In the late 19th and early 20th century, a vast amount of logging began in the state, and the number of red spruce dwindled to . Silviculture is being used to help restore the population of the lost red spruce.
Significant efforts have been made to increase the growth of red spruce trees in western North Carolina, most notably by Molly Tartt on behalf of the Daughters of the American Revolution (DAR). Tartt embarked on a mission to find the lost red spruce forest that had been planted by the DAR as a memorial to the lives lost during the American Revolution. The forest, consisting of 50,000 trees was dedicated in 1940 and had until recently been forgotten until Tartt located and identified the forest near Devil's Courthouse. | Picea rubens | Wikipedia | 325 | 1495725 | https://en.wikipedia.org/wiki/Picea%20rubens | Biology and health sciences | Pinaceae | Plants |
A funnel cloud is a funnel-shaped cloud of condensed water droplets, associated with a rotating column of wind and extending from the base of a cloud (usually a cumulonimbus or towering cumulus cloud) but not reaching the ground or a water surface. A funnel cloud is usually visible as a cone-shaped or needle like protuberance from the main cloud base. Funnel clouds form most frequently in association with supercell thunderstorms, and are often, but not always, a visual precursor to tornadoes. Funnel clouds are visual phenomena, but these are not the vortex of wind itself.
"Classic" funnel clouds
If a funnel cloud touches the surface, the feature is considered a tornado, although ground level circulations begin before the visible condensation cloud appears. Most tornadoes begin as funnel clouds, but some funnel clouds do not make surface contact and these cannot be counted as tornadoes from the perspective of a naked eye observer, even as tornadic circulations of some intensity almost always are detectable when low-level radar observations are available. Also, tornadoes occur with some frequency without an associated condensation funnel. The term condensation funnel may refer to either a tornadic cloud or a funnel cloud aloft, but the term funnel cloud exclusively refers to a rotating condensation funnel not reaching the surface. If strong cyclonic winds are occurring at the surface and are connected to a cloud base, regardless of condensation, then the feature is a tornado.
Funnel clouds result from the low air pressures found within tornadoes. The low air pressure causes air flowing towards the vortex to expand and cool. If the air is sufficiently moist and cools to the dew point, a funnel cloud is produced. The air pressure around the outer edge of the funnel cloud generally corresponds to the air pressure of the cloud base of the parent cloud.
Debris swirls are usually evident prior to the condensation funnel reaching the surface. Some tornadoes may appear only as a debris swirl, with no obvious funnel cloud extending below the rotating cloud base at any time during the tornadic life cycle. The surface level vortex tends to strengthen over time following initial formation, making the debris swirls and the condensation more apparent. | Funnel cloud | Wikipedia | 449 | 1499520 | https://en.wikipedia.org/wiki/Funnel%20cloud | Physical sciences | Clouds | Earth science |
In cloud nomenclature, any funnel- or inverted-funnel-shaped cloud descending from cumulus or cumulonimbus clouds is technically described as an accessory feature called tuba. The terms tuba and funnel cloud are nearly but not exactly synonymous; a wall cloud, for example, is also a form of tuba. Funnel clouds associated with supercells usually form within and under wall clouds.
Cold-air funnel clouds
Cold-air funnel clouds (vortices) are generally much weaker than the vortices produced by supercells. Although cold-air funnels rarely make ground contact, surface level vortices sometimes become strong enough for condensation cloud to "touch down" briefly, becoming visible as weak tornadoes or waterspouts.
Unlike the related phenomenon associated with severe thunderstorms, cold-air funnels are generally associated with partly cloudy skies in the wake of cold fronts, especially associated with certain low pressure systems, or in association with atmospheric boundaries such as lake and sea breezes or outflow boundaries.
The larger scale weather conditions are characterized by particularly cold air aloft over relatively warmer low level air, leading to high lapse rates, and often by high environmental vorticity yet relatively meager vertical wind shear. The funnels develop where atmospheric instability and moisture are sufficient to support towering cumulus clouds but typically limited to no or to little precipitation. Cold-air funnels, although weak, may persist for several minutes, and areas of intermittently forming funnel clouds may occur for tens of minutes.
Multiple such areas of activity may form within the same region during afternoon heating. Cold-air funnels appears to be a diurnal phenomenon, weakening and eventually dissipating with loss of solar heating. When precipitation does develop, the associated downdraft tends to cause rapid demise of the cold-air funnels. The mixing of cooler air in the lower troposphere with air flowing in a different direction in the middle troposphere causes the rotation on a horizontal axis, which, when deflected and tightened vertically by convective updrafts, forms a vertical rotation that can cause condensation to form a funnel cloud.
Cold-air funnel clouds are a common sight along the Pacific Coast of the United States, particularly in the spring or fall. | Funnel cloud | Wikipedia | 461 | 1499520 | https://en.wikipedia.org/wiki/Funnel%20cloud | Physical sciences | Clouds | Earth science |
On July 29, 2013, a cold-core funnel cloud touched down as an EF0 tornado in Ottawa, Ontario, Canada, causing extensive damage in the form of downed trees on a golf course. No advance weather watches or warnings were issued by Environment Canada, and the tornado was spawned from one of the few non-severe storm clouds moving through the area.
Other funnel clouds
Other funnel clouds include shear or "high based" funnels, which are ephemeral, small, and weak funnels associated with small cumulus clouds, often even those rooted aloft above the boundary or surface layer, and in "fair weather" conditions. Small funnel clouds, such as some occurring within vicinity of mountains, occur by unknown processes.
Shear funnels might also occur with weak transient circulations at the cloud base of thunderstorms. Mesoanticyclones, which accompany mesocyclones, often exhibit these funnel clouds. Brief funnels also are observed with some rear flank downdrafts (RFDs) (within inflow or outflow areas, and especially within inflow-outflow interchange areas as RFDs interact with mesocyclones or flanking line updrafts) and streamwise vorticity currents (SVCs) feeding into mesocyclones.
Although not considered a separate kind of funnel cloud, some funnel clouds form with mesovortices associated with squall lines, which also can become tornadoes but are often not visible as funnel clouds or tornadoes because they usually occur within obscuring precipitation. Other "fair weather" funnel clouds include horseshoe clouds which are a very transient phenomena associated with extremely weak vortices. | Funnel cloud | Wikipedia | 338 | 1499520 | https://en.wikipedia.org/wiki/Funnel%20cloud | Physical sciences | Clouds | Earth science |
Toxodon (meaning "bow tooth" in reference to the curvature of the teeth) is an extinct genus of large ungulate native to South America from the Pliocene to the end of the Late Pleistocene. Toxodon is a member of Notoungulata, an order of extinct South American native ungulates distinct from the two living ungulate orders that had been indigenous to the continent for over 60 million years since the early Cenozoic, prior to the arrival of living ungulates into South America around 2.5 million years ago during the Great American Interchange. Toxodon is a member of the family Toxodontidae, which includes medium to large sized herbivores. Toxodon was one of the largest members of Toxodontidae and Notoungulata, with Toxodon platensis having an estimated body mass of .
Remains of Toxodon were first collected by Charles Darwin during the voyage of the Beagle in 1832-33, and later scientifically named by Richard Owen in 1837. Both Darwin and Owen were puzzled by Toxodon's unusual anatomical features, including its long, ever-growing cheek teeth.
Toxodon has been found across much of South America, excluding southern Patagonia, the Andes and northeastern-most region of the continent, inhabiting steppe, savanna and sometimes woodland habitats. Evidence suggests that Toxodon was ecologically plastic and able to adapt its diet to local conditions. While some authors have suggested that Toxodon was semiaquatic, isotopic analysis has supported a terrestrial lifestyle.
Toxodon became extinct as part of the end-Pleistocene extinctions around 12,000 years ago, along with most large mammals across the Americas. The extinctions followed the arrival of humans to South America, who may have been a contributory factor in the extinctions. Several sites have been found suggesting human interaction with Toxodon.
Taxonomy and evolution | Toxodon | Wikipedia | 392 | 2145666 | https://en.wikipedia.org/wiki/Toxodon | Biology and health sciences | Mammals: General | Animals |
Charles Darwin, who was in South America as part of the second voyaging expedition of HMS Beagle, was one of the first to collect Toxodon fossils. In September–October 1832 and October 1833, Darwin collected several isolated teeth as well as a mandible from various localities in northern Argentina. On November 26, 1833, Darwin paid 18 pence (equivalent to £6.40 in 2018) for a T. platensis skull from a farmer in Uruguay. In his book covering the expedition, The Voyage of the Beagle. Darwin wrote, "November 26th – I set out on my return in a direct line for Montevideo. Having heard of some giant's bones at a neighbouring farm-house on the Sarandis, a small stream entering the Rio Negro, I rode there accompanied by my host, and purchased for the value of eighteen pence the head of the Toxodon." The skull had been propped up against a fence and been used as target practice for throwing stones by local children, who had knocked out its teeth. Since Darwin discovered that the fossils of similar mammals of South America were different from those in Europe, he invoked many debates about the evolution and natural selection of animals.
In his own words, Darwin wrote down in his journal,
Toxodon and its type species, T. platensis, were described in 1837 by Richard Owen based on remains collected by Darwin, in a paper titled "A description of the cranium of the Toxodon platensis, a gigantic extinct mammiferous species, referrible by its dentition to the Rodentia, but with affinities to the Pachydermata and the herbivorous Cetacea", reflecting the many unusual characteristics of its anatomy.
Evolution
Toxodon is a member of Notoungulata, a group of South American native ungulates that had been part of the fauna of South America since the Paleocene, over 60 million years ago, and had evolved in isolation in South America, prior to the arrival of living ungulates in South America around 2.5 million years ago as part of the Great American Interchange. Notoungulata represents the most diverse group of indigenous South American ungulates, with over 150 described genera in 13 different families. Notoungulates are morphologically diverse, including forms morphologically distant from Toxodon such as rodent and rabbit-like forms. | Toxodon | Wikipedia | 489 | 2145666 | https://en.wikipedia.org/wiki/Toxodon | Biology and health sciences | Mammals: General | Animals |
Analysis of collagen sequences obtained from Toxodon as well as from Macrauchenia, a member of another indigenous South American ungulate order, Litopterna, found that notoungulates and litopterns were closely related to each other, and form a sister group to perissodactyls (which contains equids, rhinoceroses and tapirs) as part of the clade Panperissodactyla, making them true ungulates. This finding has been corroborated by an analysis of mitochondrial DNA extracted from a Macrauchenia fossil, which yielded a date of 66 million years ago for the time of the split from perissodactyls.
Toxodon belongs to Toxodontidae, a large bodied group of notoungulates which first appeared in the Late Oligocene (Deseadan), ~28-23 million years ago, and underwent a great radiation during the Miocene epoch (~23-5.3 million years ago), when they reached their apex of diversity. The diversity of toxodontids, along with other notoungulates began to decline from around the Pliocene onwards, possibly as a result of climate change, as well as the arrival of competitors and predators from North America during the Great American Interchange following formation of the Isthmus of Panama. By the Late Pleistocene (Lujanian), the once great diversity of notoungulates had declined to only a few of species of toxodontids, with all other notoungulate families having become extinct.
Cladogram of Toxodontidae, showing the position of Toxodon relative to other toxodontids, after Forasiepi et al., 2014:
Species
There has not been a recent taxonomic revision of the genus Toxodon, leaving the number of valid species uncertain. | Toxodon | Wikipedia | 385 | 2145666 | https://en.wikipedia.org/wiki/Toxodon | Biology and health sciences | Mammals: General | Animals |
The species Toxodon chapalmalensis is known from the Pliocene (Montehermosan-Chapadmalalan) of Argentina, while Toxodon platensis, the type species, is known from the Pleistocene. The validity of other potential species like Toxodon darwini Burmeister, 1866, and Toxodon ensenadensis Ameghino, 1887 from the Early Pleistocene of Argentina is uncertain, and the species Toxodon gezi C. Ameghino, 1917 and Toxodon aguirrei Ameghino, 1917 have been considered junior synonyms of Toxodon platensis by recent authors. Some recent authors have argued that Toxodon gracilis Gervais and Ameghino, 1880, should be recognised as a distinct species from the Pleistocene of the Pampas significantly smaller than T. platensis, with these authors suggesting that T. platensis and T. gracilis represent the only valid species of Toxodon in the Pleistocene of the Pampas region. Other authors have argued that all Pleistocene Toxodon species should be considered synonymous with T. platensis.
Description
The bodyform of Toxodon and other toxodontids have been compared to those of hippopotamuses and rhinoceroses. Toxodon platensis is one of the largest known toxodontids and notoungulates, with an estimated body mass of approximately , and a body length of around . | Toxodon | Wikipedia | 303 | 2145666 | https://en.wikipedia.org/wiki/Toxodon | Biology and health sciences | Mammals: General | Animals |
The skull of Toxodon is proportionally large, and triangular in shape when viewed from above. All of the teeth in the jaws are high-crowned (hypsodont). Like other toxodontids, the upper and lower first incisors (I1 and i1) are large and protrude, with the second upper incisors (I2) and lower third incisors (i3) being modified into evergrowing tusks. The upper incisors display an arched shape, while the lower incisors project horizontally forwards at the front of the lower jaw. The wide front of the lower jaw with the horizontally-arranged incisors has been described as "spade-like". There is a gap (diastema) between the incisors and the cheek teeth. Like other derived toxodontids, Toxodon had long, ever-growing (hypselodont) cheek (premolar and molar) teeth, with the name Toxodon deriving from the curved shape of the upper molars, which are bowed inwards towards the midline of the skull to fit in the upper jaw. Evergrowing cheek teeth are unknown in any living ungulates, but are present in some other mammal groups like wombats and rodents. The surface of the cheek teeth is primarily composed of dentine.
The thoracic vertebrae of Toxodon have elongate neural spines, which likely anchored muscles which supported the large head. The legs of Toxodon are relatively short, with their bones being robust. The hindlimb is considerably longer than the forelimb, resulting in the back being elevated and the shoulder, neck and head being relatively low. The ulna has a strongly backwardly projecting olecranon process similar to that of rhinos, suggesting that the front leg was held extended when standing. The (distal) part of the femur closest to the foot shows a pronounced medial trochlear ridge, which likely served along with the patella (kneecap) to allow the knees to be locked when standing akin to the stay apparatus of living horses as an energy saving mechanism. There are three functional digits on each foot, which are tipped with hoof-like phalanges. | Toxodon | Wikipedia | 468 | 2145666 | https://en.wikipedia.org/wiki/Toxodon | Biology and health sciences | Mammals: General | Animals |
Distribution
Toxodon had a widespread distribution in South America east of the Andes, ranging from northern Argentina and Bolivia to the western Amazon on the Peru-Brazil border, to Northeast Brazil. Although some authors suggest that the distribution of Toxodon extended into Venezuela, other authors suggest that the related Mixotoxodon (which ranged as far north as the southern United States) was the only toxodontid present in the region during the Pleistocene.
Palaeobiology
Although some authors have suggested that Toxodon was semiaquatic based on the similarity of some aspects of its anatomy to hippopotamuses, this has been disputed by other authors, and analysis of oxygen isotope ratios (which differ between terrestrial and aquatic animals) suggests a terrestrial lifestyle for Toxodon. As such, it has been suggested that Toxodon was probably more ecologically comparable to rhinoceroses.
Toxodon is suggested to have been capable of moving at considerable speed. Toxodon is believed to have been ecologically plastic and have had a wide niche breadth, with its diet varying according to local conditions, with an almost totally C3 browsing diet in the Amazon rainforest, mixed feeding C3 in Bahia and the Pampas to almost completely C4 dominated grazing diet in the Chaco. Within the Brazilian Intertropical Region, local climate had little impact on the diet of T. platensis. Although Toxodon is thought to have inhabited open landscapes like steppe and savannah, in some areas like the southwestern Brazilian Amazon, it is suggested to have inhabited woodland.
Like living animals of similar size, it has been suggested that Toxodon probably only gave birth to a single offspring at a time.
T. platensis bones have been found displaying signs of disease like osteomyelitis and spondyloarthropathies. The teeth of Toxodon often display enamel hypoplasia (loss of tooth enamel) in the form of grooves and pits, which is likely due to their evergrowing nature and/or environmental stresses.
Tracks probably attributable to Toxodon have been reported from eastern Pernambuco in Northeast Brazil.
Isotopic analysis suggests that Toxodon may have been predated upon by the large sabertooth cat Smilodon populator, the apex predator of South American ecosystems during much of the Pleistocene. | Toxodon | Wikipedia | 476 | 2145666 | https://en.wikipedia.org/wiki/Toxodon | Biology and health sciences | Mammals: General | Animals |
Extinction
Toxodon became extinct at the end of the Late Pleistocene around 12,000 years as part of the end-Pleistocene extinction event alongside almost all other large animals in South America. Previous mid-Holocene dates are now thought to be in error. These extinctions followed the first arrival of humans in the Americas, and it has been suggested human hunting may have been a casual factor in the extinctions. Several sites record apparent interactions between Toxodon and humans. Remains of Toxodon from the Arroyo Seco 2 site in the Pampas are associated with unambiguously butchered megafauna, but it is unclear if the Toxodon itself was actually butchered or the remains were naturally transported to the site. At the Paso Otero 5 site in the Pampas of northeast Argentina, burned bones of Toxodon alongside those of numerous other extinct megafauna species are associated with Fishtail points (a type of knapped stone spear point common across South America at the end of the Pleistocene, suggested to be used to hunt large mammals). The bones of the megafauna were probably deliberately burned as fuel. No cut marks are visible on the vast majority of bones at the site (with only one bone of a llama possibly displaying any butchery marks), which may be due to the burning degrading the bones. Various remains of Toxodon platensis in the collection of the Museum national d'Histoire naturelle collected from the Pampas region in the 19th century including a femur, an iliac fragment, a tibia, as well as a mandible (the latter of which has been radiocarbon dated to around 13,000 years ago), have been found to display cut marks indicative of butchery. | Toxodon | Wikipedia | 356 | 2145666 | https://en.wikipedia.org/wiki/Toxodon | Biology and health sciences | Mammals: General | Animals |
CRISPR () (an acronym for clustered regularly interspaced short palindromic repeats) is a family of DNA sequences found in the genomes of prokaryotic organisms such as bacteria and archaea. Each sequence within an individual prokaryotic cell is derived from a DNA fragment of a bacteriophage that had previously infected the prokaryote or one of its ancestors. These sequences are used to detect and destroy DNA from similar bacteriophages during subsequent infections. Hence these sequences play a key role in the antiviral (i.e. anti-phage) defense system of prokaryotes and provide a form of heritable, acquired immunity. CRISPR is found in approximately 50% of sequenced bacterial genomes and nearly 90% of sequenced archaea.
Cas9 (or "CRISPR-associated protein 9") is an enzyme that uses CRISPR sequences as a guide to recognize and open up specific strands of DNA that are complementary to the CRISPR sequence. Cas9 enzymes together with CRISPR sequences form the basis of a technology known as CRISPR-Cas9 that can be used to edit genes within living organisms. This editing process has a wide variety of applications including basic biological research, development of biotechnological products, and treatment of diseases. The development of the CRISPR-Cas9 genome editing technique was recognized by the Nobel Prize in Chemistry in 2020 awarded to Emmanuelle Charpentier and Jennifer Doudna.
History
Repeated sequences
The discovery of clustered DNA repeats took place independently in three parts of the world. The first description of what would later be called CRISPR is from Osaka University researcher Yoshizumi Ishino and his colleagues in 1987. They accidentally cloned part of a CRISPR sequence together with the "iap" gene (isozyme conversion of alkaline phosphatase) from their target genome, that of Escherichia coli. The organization of the repeats was unusual. Repeated sequences are typically arranged consecutively, without interspersing different sequences. They did not know the function of the interrupted clustered repeats.
In 1993, researchers of Mycobacterium tuberculosis in the Netherlands published two articles about a cluster of interrupted direct repeats (DR) in that bacterium. They recognized the diversity of the sequences that intervened in the direct repeats among different strains of M. tuberculosis and used this property to design a typing method called spoligotyping, still in use today. | CRISPR | Wikipedia | 502 | 2146034 | https://en.wikipedia.org/wiki/CRISPR | Technology | Biotechnology | null |
Francisco Mojica at the University of Alicante in Spain studied the function of repeats in the archaeal species Haloferax and Haloarcula. Mojica's supervisor surmised that the clustered repeats had a role in correctly segregating replicated DNA into daughter cells during cell division, because plasmids and chromosomes with identical repeat arrays could not coexist in Haloferax volcanii. Transcription of the interrupted repeats was also noted for the first time; this was the first full characterization of CRISPR. By 2000, Mojica and his students, after an automated search of published genomes, identified interrupted repeats in 20 species of microbes as belonging to the same family. Because those sequences were interspaced, Mojica initially called these sequences "short regularly spaced repeats" (SRSR). In 2001, Mojica and Ruud Jansen, who were searching for an additional interrupted repeats, proposed the acronym CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) to unify the numerous acronyms used to describe these sequences. In 2002, Tang, et al. showed evidence that CRISPR repeat regions from the genome of Archaeoglobus fulgidus were transcribed into long RNA molecules subsequently processed into unit-length small RNAs, plus some longer forms of 2, 3, or more spacer-repeat units.
In 2005, yogurt researcher Rodolphe Barrangou discovered that Streptococcus thermophilus, after iterative phage infection challenges, develops increased phage resistance due to the incorporation of additional CRISPR spacer sequences. Barrangou's employer, the Danish food company Danisco, then developed phage-resistant S. thermophilus strains for yogurt production. Danisco was later bought by DuPont, which owns about 50 percent of the global dairy culture market, and the technology spread widely.
CRISPR-associated systems
A major advance in understanding CRISPR came with Jansen's observation that the prokaryote repeat cluster was accompanied by four homologous genes that make up CRISPR-associated systems, cas 1–4. The Cas proteins showed helicase and nuclease motifs, suggesting a role in the dynamic structure of the CRISPR loci. In this publication, the acronym CRISPR was used as the universal name of this pattern, but its function remained enigmatic. | CRISPR | Wikipedia | 497 | 2146034 | https://en.wikipedia.org/wiki/CRISPR | Technology | Biotechnology | null |
In 2005, three independent research groups showed that some CRISPR spacers are derived from phage DNA and extrachromosomal DNA such as plasmids. In effect, the spacers are fragments of DNA gathered from viruses that previously attacked the cell. The source of the spacers was a sign that the CRISPR-cas system could have a role in adaptive immunity in bacteria. All three studies proposing this idea were initially rejected by high-profile journals, but eventually appeared in other journals.
The first publication proposing a role of CRISPR-Cas in microbial immunity, by Mojica and collaborators at the University of Alicante, predicted a role for the RNA transcript of spacers on target recognition in a mechanism that could be analogous to the RNA interference system used by eukaryotic cells. Koonin and colleagues extended this RNA interference hypothesis by proposing mechanisms of action for the different CRISPR-Cas subtypes according to the predicted function of their proteins. | CRISPR | Wikipedia | 197 | 2146034 | https://en.wikipedia.org/wiki/CRISPR | Technology | Biotechnology | null |
Experimental work by several groups revealed the basic mechanisms of CRISPR-Cas immunity. In 2007, the first experimental evidence that CRISPR was an adaptive immune system was published. A CRISPR region in Streptococcus thermophilus acquired spacers from the DNA of an infecting bacteriophage. The researchers manipulated the resistance of S. thermophilus to different types of phages by adding and deleting spacers whose sequence matched those found in the tested phages. In 2008, Brouns and Van der Oost identified a complex of Cas proteins called Cascade, that in E. coli cut the CRISPR RNA precursor within the repeats into mature spacer-containing RNA molecules called CRISPR RNA (crRNA), which remained bound to the protein complex. Moreover, it was found that Cascade, crRNA and a helicase/nuclease (Cas3) were required to provide a bacterial host with immunity against infection by a DNA virus. By designing an anti-virus CRISPR, they demonstrated that two orientations of the crRNA (sense/antisense) provided immunity, indicating that the crRNA guides were targeting dsDNA. That year Marraffini and Sontheimer confirmed that a CRISPR sequence of S. epidermidis targeted DNA and not RNA to prevent conjugation. This finding was at odds with the proposed RNA-interference-like mechanism of CRISPR-Cas immunity, although a CRISPR-Cas system that targets foreign RNA was later found in Pyrococcus furiosus. A 2010 study showed that CRISPR-Cas cuts strands of both phage and plasmid DNA in S. thermophilus.
Cas9 | CRISPR | Wikipedia | 353 | 2146034 | https://en.wikipedia.org/wiki/CRISPR | Technology | Biotechnology | null |
A simpler CRISPR system from Streptococcus pyogenes relies on the protein Cas9. The Cas9 endonuclease is a four-component system that includes two small molecules: crRNA and trans-activating CRISPR RNA (tracrRNA). In 2012, Jennifer Doudna and Emmanuelle Charpentier re-engineered the Cas9 endonuclease into a more manageable two-component system by fusing the two RNA molecules into a "single-guide RNA" that, when combined with Cas9, could find and cut the DNA target specified by the guide RNA. This contribution was so significant that it was recognized by the Nobel Prize in Chemistry in 2020. By manipulating the nucleotide sequence of the guide RNA, the artificial Cas9 system could be programmed to target any DNA sequence for separation. Another collaboration comprising Virginijus Šikšnys, Gasiūnas, Barrangou, and Horvath showed that Cas9 from the S. thermophilus CRISPR system can also be reprogrammed to target a site of their choosing by changing the sequence of its crRNA. These advances fueled efforts to edit genomes with the modified CRISPR-Cas9 system.
Groups led by Feng Zhang and George Church simultaneously published descriptions of genome editing in human cell cultures using CRISPR-Cas9 for the first time. It has since been used in a wide range of organisms, including baker's yeast (Saccharomyces cerevisiae), the opportunistic pathogen Candida albicans, zebrafish (Danio rerio), fruit flies (Drosophila melanogaster), ants (Harpegnathos saltator and Ooceraea biroi), mosquitoes (Aedes aegypti), nematodes (Caenorhabditis elegans), plants, mice (Mus musculus domesticus), monkeys and human embryos.
CRISPR has been modified to make programmable transcription factors that allows activation or silencing of targeted genes. | CRISPR | Wikipedia | 429 | 2146034 | https://en.wikipedia.org/wiki/CRISPR | Technology | Biotechnology | null |
The CRISPR-Cas9 system has been shown to make effective gene edits in Human tripronuclear zygotes, as first described in a 2015 paper by Chinese scientists P. Liang and Y. Xu. The system made a successful cleavage of mutant Beta-Hemoglobin (HBB) in 28 out of 54 embryos. Four out of the 28 embryos were successfully recombined using a donor template. The scientists showed that during DNA recombination of the cleaved strand, the homologous endogenous sequence HBD competes with the exogenous donor template. DNA repair in human embryos is much more complicated and particular than in derived stem cells.
Cas12a
In 2015, the nuclease Cas12a (formerly called ) was characterized in the CRISPR-Cpf1 system of the bacterium Francisella novicida. Its original name, from a TIGRFAMs protein family definition built in 2012, reflects the prevalence of its CRISPR-Cas subtype in the Prevotella and Francisella lineages. Cas12a showed several key differences from Cas9 including: causing a 'staggered' cut in double stranded DNA as opposed to the 'blunt' cut produced by Cas9, relying on a 'T rich' PAM (providing alternative targeting sites to Cas9), and requiring only a CRISPR RNA (crRNA) for successful targeting. By contrast, Cas9 requires both crRNA and a trans-activating crRNA (tracrRNA). | CRISPR | Wikipedia | 313 | 2146034 | https://en.wikipedia.org/wiki/CRISPR | Technology | Biotechnology | null |
These differences may give Cas12a some advantages over Cas9. For example, Cas12a's small crRNAs are ideal for multiplexed genome editing, as more of them can be packaged in one vector than can Cas9's sgRNAs. The sticky 5′ overhangs left by Cas12a can also be used for DNA assembly that is much more target-specific than traditional restriction enzyme cloning. Finally, Cas12a cleaves DNA 18–23 base pairs downstream from the PAM site. This means there is no disruption to the recognition sequence after repair, and so Cas12a enables multiple rounds of DNA cleavage. By contrast, since Cas9 cuts only 3 base pairs upstream of the PAM site, the NHEJ pathway results in indel mutations that destroy the recognition sequence, thereby preventing further rounds of cutting. In theory, repeated rounds of DNA cleavage should cause an increased opportunity for the desired genomic editing to occur. A distinctive feature of Cas12a, as compared to Cas9, is that after cutting its target, Cas12a remains bound to the target and then cleaves other ssDNA molecules non-discriminately. This property is called "collateral cleavage" or "trans-cleavage" activity and has been exploited for the development of various diagnostic technologies.
Cas13
In 2016, the nuclease (formerly known as ) from the bacterium Leptotrichia shahii was characterized. Cas13 is an RNA-guided RNA endonuclease, which means that it does not cleave DNA, but only single-stranded RNA. Cas13 is guided by its crRNA to a ssRNA target and binds and cleaves the target. Similar to Cas12a, the Cas13 remains bound to the target and then cleaves other ssRNA molecules non-discriminately. This collateral cleavage property has been exploited for the development of various diagnostic technologies.
In 2021, Dr. Hui Yang characterized novel miniature Cas13 protein (mCas13) variants, Cas13X and Cas13Y. Using a small portion of N gene sequence from SARS-CoV-2 as a target in characterization of mCas13, revealed the sensitivity and specificity of mCas13 coupled with RT-LAMP for detection of SARS-CoV-2 in both synthetic and clinical samples over other available standard tests like RT-qPCR (1 copy/μL).
Locus structure | CRISPR | Wikipedia | 496 | 2146034 | https://en.wikipedia.org/wiki/CRISPR | Technology | Biotechnology | null |
Repeats and spacers
The CRISPR array is made up of an AT-rich leader sequence followed by short repeats that are separated by unique spacers. CRISPR repeats typically range in size from 28 to 37 base pairs (bps), though there can be as few as 23 bp and as many as 55 bp. Some show dyad symmetry, implying the formation of a secondary structure such as a stem-loop ('hairpin') in the RNA, while others are designed to be unstructured. The size of spacers in different CRISPR arrays is typically 32 to 38 bp (range 21 to 72 bp). New spacers can appear rapidly as part of the immune response to phage infection. There are usually fewer than 50 units of the repeat-spacer sequence in a CRISPR array.
CRISPR RNA structures
Cas genes and CRISPR subtypes
Small clusters of cas genes are often located next to CRISPR repeat-spacer arrays. Collectively the 93 cas genes are grouped into 35 families based on sequence similarity of the encoded proteins. 11 of the 35 families form the cas core, which includes the protein families Cas1 through Cas9. A complete CRISPR-Cas locus has at least one gene belonging to the cas core.
CRISPR-Cas systems fall into two classes. Class 1 systems use a complex of multiple Cas proteins to degrade foreign nucleic acids. Class 2 systems use a single large Cas protein for the same purpose. Class 1 is divided into types I, III, and IV; class 2 is divided into types II, V, and VI. The 6 system types are divided into 33 subtypes. Each type and most subtypes are characterized by a "signature gene" found almost exclusively in the category. Classification is also based on the complement of cas genes that are present. Most CRISPR-Cas systems have a Cas1 protein. The phylogeny of Cas1 proteins generally agrees with the classification system, but exceptions exist due to module shuffling. Many organisms contain multiple CRISPR-Cas systems suggesting that they are compatible and may share components. The sporadic distribution of the CRISPR-Cas subtypes suggests that the CRISPR-Cas system is subject to horizontal gene transfer during microbial evolution.
Mechanism
CRISPR-Cas immunity is a natural process of bacteria and archaea. CRISPR-Cas prevents bacteriophage infection, conjugation and natural transformation by degrading foreign nucleic acids that enter the cell. | CRISPR | Wikipedia | 501 | 2146034 | https://en.wikipedia.org/wiki/CRISPR | Technology | Biotechnology | null |
Spacer acquisition
When a microbe is invaded by a bacteriophage, the first stage of the immune response is to capture phage DNA and insert it into a CRISPR locus in the form of a spacer. Cas1 and Cas2 are found in both types of CRISPR-Cas immune systems, which indicates that they are involved in spacer acquisition. Mutation studies confirmed this hypothesis, showing that removal of Cas1 or Cas2 stopped spacer acquisition, without affecting CRISPR immune response.
Multiple Cas1 proteins have been characterised and their structures resolved. Cas1 proteins have diverse amino acid sequences. However, their crystal structures are similar and all purified Cas1 proteins are metal-dependent nucleases/integrases that bind to DNA in a sequence-independent manner. Representative Cas2 proteins have been characterised and possess either (single strand) ssRNA- or (double strand) dsDNA- specific endoribonuclease activity.
In the I-E system of E. coli Cas1 and Cas2 form a complex where a Cas2 dimer bridges two Cas1 dimers. In this complex Cas2 performs a non-enzymatic scaffolding role, binding double-stranded fragments of invading DNA, while Cas1 binds the single-stranded flanks of the DNA and catalyses their integration into CRISPR arrays. New spacers are usually added at the beginning of the CRISPR next to the leader sequence creating a chronological record of viral infections. In E. coli a histone like protein called integration host factor (IHF), which binds to the leader sequence, is responsible for the accuracy of this integration. IHF also enhances integration efficiency in the type I-F system of Pectobacterium atrosepticum. but in other systems, different host factors may be required
Protospacer adjacent motifs (PAM) | CRISPR | Wikipedia | 382 | 2146034 | https://en.wikipedia.org/wiki/CRISPR | Technology | Biotechnology | null |
Bioinformatic analysis of regions of phage genomes that were excised as spacers (termed protospacers) revealed that they were not randomly selected but instead were found adjacent to short (3–5 bp) DNA sequences termed protospacer adjacent motifs (PAM). Analysis of CRISPR-Cas systems showed PAMs to be important for type I and type II, but not type III systems during acquisition. In type I and type II systems, protospacers are excised at positions adjacent to a PAM sequence, with the other end of the spacer cut using a ruler mechanism, thus maintaining the regularity of the spacer size in the CRISPR array. The conservation of the PAM sequence differs between CRISPR-Cas systems and appears to be evolutionarily linked to Cas1 and the leader sequence.
New spacers are added to a CRISPR array in a directional manner, occurring preferentially, but not exclusively, adjacent to the leader sequence. Analysis of the type I-E system from E. coli demonstrated that the first direct repeat adjacent to the leader sequence is copied, with the newly acquired spacer inserted between the first and second direct repeats.
The PAM sequence appears to be important during spacer insertion in type I-E systems. That sequence contains a strongly conserved final nucleotide (nt) adjacent to the first nt of the protospacer. This nt becomes the final base in the first direct repeat. This suggests that the spacer acquisition machinery generates single-stranded overhangs in the second-to-last position of the direct repeat and in the PAM during spacer insertion. However, not all CRISPR-Cas systems appear to share this mechanism as PAMs in other organisms do not show the same level of conservation in the final position. It is likely that in those systems, a blunt end is generated at the very end of the direct repeat and the protospacer during acquisition.
Insertion variants
Analysis of Sulfolobus solfataricus CRISPRs revealed further complexities to the canonical model of spacer insertion, as one of its six CRISPR loci inserted new spacers randomly throughout its CRISPR array, as opposed to inserting closest to the leader sequence. | CRISPR | Wikipedia | 449 | 2146034 | https://en.wikipedia.org/wiki/CRISPR | Technology | Biotechnology | null |
Multiple CRISPRs contain many spacers to the same phage. The mechanism that causes this phenomenon was discovered in the type I-E system of E. coli. A significant enhancement in spacer acquisition was detected where spacers already target the phage, even mismatches to the protospacer. This 'priming' requires the Cas proteins involved in both acquisition and interference to interact with each other. Newly acquired spacers that result from the priming mechanism are always found on the same strand as the priming spacer. This observation led to the hypothesis that the acquisition machinery slides along the foreign DNA after priming to find a new protospacer.
Biogenesis
CRISPR-RNA (crRNA), which later guides the Cas nuclease to the target during the interference step, must be generated from the CRISPR sequence. The crRNA is initially transcribed as part of a single long transcript encompassing much of the CRISPR array. This transcript is then cleaved by Cas proteins to form crRNAs. The mechanism to produce crRNAs differs among CRISPR-Cas systems. In type I-E and type I-F systems, the proteins Cas6e and Cas6f respectively, recognise stem-loops created by the pairing of identical repeats that flank the crRNA. These Cas proteins cleave the longer transcript at the edge of the paired region, leaving a single crRNA along with a small remnant of the paired repeat region.
Type III systems also use Cas6, however, their repeats do not produce stem-loops. Cleavage instead occurs by the longer transcript wrapping around the Cas6 to allow cleavage just upstream of the repeat sequence.
Type II systems lack the Cas6 gene and instead utilize RNaseIII for cleavage. Functional type II systems encode an extra small RNA that is complementary to the repeat sequence, known as a trans-activating crRNA (tracrRNA). Transcription of the tracrRNA and the primary CRISPR transcript results in base pairing and the formation of dsRNA at the repeat sequence, which is subsequently targeted by RNaseIII to produce crRNAs. Unlike the other two systems, the crRNA does not contain the full spacer, which is instead truncated at one end. | CRISPR | Wikipedia | 453 | 2146034 | https://en.wikipedia.org/wiki/CRISPR | Technology | Biotechnology | null |
CrRNAs associate with Cas proteins to form ribonucleotide complexes that recognize foreign nucleic acids. CrRNAs show no preference between the coding and non-coding strands, which is indicative of an RNA-guided DNA-targeting system. The type I-E complex (commonly referred to as Cascade) requires five Cas proteins bound to a single crRNA.
Interference
During the interference stage in type I systems, the PAM sequence is recognized on the crRNA-complementary strand and is required along with crRNA annealing. In type I systems correct base pairing between the crRNA and the protospacer signals a conformational change in Cascade that recruits Cas3 for DNA degradation.
Type II systems rely on a single multifunctional protein, Cas9, for the interference step. Cas9 requires both the crRNA and the tracrRNA to function and cleave DNA using its dual HNH and RuvC/RNaseH-like endonuclease domains. Basepairing between the PAM and the phage genome is required in type II systems. However, the PAM is recognized on the same strand as the crRNA (the opposite strand to type I systems).
Type III systems, like type I require six or seven Cas proteins binding to crRNAs. The type III systems analysed from S. solfataricus and P. furiosus both target the mRNA of phages rather than phage DNA genome, which may make these systems uniquely capable of targeting RNA-based phage genomes. Type III systems were also found to target DNA in addition to RNA using a different Cas protein in the complex, Cas10. The DNA cleavage was shown to be transcription dependent.
The mechanism for distinguishing self from foreign DNA during interference is built into the crRNAs and is therefore likely common to all three systems. Throughout the distinctive maturation process of each major type, all crRNAs contain a spacer sequence and some portion of the repeat at one or both ends. It is the partial repeat sequence that prevents the CRISPR-Cas system from targeting the chromosome as base pairing beyond the spacer sequence signals self and prevents DNA cleavage. RNA-guided CRISPR enzymes are classified as type V restriction enzymes.
Evolution | CRISPR | Wikipedia | 454 | 2146034 | https://en.wikipedia.org/wiki/CRISPR | Technology | Biotechnology | null |
The cas genes in the adaptor and effector modules of the CRISPR-Cas system are believed to have evolved from two different ancestral modules. A transposon-like element called casposon encoding the Cas1-like integrase and potentially other components of the adaptation module was inserted next to the ancestral effector module, which likely functioned as an independent innate immune system. The highly conserved cas1 and cas2 genes of the adaptor module evolved from the ancestral module while a variety of class 1 effector cas genes evolved from the ancestral effector module. The evolution of these various class 1 effector module cas genes was guided by various mechanisms, such as duplication events. On the other hand, each type of class 2 effector module arose from subsequent independent insertions of mobile genetic elements. These mobile genetic elements took the place of the multiple gene effector modules to create single gene effector modules that produce large proteins which perform all the necessary tasks of the effector module. The spacer regions of CRISPR-Cas systems are taken directly from foreign mobile genetic elements and thus their long-term evolution is hard to trace. The non-random evolution of these spacer regions has been found to be highly dependent on the environment and the particular foreign mobile genetic elements it contains.
CRISPR-Cas can immunize bacteria against certain phages and thus halt transmission. For this reason, Koonin described CRISPR-Cas as a Lamarckian inheritance mechanism. However, this was disputed by a critic who noted, "We should remember [Lamarck] for the good he contributed to science, not for things that resemble his theory only superficially. Indeed, thinking of CRISPR and other phenomena as Lamarckian only obscures the simple and elegant way evolution really works". But as more recent studies have been conducted, it has become apparent that the acquired spacer regions of CRISPR-Cas systems are indeed a form of Lamarckian evolution because they are genetic mutations that are acquired and then passed on. On the other hand, the evolution of the Cas gene machinery that facilitates the system evolves through classic Darwinian evolution.
Coevolution
Analysis of CRISPR sequences revealed coevolution of host and viral genomes. | CRISPR | Wikipedia | 453 | 2146034 | https://en.wikipedia.org/wiki/CRISPR | Technology | Biotechnology | null |
The basic model of CRISPR evolution is newly incorporated spacers driving phages to mutate their genomes to avoid the bacterial immune response, creating diversity in both the phage and host populations. To resist a phage infection, the sequence of the CRISPR spacer must correspond perfectly to the sequence of the target phage gene. Phages can continue to infect their hosts' given point mutations in the spacer. Similar stringency is required in PAM or the bacterial strain remains phage sensitive.
Rates
A study of 124 S. thermophilus strains showed that 26% of all spacers were unique and that different CRISPR loci showed different rates of spacer acquisition. Some CRISPR loci evolve more rapidly than others, which allowed the strains' phylogenetic relationships to be determined. A comparative genomic analysis showed that E. coli and S. enterica evolve much more slowly than S. thermophilus. The latter's strains that diverged 250,000 years ago still contained the same spacer complement.
Metagenomic analysis of two acid-mine-drainage biofilms showed that one of the analyzed CRISPRs contained extensive deletions and spacer additions versus the other biofilm, suggesting a higher phage activity/prevalence in one community than the other. In the oral cavity, a temporal study determined that 7–22% of spacers were shared over 17 months within an individual while less than 2% were shared across individuals.
From the same environment, a single strain was tracked using PCR primers specific to its CRISPR system. Broad-level results of spacer presence/absence showed significant diversity. However, this CRISPR added three spacers over 17 months, suggesting that even in an environment with significant CRISPR diversity some loci evolve slowly.
CRISPRs were analysed from the metagenomes produced for the Human Microbiome Project. Although most were body-site specific, some within a body site are widely shared among individuals. One of these loci originated from streptococcal species and contained ≈15,000 spacers, 50% of which were unique. Similar to the targeted studies of the oral cavity, some showed little evolution over time. | CRISPR | Wikipedia | 450 | 2146034 | https://en.wikipedia.org/wiki/CRISPR | Technology | Biotechnology | null |
CRISPR evolution was studied in chemostats using S. thermophilus to directly examine spacer acquisition rates. In one week, S. thermophilus strains acquired up to three spacers when challenged with a single phage. During the same interval, the phage developed single-nucleotide polymorphisms that became fixed in the population, suggesting that targeting had prevented phage replication absent these mutations.
Another S. thermophilus experiment showed that phages can infect and replicate in hosts that have only one targeting spacer. Yet another showed that sensitive hosts can exist in environments with high-phage titres. The chemostat and observational studies suggest many nuances to CRISPR and phage (co)evolution.
Identification
CRISPRs are widely distributed among bacteria and archaea and show some sequence similarities. Their most notable characteristic is their repeating spacers and direct repeats. This characteristic makes CRISPRs easily identifiable in long sequences of DNA, since the number of repeats decreases the likelihood of a false positive match.
Analysis of CRISPRs in metagenomic data is more challenging, as CRISPR loci do not typically assemble, due to their repetitive nature or through strain variation, which confuses assembly algorithms. Where many reference genomes are available, polymerase chain reaction (PCR) can be used to amplify CRISPR arrays and analyse spacer content. However, this approach yields information only for specifically targeted CRISPRs and for organisms with sufficient representation in public databases to design reliable polymerase PCR primers. Degenerate repeat-specific primers can be used to amplify CRISPR spacers directly from environmental samples; amplicons containing two or three spacers can be then computationally assembled to reconstruct long CRISPR arrays.
The alternative is to extract and reconstruct CRISPR arrays from shotgun metagenomic data. This is computationally more difficult, particularly with second generation sequencing technologies (e.g. 454, Illumina), as the short read lengths prevent more than two or three repeat units appearing in a single read. CRISPR identification in raw reads has been achieved using purely de novo identification or by using direct repeat sequences in partially assembled CRISPR arrays from contigs (overlapping DNA segments that together represent a consensus region of DNA) and direct repeat sequences from published genomes as a hook for identifying direct repeats in individual reads. | CRISPR | Wikipedia | 490 | 2146034 | https://en.wikipedia.org/wiki/CRISPR | Technology | Biotechnology | null |
Use by phages
Another way for bacteria to defend against phage infection is by having chromosomal islands. A subtype of chromosomal islands called phage-inducible chromosomal island (PICI) is excised from a bacterial chromosome upon phage infection and can inhibit phage replication. PICIs are induced, excised, replicated, and finally packaged into small capsids by certain staphylococcal temperate phages. PICIs use several mechanisms to block phage reproduction. In the first mechanism, PICI-encoded Ppi differentially blocks phage maturation by binding or interacting specifically with phage TerS, hence blocking phage TerS/TerL complex formation responsible for phage DNA packaging. In the second mechanism PICI CpmAB redirects the phage capsid morphogenetic protein to make 95% of SaPI-sized capsid and phage DNA can package only 1/3rd of their genome in these small capsids and hence become nonviable phage. The third mechanism involves two proteins, PtiA and PtiB, that target the LtrC, which is responsible for the production of virion and lysis proteins. This interference mechanism is modulated by a modulatory protein, PtiM, binds to one of the interference-mediating proteins, PtiA, and hence achieves the required level of interference.
One study showed that lytic ICP1 phage, which specifically targets Vibrio cholerae serogroup O1, has acquired a CRISPR-Cas system that targets a V. cholera PICI-like element. The system has 2 CRISPR loci and 9 Cas genes. It seems to be homologous to the I-F system found in Yersinia pestis. Moreover, like the bacterial CRISPR-Cas system, ICP1 CRISPR-Cas can acquire new sequences, which allows phage and host to co-evolve.
Certain archaeal viruses were shown to carry mini-CRISPR arrays containing one or two spacers. It has been shown that spacers within the virus-borne CRISPR arrays target other viruses and plasmids, suggesting that mini-CRISPR arrays represent a mechanism of heterotypic superinfection exclusion and participate in interviral conflicts.
Applications | CRISPR | Wikipedia | 472 | 2146034 | https://en.wikipedia.org/wiki/CRISPR | Technology | Biotechnology | null |
CRISPR gene editing is a revolutionary technology that allows for precise, targeted modifications to the DNA of living organisms. Developed from a natural defense mechanism found in bacteria, CRISPR-Cas9 is the most commonly used system, that allows "cutting" of DNA at specific locations and either delete, modify, or insert genetic material. This technology has transformed fields such as genetics, medicine, and agriculture, offering potential treatments for genetic disorders, advancements in crop engineering, and research into the fundamental workings of life. However, its ethical implications and potential unintended consequences have sparked significant debate. | CRISPR | Wikipedia | 118 | 2146034 | https://en.wikipedia.org/wiki/CRISPR | Technology | Biotechnology | null |
SN 1572 (Tycho's Star, Tycho's Nova, Tycho's Supernova), or B Cassiopeiae (B Cas), was a supernova of Type Ia in the constellation Cassiopeia, one of eight supernovae visible to the naked eye in historical records. It appeared in early November 1572 and was independently discovered by many individuals.
Its supernova remnant has been observed optically but was first detected at radio wavelengths. It is often known as 3C 10, a radio-source designation, although increasingly as Tycho's supernova remnant.
Historic description
The appearance of the Milky Way supernova of 1572 belongs among the most important observation events in the history of astronomy. The appearance of the "new star" helped to revise ancient models of the heavens and to speed on a revolution in astronomy that began with the realisation of the need to produce better astrometric star catalogues, and thus the need for more precise astronomical observing instruments. It also challenged the Aristotelian dogma of the unchangeability of the realm of stars.
The supernova of 1572 is often called "Tycho's supernova", because of Tycho Brahe's extensive work De nova et nullius aevi memoria prius visa stella ("Concerning the Star, new and never before seen in the life or memory of anyone", published in 1573 with reprints overseen by Johannes Kepler in 1602 and 1610), a work containing both Brahe's own observations and the analysis of sightings from many other observers. Comparisons between Brahe's observations and those of Spanish scientist Jerónimo Muñoz revealed that the object was more distant than the Moon. This led Brahe to approach the Great Comet of 1577 as an astronomical body as well. Other Europeans to sight the supernova included Wolfgang Schuler, Christopher Clavius, Thomas Digges, John Dee, Francesco Maurolico, Tadeáš Hájek and .
In England, Queen Elizabeth had the mathematician and astrologer Thomas Allen come and visit "to have his advice about the new star that appeared in the Swan or Cassiopeia ... to which he gave his judgement very learnedly", as the antiquary John Aubrey recorded in his memoranda a century later. | SN 1572 | Wikipedia | 478 | 2146163 | https://en.wikipedia.org/wiki/SN%201572 | Physical sciences | Notable transient events | Astronomy |
In Ming dynasty China, the star became an issue between Zhang Juzheng and the young Wanli Emperor: in accordance with the cosmological tradition, the emperor was warned to consider his misbehavior, since the new star was interpreted as an evil omen.
The more reliable contemporary reports state that the new star itself burst forth soon after November 2, 1572 and by November 11 it was already brighter than Jupiter. Around November 16, 1572, it reached its peak brightness at about magnitude −4.0, with some descriptions giving it as equal to Venus when that planet was at its brightest. Contrarily, Brahe described the supernova as "brighter than Venus". The supernova remained visible to the naked eye into early 1574, gradually fading until it disappeared from view.
Supernova
The supernova was classified as type I on the basis of its historical light curve soon after type I and type II supernovae were first defined on the basis of their spectra. The X-ray spectrum of the remnant showed that it was almost certainly of type Ia, but its detailed classification within the type Ia class continued to be debated until the spectrum of its light at peak luminosity was measured in a light echo in 2008. This gave final confirmation that it was a normal type Ia.
The classification as a type Ia supernova of normal luminosity allows an accurate measure of the distance to SN 1572. The peak absolute magnitude can be calculated from the B-band decline rate to be . Given estimates of the peak apparent magnitude and the known extinction of magnitudes, the distance is kpc.
Supernova remnant
The distance to the supernova remnant has been estimated to between 2 and 5 kpc (approx. 6,500 and 16,300 light-years), with recent studies suggesting a narrower range of 2.5 and 3 kpc (approximately 8,000 and 9,800 light-years). Tycho's SNR has a roughly spherical morphology and spreads over an angular diameter of about 8 arcminutes. Its physical size corresponds to radius of the order of a few parsecs. Its measured expansion rate is about 11–12%/year in radio and X-ray. The average forward shock speed is between 4,000 and 5,000 km/s, dropping to lower speed when encountering local interstellar clouds. An older source says that the gas shell has reached an apparent diameter of 3.7 arcminutes. | SN 1572 | Wikipedia | 503 | 2146163 | https://en.wikipedia.org/wiki/SN%201572 | Physical sciences | Notable transient events | Astronomy |
Initial radio detection
The search for a supernova remnant was futile until 1952, when Robert Hanbury Brown and Cyril Hazard reported a radio detection at 158.5 MHz, obtained at the Jodrell Bank Observatory. This was confirmed, and its position more accurately measured in 1957 by Baldwin and Edge using the Cambridge Radio Telescope working at a wavelength of . The remnant was also identified tentatively in the second Cambridge Catalogue of Radio Sources as object "2C 34", and more firmly as "3C 10" in the third Cambridge list.
There is no dispute that 3C 10 is the remnant of the supernova observed in 1572–1573. Following a 1964 review article by Minkowski, the designation 3C 10 appears to be that most commonly used in the literature when referring to the radio remnant of B Cas, although some authors use the tabulated galactic designation G120.7+2.1 and many authors commonly refer to it as Tycho's supernova remnant. Because the radio remnant was reported before the optical supernova-remnant wisps were discovered, the designation 3C 10 is used by some to signify the remnant at all wavelengths.
X-ray observation
An X-ray source designated Cepheus X-1 (or Cep X-1) was detected by the Uhuru X-ray observatory at 4U 0022+63. Earlier catalog designations are X120+2 and XRS 00224+638. Cepheus X-1 is actually in the constellation Cassiopeia, and it is SN 1572, the Tycho SNR.
Optical detection
The supernova remnant of B Cas was discovered in the 1960s by scientists with a Palomar Mountain telescope as a very faint nebula. It was later photographed by a telescope on the international ROSAT spacecraft. The supernova has been confirmed as Type Ia, in which a white dwarf star has accreted matter from a companion until it approaches the Chandrasekhar limit and explodes. This type of supernova does not typically create the spectacular nebula more typical of Type II supernovas, such as SN 1054 which created the Crab Nebula. A shell of gas is still expanding from its center at about 9,000 km/s. A recent study indicates a rate of expansion below 5,000 km/s. | SN 1572 | Wikipedia | 468 | 2146163 | https://en.wikipedia.org/wiki/SN%201572 | Physical sciences | Notable transient events | Astronomy |
Companion star
In October 2004, a letter in Nature reported the discovery of a G2 star, similar in type to our own Sun and named Tycho G. It is thought to be the companion star that contributed mass to the white dwarf that ultimately resulted in the supernova. A subsequent study, published in March 2005, revealed further details about this star: Tycho G was probably a main-sequence star or subgiant before the explosion, but some of its mass was stripped away and its outer layers were shock-heated by the supernova.
Tycho G's current velocity is perhaps the strongest evidence that it was the companion star to the white dwarf, as it is traveling at a rate of 136 km/s, which is more than four times faster than the mean velocity of other stars in its stellar neighbourhood.
This find has been challenged in recent years. The star is relatively far away from the center and does not show rotation which might be expected of a companion star.
In Gaia DR2, the star was calculated to be light-years away, on the lower end of SN 1572's possible range of distances, which in turn lowered the calculated velocity from 136 km/s to only 56 km/s.
In literature
In the ninth episode of James Joyce's Ulysses, Stephen Dedalus associates the appearance of the supernova with the youthful William Shakespeare, and in the November 1998 issue of Sky & Telescope, three researchers from Southwest Texas State University, Don Olson and Russell Doescher of the Physics Department and Marilynn Olson of the English Department, argued that this supernova is described in Shakespeare's Hamlet, specifically by Bernardo in Act I, Scene i.
The supernova inspired the poem "Al Aaraaf" by Edgar Allan Poe.
The protagonist in Arthur C. Clarke's 1955 short story "The Star" casually mentions the supernova. It is a major element in Frederik Pohl's spoof science article, "The Martian Star-Gazers", first published in Galaxy Science Fiction Magazine in 1962. | SN 1572 | Wikipedia | 415 | 2146163 | https://en.wikipedia.org/wiki/SN%201572 | Physical sciences | Notable transient events | Astronomy |
Reptiliomorpha (meaning reptile-shaped; in PhyloCode known as Pan-Amniota) is a clade containing the amniotes and those tetrapods that share a more recent common ancestor with amniotes than with living amphibians (lissamphibians). It was defined by Michel Laurin (2001) and Vallin and Laurin (2004) as the largest clade that includes Homo sapiens, but not Ascaphus truei (tailed frog). Laurin and Reisz (2020) defined Pan-Amniota as the largest total clade containing Homo sapiens, but not Pipa pipa, Caecilia tentaculata, and Siren lacertina.
The informal variant of the name, "reptiliomorphs", is also occasionally used to refer to stem-amniotes, i.e. a grade of reptile-like tetrapods that are more closely related to amniotes than they are to lissamphibians, but are not amniotes themselves; the name is used in this meaning e.g. by Ruta, Coates and Quicke (2003). An alternative name, "Anthracosauria", is also commonly used for the group, but is confusingly also used for a more primitive grade of reptiliomorphs (Embolomeri) by Benton. While both anthracosaurs and/or embolomeres are suggested to be reptiliomorphs closer to amniotes, some recent studies either retain them as amphibians or argue that their relationships are still ambiguous and are more likely to be stem-tetrapods. | Reptiliomorpha | Wikipedia | 348 | 2149536 | https://en.wikipedia.org/wiki/Reptiliomorpha | Biology and health sciences | Reptiliomorphs | Animals |
As the exact phylogenetic position of Lissamphibia within Tetrapoda remains uncertain, it also remains controversial which fossil tetrapods are more closely related to amniotes than to lissamphibians, and thus, which ones of them were reptiliomorphs in any meaning of the word. The two major hypotheses for lissamphibian origins are that they are either descendants of dissorophoid temnospondyls or microsaurian "lepospondyls". If the former (the "temnospondyl hypothesis") is true, then Reptiliomorpha includes all tetrapod groups that are closer to amniotes than to temnospondyls. These include the diadectomorphs, seymouriamorphs, most or all "lepospondyls", gephyrostegids, and possibly the embolomeres and chroniosuchians. In addition, several "anthracosaur" genera of uncertain taxonomic placement would also probably qualify as reptiliomorphs, including Solenodonsaurus, Eldeceeon, Silvanerpeton, and Casineria. However, if lissamphibians originated among the lepospondyls according to the "lepospondyl hypothesis", then Reptiliomorpha refers to groups that are closer to amniotes than to lepospondyls. Few non-amniote groups would count as reptiliomorphs under this definition, although the diadectomorphs are among those that qualify. | Reptiliomorpha | Wikipedia | 338 | 2149536 | https://en.wikipedia.org/wiki/Reptiliomorpha | Biology and health sciences | Reptiliomorphs | Animals |
Changing definitions
The name Reptiliomorpha was coined by Professor Gunnar Säve-Söderbergh in 1934 to designate amniotes and various types of late Paleozoic tetrapods that were more closely related to amniotes than to living amphibians. In his view, the amphibians had evolved from fish twice, with one group composed of the ancestors of modern salamanders and the other, which Säve-Söderbergh referred to as Eutetrapoda, consisting of anurans (frogs), amniotes, and their ancestors, with the origin of caecilians being uncertain. Säve-Söderbergh's Eutetrapoda consisted of two sister-groups: Batrachomorpha, containing anurans and their ancestors, and Reptiliomorpha, containing anthracosaurs and amniotes. Säve-Söderbergh subsequently added Seymouriamorpha to his Reptiliomorpha as well.
Alfred Sherwood Romer rejected Säve-Söderbergh's theory of a biphyletic amphibia and used the name Anthracosauria to describe the "labyrinthodont" lineage from which amniotes evolved. In 1970, the German paleontologist Alec Panchen took up Säve-Söderbergh's name for this group as having priority, but Romer's terminology is still in use, e.g. by Carroll (1988 and 2002) and by Hildebrand & Goslow (2001). Some writers preferring phylogenetic nomenclature use Anthracosauria.
In 1956, Friedrich von Huene included both amphibians and anapsid reptiles in the Reptiliomorpha. This included the following orders: Anthracosauria, Seymouriamorpha, Microsauria, Diadectomorpha, Procolophonia, Pareiasauria, Captorhinidia, Testudinata. | Reptiliomorpha | Wikipedia | 403 | 2149536 | https://en.wikipedia.org/wiki/Reptiliomorpha | Biology and health sciences | Reptiliomorphs | Animals |
Michael Benton (2000, 2004) made it the sister-clade to Lepospondyli, containing "anthracosaurs" (in the strict sense, i.e. Embolomeri), seymouriamorphs, diadectomorphs and amniotes. Subsequently, Benton included lepospondyls in Reptiliomorpha as well. However, when considered in a Linnean framework, Reptiliomorpha is given the rank of superorder and includes only reptile-like amphibians, not their amniote descendants.
Several phylogenetic studies indicate that amniotes and diadectomorphs share a more recent common ancestor with lepospondyls than with seymouriamorphs, Gephyrostegus and Embolomeri (e.g. Laurin and Reisz, 1997, 1999; Ruta, Coates and Quicke, 2003; Vallin and Laurin, 2004; Ruta and Coates, 2007). Lepospondyls are one of the groups of tetrapods suggested to be ancestors of living amphibians; as such, their potential close relationship to amniotes has important implications for the content of Reptiliomorpha. Assuming that lissamphibians aren't descended from lepospondyls but from a different group of tetrapods, e.g. from temnospondyls, it would mean that Lepospondyli belonged to Reptiliomorpha sensu Laurin (2001), as it would make them more closely related to amniotes than to lissamphibians. On the other hand, if lissamphibians are descended from lepospondyls, then not only Lepospondyli would have to be excluded from Reptiliomorpha, but seymouriamorphs, Gephyrostegus and Embolomeri would also have to be excluded from this group, as this would make them more distantly related to amniotes than living amphibians are. In that case, the clade Reptiliomorpha sensu Laurin would contain, apart from Amniota, only diadectomorphs and possibly also Solenodonsaurus. | Reptiliomorpha | Wikipedia | 474 | 2149536 | https://en.wikipedia.org/wiki/Reptiliomorpha | Biology and health sciences | Reptiliomorphs | Animals |
Characteristics
Gephyrostegids, seymouriamorphs and diadectomorphs were land-based, reptile-like amphibians, while embolomeres were aquatic amphibians with long body and short limbs. Their anatomy falls between the mainly aquatic Devonian labyrinthodonts and the first reptiles. University of Bristol paleontologist Professor Michael J. Benton gives the following characteristics for the Reptiliomorpha (in which he includes embolomeres, seymouriamorphs and diadectomorphs):
narrow premaxillae (less than half the skull width)
vomers taper forward
phalangeal formulae (number of joints in each toe) of foot 2.3.4.5.4–5
Cranial morphology
The groups traditionally assigned to Reptiliomorpha, i.e. embolomeres, seymouriamorphs and diadectomorphs, differed from their contemporaries, the non-reptiliomorph temnospondyls, in having a deeper and taller skull, but retained the primitive kinesis (loose attachment) between the skull roof and the cheek (with exception of some specialized taxa, such as Seymouria, in which the cheek was solidly attached to the skull roof). The deeper skull allowed for laterally placed eyes, contrary to the dorsally placed eyes commonly found in amphibians. The skulls of the group are usually found with fine radiating grooves. The quadrate bone in the back of the skull held a deep otic notch, likely holding a spiracle rather than a tympanum.
Postcranial skeleton | Reptiliomorpha | Wikipedia | 337 | 2149536 | https://en.wikipedia.org/wiki/Reptiliomorpha | Biology and health sciences | Reptiliomorphs | Animals |
The vertebrae showed the typical multi-element construction seen in labyrinthodonts. According to Benton, in the vertebrae of "anthracosaurs" (i.e. Embolomeri) the intercentrum and pleurocentrum may be of equal size, while in the vertebrae of seymouriamorphs the pleurocentrum is the dominant element and the intercentrum is reduced to a small wedge. The intercentrum gets further reduced in the vertebrae of amniotes, where it becomes a thin plate or disappears altogether. Unlike most labyrinthodonts, the body was moderately deep rather than flat, and the limbs were well-developed and ossified, indicating a predominantly terrestrial lifestyle except in secondarily aquatic groups. Each foot held five digits, the pattern seen in their amniote descendants. They did, however, lack the reptilian type of ankle bone that would have allowed the use of the feet as levers for propulsion rather than as holdfasts.
Physiology
The general build was heavy in all forms, though otherwise very similar to that of early reptiles. The skin, at least in the more advanced forms probably had a water-tight epidermal horny overlay, similar to the one seen in today's reptiles, though they lacked horny claws. In chroniosuchians and some seymouriamorphs, like Discosauriscus, dermal scales are found in post-metamorphic specimens, indicating they may have had a "knobbly", if not scaly, appearance. With reptiliomorph anthracosaurs having evolved small near-circular keratinous scales, their amniote descendants further covered almost their entire body with them, and also formed claws of keratin, with both scales and claws making cutaneous respiration and water absorption impossible, making them breathe through their mouths and nostrils, and drink water through mouth.
Seymouriamorphs reproduced in amphibian fashion with aquatic eggs that hatched into larvae (tadpoles) with external gills; it is unknown how other tetrapods traditionally assigned to Reptiliomorpha reproduced.
Evolutionary history
Early reptiliomorphs | Reptiliomorpha | Wikipedia | 446 | 2149536 | https://en.wikipedia.org/wiki/Reptiliomorpha | Biology and health sciences | Reptiliomorphs | Animals |
During the Carboniferous and Permian periods, some tetrapods started to evolve towards a reptilian condition. Some of these tetrapods (e.g. Archeria, Eogyrinus) were elongate, eel-like aquatic forms with diminutive limbs, while others (e.g. Seymouria, Solenodonsaurus, Diadectes, Limnoscelis) were so reptile-like that until quite recently they actually had been considered to be true reptiles, and it is likely that to a modern observer they would have appeared as large to medium-sized, heavy-set lizards. Several groups however remained aquatic or semiaquatic. Some of the chroniosuchians show the build and presumably habits of modern crocodiles and were probably also similar to crocodylians in that they were river-side predators. While some other Chroniosuchians possessed elongated newt- or eel-like bodies. The two most terrestrially adapted groups were the medium-sized insectivorous or carnivorous Seymouriamorpha and the mainly herbivorous Diadectomorpha, with many large forms. The latter group has, in most analysis, the closest relatives of the Amniotes.
From aquatic to terrestrial eggs
Their terrestrial life style combined with the need to return to the water to lay eggs hatching to larvae (tadpoles) led to a drive to abandon the larval stage and aquatic eggs. A possible reason may have been competition for breeding ponds, to exploit drier environments with less access to open water, or to avoid predation on tadpoles by fish, a problem still plaguing modern amphibians. Whatever the reason, the drive led to internal fertilization and direct development (completing the tadpole stage within the egg). A striking parallel can be seen in the frog family Leptodactylidae, which has a very diverse reproductive system, including foam nests, non-feeding terrestrial tadpoles and direct development. The Diadectomorphans generally being large animals would have had correspondingly large eggs, unable to survive on land. | Reptiliomorpha | Wikipedia | 435 | 2149536 | https://en.wikipedia.org/wiki/Reptiliomorpha | Biology and health sciences | Reptiliomorphs | Animals |
Fully terrestrial life was achieved with the development of the amniote egg, where a number of membranous sacks protect the embryo and facilitate gas exchange between the egg and the atmosphere. The first to evolve was probably the allantois, a sack that develops from the gut/yolk-sack. This sack contains the embryo's nitrogenous waste (urea) during development, stopping it from poisoning the embryo. A very small allantois is found in modern amphibians. Later came the amnion surrounding the fetus proper, and the chorion, encompassing the amnion, allantois, and yolk-sack.
Origin of amniotes
Exactly where the border between reptile-like amphibians (non-amniote reptiliomorphs) and amniotes lies will probably never be known, as the reproductive structures involved fossilize poorly, but various small, advanced reptiliomorphs have been suggested as the first true amniotes, including Solenodonsaurus, Casineria and Westlothiana. Such small animals laid small eggs, 1 cm in diameter or less. Small eggs would have a small enough volume to surface ratio to be able to develop on land without the amnion and chorion actively affecting gas exchange, setting the stage for the evolution of true amniotic eggs. Although the first true amniotes probably appeared as early as the Middle Mississippian sub-epoch, non-amniote (or amphibian) reptiliomorph lineages coexisted alongside their amniote descendants for many millions of years. By the middle Permian the non-amniote terrestrial forms had died out, but several aquatic non-amniote groups continued to the end of the Permian, and in the case of the chroniosuchians survived the end Permian mass extinction, only to die out prior to the end of the Triassic. Meanwhile, the single most successful daughter-clade of the reptiliomorphs, the amniotes, continued to flourish and evolve into a staggering diversity of tetrapods including mammals, reptiles, and birds.
Gallery | Reptiliomorpha | Wikipedia | 443 | 2149536 | https://en.wikipedia.org/wiki/Reptiliomorpha | Biology and health sciences | Reptiliomorphs | Animals |
Quantifier elimination is a concept of simplification used in mathematical logic, model theory, and theoretical computer science. Informally, a quantified statement " such that " can be viewed as a question "When is there an such that ?", and the statement without quantifiers can be viewed as the answer to that question.
One way of classifying formulas is by the amount of quantification. Formulas with less depth of quantifier alternation are thought of as being simpler, with the quantifier-free formulas as the simplest.
A theory has quantifier elimination if for every formula , there exists another formula without quantifiers that is equivalent to it (modulo this theory).
Examples
An example from mathematics says that a single-variable quadratic polynomial has a real root if and only if its discriminant is non-negative:
Here the sentence on the left-hand side involves a quantifier , whereas the equivalent sentence on the right does not.
Examples of theories that have been shown decidable using quantifier elimination are Presburger arithmetic, algebraically closed fields, real closed fields, atomless Boolean algebras, term algebras, dense linear orders, abelian groups, random graphs, as well as many of their combinations such as Boolean algebra with Presburger arithmetic, and term algebras with queues.
Quantifier eliminator for the theory of the real numbers as an ordered additive group is Fourier–Motzkin elimination; for the theory of the field of real numbers it is the Tarski–Seidenberg theorem.
Quantifier elimination can also be used to show that "combining" decidable theories leads to new decidable theories (see Feferman–Vaught theorem).
Algorithms and decidability
If a theory has quantifier elimination, then a specific question can be addressed: Is there a method of determining for each ? If there is such a method we call it a quantifier elimination algorithm. If there is such an algorithm, then decidability for the theory reduces to deciding the truth of the quantifier-free sentences. Quantifier-free sentences have no variables, so their validity in a given theory can often be computed, which enables the use of quantifier elimination algorithms to decide validity of sentences.
Related concepts | Quantifier elimination | Wikipedia | 491 | 2150441 | https://en.wikipedia.org/wiki/Quantifier%20elimination | Mathematics | Model theory | null |
Various model-theoretic ideas are related to quantifier elimination, and there are various equivalent conditions.
Every first-order theory with quantifier elimination is model complete. Conversely, a model-complete theory, whose theory of universal consequences has the amalgamation property, has quantifier elimination.
The models of the theory of the universal consequences of a theory are precisely the substructures of the models of . The theory of linear orders does not have quantifier elimination. However the theory of its universal consequences has the amalgamation property.
Basic ideas
To show constructively that a theory has quantifier elimination, it suffices to show that we can eliminate an existential quantifier applied to a conjunction of literals, that is, show that each formula of the form:
where each is a literal, is equivalent to a quantifier-free formula. Indeed, suppose we know how to eliminate quantifiers from conjunctions of literals, then if is a quantifier-free formula, we can write it in disjunctive normal form
and use the fact that
is equivalent to
Finally, to eliminate a universal quantifier
where is quantifier-free, we transform
into disjunctive normal form, and use the fact that
is equivalent to
Relationship with decidability
In early model theory, quantifier elimination was used to demonstrate that various theories possess properties like decidability and completeness. A common technique was to show first that a theory admits elimination of quantifiers and thereafter prove decidability or completeness by considering only the quantifier-free formulas. This technique can be used to show that Presburger arithmetic is decidable.
Theories could be decidable yet not admit quantifier elimination. Strictly speaking, the theory of the additive natural numbers did not admit quantifier elimination, but it was an expansion of the additive natural numbers that was shown to be decidable. Whenever a theory is decidable, and the language of its valid formulas is countable, it is possible to extend the theory with countably many relations to have quantifier elimination (for example, one can introduce, for each formula of the theory, a relation symbol that relates the free variables of the formula).
Example: Nullstellensatz for algebraically closed fields and for differentially closed fields. | Quantifier elimination | Wikipedia | 496 | 2150441 | https://en.wikipedia.org/wiki/Quantifier%20elimination | Mathematics | Model theory | null |
The green sea turtle (Chelonia mydas), also known as the green turtle, black (sea) turtle or Pacific green turtle, is a species of large sea turtle of the family Cheloniidae. It is the only species in the genus Chelonia. Its range extends throughout tropical and subtropical seas around the world, with two distinct populations in the Atlantic and Pacific Oceans, but it is also found in the Indian Ocean. The common name refers to the usually green fat found beneath its carapace, due to its diet strictly being seagrass, not to the color of its carapace, which is olive to black.
The dorsoventrally flattened body of C. mydas is covered by a large, teardrop-shaped carapace; it has a pair of large, paddle-like flippers. It is usually lightly colored, although in the eastern Pacific populations, parts of the carapace can be almost black. Unlike other members of its family, such as the hawksbill sea turtle, C. mydas is mostly herbivorous. The adults usually inhabit shallow lagoons, feeding mostly on various species of seagrasses. The turtles bite off the tips of the blades of seagrass, which keeps the grass healthy.
Like other sea turtles, green sea turtles migrate long distances between feeding grounds and hatching beaches. Many islands worldwide are known as Turtle Island due to green sea turtles nesting on their beaches. Females crawl out on beaches, dig nests, and lay eggs during the night. Later, hatchlings emerge, and scramble into the water. Those that reach maturity may live to 90 years in the wild.
C. mydas is listed as endangered by the IUCN and CITES and is protected from exploitation in most countries. It is illegal to collect, harm, or kill them. In addition, many countries have laws and ordinances to protect nesting areas. However, turtles are still in danger due to human activity. In some countries, turtles and their eggs are still hunted for food. Pollution indirectly harms turtles at both population and individual scales. Many turtles die after being caught in fishing nets. In addition, real estate development often causes habitat loss by eliminating nesting beaches. | Green sea turtle | Wikipedia | 441 | 2150513 | https://en.wikipedia.org/wiki/Green%20sea%20turtle | Biology and health sciences | Turtles | Animals |
Taxonomy
The green sea turtle is a member of the tribe Cheloniini. A 1993 study clarified the status of genus Chelonia with respect to the other marine turtles. The carnivorous Eretmochelys (hawksbill), Caretta (loggerhead) and Lepidochelys (ridley) were assigned to the tribe Carettini. Herbivorous Chelonia warranted their status as a genus, while Natator (flatback) was further removed from the other genera than previously believed.
The species was originally described by Carl Linnaeus in his landmark 1758 10th edition of Systema Naturae as Testudo mydas. In 1868, Marie Firmin Bocourt named a particular species of sea turtle Chelonia agassizii, in honor of Swiss-American zoologist Louis Agassiz. This "species" was referred to as the "black sea turtle". Later research determined Bocourt's "black sea turtle" was not genetically distinct from C. mydas, and thus taxonomically not a separate species. These two "species" were then united as Chelonia mydas and populations were given subspecies status: C. mydas mydas referred to the originally described population, while C. mydas agassizi referred only to the Pacific population known as the Galápagos green turtle. This subdivision was later determined to be invalid and all species members were then designated Chelonia mydas. The oft-mentioned name C. agassizi remains an invalid junior synonym of C. mydas.
The species' common name does not derive from any particular green external coloration of the turtle. Its name comes from the greenish color of the turtles' fat, which is only found in a layer between their inner organs and their shell. As a species found worldwide, the green turtle has many local names. In the Hawaiian language it is called honu, and it is locally known as a symbol of good luck and longevity.
Description | Green sea turtle | Wikipedia | 401 | 2150513 | https://en.wikipedia.org/wiki/Green%20sea%20turtle | Biology and health sciences | Turtles | Animals |
Its appearance is that of a typical sea turtle. C. mydas has a dorsoventrally flattened body, a beaked head at the end of a short neck, and paddle-like arms well-adapted for swimming. Adult green turtles grow to long. The average weight of mature individuals is and the average carapace length is . They are considered the second largest sea turtle in the United States, after the leatherback sea turtle.
Exceptional specimens can weigh or even more, with the largest known C. mydas having weighed and measured in carapace length.
Anatomically, a few characteristics distinguish the green turtle from the other members of its family. Unlike its close relative the hawksbill turtle, the green turtle's snout is very short and its beak is unhooked. The neck cannot be pulled into the shell. The sheath of the turtle's upper jaw possesses a denticulated edge, while its lower jaw has stronger, serrated, more defined denticulation. The dorsal surface of the turtle's head has a single pair of prefrontal scales. Its carapace is composed of five central scutes flanked by four pairs of lateral scutes. Underneath, the green turtle has four pairs of inframarginal scutes covering the area between the turtle's plastron and its shell. Mature C. mydas front appendages have only a single claw (as opposed to the hawksbill two), although a second claw is sometimes prominent in young specimens.
The carapace of the turtle has various color patterns that change over time. Hatchlings of Chelonia mydas, like those of other marine turtles, have mostly black carapaces and light-colored plastrons. Carapaces of juveniles turn dark brown to olive, while those of mature adults are either entirely brown, spotted or marbled with variegated rays. Underneath, the turtle's plastron is hued yellow. C. mydas limbs are dark-colored and lined with yellow, and are usually marked with a large dark brown spot in the center of each appendage.
Distribution | Green sea turtle | Wikipedia | 429 | 2150513 | https://en.wikipedia.org/wiki/Green%20sea%20turtle | Biology and health sciences | Turtles | Animals |
The range of the green sea turtle extends throughout tropical and subtropical oceans worldwide. The two major subpopulations are the Atlantic and the eastern Pacific subpopulations. Each population is genetically distinct, with its own set of nesting and feeding grounds within the population's known range. One of the genetic differences between the two subpopulations is the type of mitochondrial DNA found in individual's cells. Individuals from rookeries in the Atlantic Ocean and Mediterranean Sea have a similar type of mitochondrial DNA, and individuals from the Pacific and Indian Oceans have another type of mitochondrial DNA. Their native range includes tropical to subtropical waters along continental coasts and islands between 30°N and 30°S. Since green sea turtles are a migrating species, their global distribution spans into the open ocean. The differences in mitochondrial DNA more than likely stems from the populations being isolated from each other by the southern tips of both South America and Africa with no warm waters for the green sea turtles to migrate through. The green sea turtle is estimated to inhabit coastal areas of more than 140 countries, with nesting sites in over 80 countries worldwide throughout the year. In the United States Atlantic coast, green sea turtles can be found from Texas and north to Massachusetts. In the United States Pacific coast, they have been found from southern California north to the southernmost tip of Alaska. The largest populations of green sea turtles within the United States coastline are in the Hawaiian Islands and Florida. Globally, the largest populations of sea turtles are in the Great Barrier Reef in Australia, and the Caribbean Sea.
Atlantic subpopulation
The green sea turtle can generally be found throughout the Atlantic Ocean. Although the species is most abundant in tropical climates, green sea turtles can also be found in temperate climates, and individuals have been spotted as far north as Canada in the western Atlantic, and the Cimbrian peninsular in the east. The subpopulation's southern range is known until past the southern tip of Africa in the east and Argentina in the western Atlantic. The major nesting sites can be found on various islands in the Caribbean, along the Atlantic coast of Florida in the United States, the eastern coast of the South American continent and most notably, on isolated North Atlantic islands. | Green sea turtle | Wikipedia | 441 | 2150513 | https://en.wikipedia.org/wiki/Green%20sea%20turtle | Biology and health sciences | Turtles | Animals |
In the Caribbean, major nesting sites have been identified on Aves Island, the U.S. Virgin Islands, Puerto Rico, the Dominican Republic, and Costa Rica. In recent years, there are signs of increased nesting in the Cayman Islands. One of the region's most important nesting grounds is in Tortuguero in Costa Rica. In fact, the majority of the Caribbean region's C. mydas population hails from a few beaches in Tortuguero. Within United States waters, minor nesting sites have been noted in the states of Georgia, North Carolina, and South Carolina. Florida's east coast is the largest nesting site in the United States. Hutchinson Island in particular is a major nesting area in Florida waters. Florida has several annual nesting periods when local beaches are closed or cordoned off to protect nesting sites. According to Green Sea Turtle Watch, in 2015 more than 37,000 green sea turtle nests were documented in Florida, a record number. In addition to sporadic distribution of nesting sites, feeding grounds are much more widely distributed throughout Florida. Important feeding grounds in Florida include Indian River Lagoon, the Florida Keys, Florida Bay, Homosassa, Crystal River, and Cedar Key.
Notable locations in South America include secluded beaches in Suriname and French Guiana. In the Southern Atlantic Ocean, the most notable nesting grounds for Chelonia mydas are found on the island of Ascension, hosts 6,000–13,000 turtle nests.
Indo-Pacific subpopulation
In the Pacific, its range reaches as far north as the southern coast of Alaska and as far south as Chile in the east. The turtle's distribution in the western Pacific reaches north to Japan and southern parts of Russia's Pacific coast, and as far south as the northern tip of New Zealand and a few islands south of Tasmania. Significant nesting grounds are scattered throughout the entire Pacific region, including Mexico, the Hawaiian Islands, the South Pacific, the northern coast of Australia, and Southeast Asia. Major Indian Ocean nesting colonies include India, Pakistan, Sri Lanka and other coastal countries.
The turtles can also be found throughout the Indian Ocean; the east coast of the African continent hosts a few nesting grounds, including islands in the waters around Madagascar.
Specific nesting grounds
Nesting grounds are found all along the Mexican coast. These turtles feed in seagrass pastures in the Gulf of California. Green turtles belonging to the distinct Hawaiian subpopulation nest at the protected French Frigate Shoals some west of the Hawaiian Islands. | Green sea turtle | Wikipedia | 499 | 2150513 | https://en.wikipedia.org/wiki/Green%20sea%20turtle | Biology and health sciences | Turtles | Animals |
In the Philippines, green turtles nest in the Turtle Islands along with closely related hawksbill turtles.
In December 2007, fishermen using a hulbot-hulbot (a type of fish net) accidentally caught an , long and wide turtle off Barangay Bolong, Zamboanga City, Philippines. December is breeding season near the Bolong beach.
An annual presence is recorded in the Gulf of Panama, on the Isla Parida island. Local activists also moving some turtle nests to the coast, in the vicinity of the small town of Malena, to save and increase the turtle population in the safe place.
Indonesia has a few nesting beaches, one in the Meru Betiri National Reserve in East Java.
Off the north-eastern and northern coasts of Australia, the Great Barrier Reef has two genetically distinct populations; one north and one south. Within the reef, 20 separate locations consisting of small islands and cays were identified as nesting sites for either population of C. mydas. Of these, the most important is on Raine Island. In the Torres Strait there is a large rookery on Bramble Cay. The Coral Sea has nesting areas of world significance.
Major nesting sites are common on either side of the Arabian Sea, both in Ash Sharqiyah, Oman, and along the coast of Karachi, Pakistan. Some specific beaches there, such as Hawke's Bay and Sandspit, are common to both C. mydas and L. olivacea subpopulation. Sandy beaches along Sindh and Balochistan are nesting sites. Some off the Pakistani coast, Astola island is another nesting beach.
Galápagos green turtle | Green sea turtle | Wikipedia | 332 | 2150513 | https://en.wikipedia.org/wiki/Green%20sea%20turtle | Biology and health sciences | Turtles | Animals |
The population that has often been known as the Galápagos green turtle have been recorded and observed in the Galápagos as far back as the 17th century by William Dampier. Not much attention has been paid to them due to the overwhelming research done on the Galápagos giant tortoises. Only over the last 30 years have extensive studies been performed covering the behaviors of the Galápagos green turtles. Much of the debate that has surrounded them recently is over the binomial classification of the species. At one point the name Chelonia agassizii was applied to this population as a separate species. Analysis of mitochondrial and nuclear DNA of 15 nesting beaches, however, has demonstrated that there is not only no significant distinction of this population but that it would be paraphyletic to recognise it. As such the species name Chelonia agassizzii is considered a junior synonym of Chelonia mydas as such it is considered as a local variant of the populations of the East Pacific waters and those of other nesting areas.
The morphological distinctiveness of the Galápagos green turtle has given rise to the debate, but evidence of taxonomic distinctiveness is best served using the combination of multiple datasets. The two most notable morphological distinctions are the considerably smaller adult size and the much darker pigmentation of the carapace, plastron, and extremities. Other distinctions are the curving of the carapace above each hind flipper, the more dome-shaped carapace, and the very long tail of adult males.
Three possibilities have arisen from their unique characteristics: agassizii is a separate species from C. mydas, it is a subspecies of green sea turtle, or it is simply a color mutation. These facts have led to the debate over binomial separation however due to the significance of the DNA testing results there have been no distinctions made at this time. At a meeting for sea turtle scientists and their collaborators in 2000, the evidence for the taxonomic position of the Galápagos green turtle was reviewed and a majority among the participants supported treating it as a population or subspecies of the green turtle (instead of a separate species). However, this is possibly a case of political taxonomy. As such the three major international checklists that cover turtles of the world Reptile Database the checklist of Fritz and Havas (2007) and the IUCN Checklist (TTWG 2017) all consider this a junior synonym.
Habitat | Green sea turtle | Wikipedia | 496 | 2150513 | https://en.wikipedia.org/wiki/Green%20sea%20turtle | Biology and health sciences | Turtles | Animals |
Green sea turtles move across three habitat types, depending on their life stage. They lay eggs on beaches. Mature turtles spend most of their time in shallow, coastal waters with lush seagrass beds. Adults frequent inshore bays, lagoons, and shoals with lush seagrass meadows. Entire generations often migrate between one pair of feeding and nesting areas. Green sea turtles, Chelonia mydas, are classified as an aquatic species and are distributed around the globe in warm tropical to subtropical waters. The environmental parameter that limits the distribution of the turtles is ocean temperatures below 7 to 10 degrees Celsius. Within their geographical range, the green sea turtles generally stay near continental and island coastlines. Near the coastlines, the green sea turtles live within shallow bays and protected shores. In these protected shores and bays, the green sea turtle habitats include coral reefs, salt marshes, and nearshore seagrass beds. The coral reefs provide red, brown, and green algae for their diet and give protection from predators and rough storms within the ocean. The salt marshes and seagrass beds contain seaweed and grass vegetation, allowing ample habitat for the sea turtles.
Turtles spend most of their first five years in convergence zones within the bare open ocean that surround them. These young turtles are rarely seen as they swim in deep, pelagic waters. Green sea turtles typically swim at .
Ecology and behavior
As one of the first sea turtle species studied, much of what is known of sea turtle ecology comes from studies of green turtles. The ecology of C. mydas changes drastically with each stage of its life history. Newly emerged hatchlings are carnivorous, pelagic organisms, part of the open ocean mininekton. In contrast, immature juveniles and adults are commonly found in seagrass meadows closer inshore as herbivorous grazers. | Green sea turtle | Wikipedia | 367 | 2150513 | https://en.wikipedia.org/wiki/Green%20sea%20turtle | Biology and health sciences | Turtles | Animals |
Diet
The diet of green turtles changes with age. Juveniles are carnivorous, but as they mature they become omnivorous. Young sea turtles eat fish and their eggs, sea hare eggs, hydrozoans, bryozoans, molluscs, jellyfish, small invertebrates, echinoderms, tunicates, insects, worms, sponges, algae, sea grasses, leaves, tree bark, and crustaceans. Green sea turtles have a relatively slow growth rate because of the low nutritional value of their diet. Body fat turns green because of the consumed vegetation. This diet shift has an effect on the green turtle's skull morphology. Their serrated jaw helps them chew green and red algae (such as filamentous red alga (Bostrychia), red moss (Caloglossa), freshwater red algae (Compsopogon), lobster horns (Polysiphonia), sea lettuce (Ulva lactuca), green seaweed (Gayralia), and crinkle grass (Rhizoclonium)) and sea grasses. They also consume large quantities of wetland plants such as Avicennia schaueriana and Sporobolus alterniflorus, which are commonly found in salt marshes. Most adult sea turtles are strictly herbivorous.
Predators and parasites
Only some human beings and the larger sharks feed on C. mydas adults. Specifically, tiger sharks (Galeocerdo cuvier) hunt adults in Hawaiian waters. The tiger shark is the main predator of the green turtle as it will prey on green turtles of all sizes. The tiger shark has often been seen feeding on green turtles near their nesting beaches because they are restricted in the area of their nesting beaches and vulnerable to predation. Juveniles and new hatchlings have significantly more predators, including crabs, small marine mammals and shorebirds. Additionally, their eggs are vulnerable to predation by scavengers like red foxes and golden jackals. | Green sea turtle | Wikipedia | 405 | 2150513 | https://en.wikipedia.org/wiki/Green%20sea%20turtle | Biology and health sciences | Turtles | Animals |
Green sea turtles have a variety of parasites including barnacles, leeches, protozoans, cestodes, and nematodes. Barnacles attach to the carapace, and leeches to the flippers and skin of the turtles, causing damage to the soft tissues and leading to blood loss. Protozoans, cestodes and nematodes lead to many turtle deaths because of the infections in the liver and intestinal tract they cause. The greatest disease threat to the turtle population is fibropapilloma, which produces lethal tumor growth on scales, lungs, stomach, and kidneys. Fibropapilloma is caused by a herpesvirus that is transmitted by leeches such as Ozobranchus branchiatus, a species of leech which feeds almost entirely on green sea turtles.
Life cycle
Green sea turtles migrate long distances between feeding sites and nesting sites; some swim more than to reach their spawning grounds. Beaches in Southeast Asia, India, islands in the western Pacific, and Central America are where green sea turtles breed. Mature turtles often return to the exact beach from which they hatched. Females usually mate every two to four years. Males, on the other hand, visit the breeding areas every year, attempting to mate. Mating seasons vary between populations. For most C. mydas in the Caribbean, mating season is from June to September. The French Guiana nesting subpopulation nests from March to June. In the tropics, green turtles nest throughout the year, although some subpopulations prefer particular times of the year. In Pakistan, Indian Ocean turtles nest year-round, but prefer the months of July to December. | Green sea turtle | Wikipedia | 340 | 2150513 | https://en.wikipedia.org/wiki/Green%20sea%20turtle | Biology and health sciences | Turtles | Animals |
Sea turtles return to the beaches on which they were born to lay their own eggs. The reason for returning to native beaches may be that it guarantees the turtles an environment that has the necessary components for their nesting to be successful. These include a sandy beach, easy access for the hatchlings to get to the ocean, the right incubation temperatures, and low probability of predators that may feed on their eggs. Over time these turtles have evolved these tendencies to return to an area that has provided reproductive success for many generations. Their ability to return to their birthplace is known as natal homing. The males also return to their birthplaces in order to mate. These males that return to their homes know they will be able to find mates because the females born there also return to breed. By doing this, the green sea turtles are able to improve their reproductive success and is why they are willing to expend the energy to travel thousands of miles across the ocean in order to reproduce.
Mating behaviour is similar to other marine turtles. Female turtles control the process. A few populations practice polyandry, although this does not seem to benefit hatchlings. After mating in the water, the female moves above the beach's high tide line, where she digs a hole in depth with her hind flippers and deposits her eggs. The hole is then covered up again. Clutch size ranges between 85 and 200, depending on the age of the female. This process takes about an hour to an hour and a half. After the nest is completely covered, she returns to the sea. The female will do this 3 to 5 times in one season. | Green sea turtle | Wikipedia | 328 | 2150513 | https://en.wikipedia.org/wiki/Green%20sea%20turtle | Biology and health sciences | Turtles | Animals |
The eggs are round and white, and about in diameter. The hatchlings remain buried for days until they all emerge together at night. The temperature of the nest determines the sex of the turtles at around the 20–40 day mark. Green Sea Turtles are type 1a, meaning males develop at cooler temperatures while females are produced under hot temperatures. At around 50 to 70 days, the eggs hatch during the night, and the hatchlings instinctively head directly into the water. This is the most dangerous time in a turtle's life. As they walk, predators, such as gulls and crabs, feed on them. A significant percentage never make it to the ocean. Little is known of the initial life history of newly hatched sea turtles. Juveniles spend three to five years in the open ocean before they settle as still-immature juveniles into their permanent shallow-water lifestyle. It is speculated that they take twenty to fifty years to reach sexual maturity. Individuals live up to eighty years in the wild. It is estimated that only 1% of hatchlings reach sexual maturity.
Each year on Ascension Island in the South Atlantic, C. mydas females create 6,000 to 25,000 nests. They are among the largest green turtles in the world; many are more than in length and weigh up to .
Breathing and sleep
Sea turtles spend almost all their lives submerged, but must breathe air for the oxygen needed to meet the demands of vigorous activity. With a single explosive exhalation and rapid inhalation, sea turtles can quickly replace the air in their lungs. The lungs permit a rapid exchange of oxygen and prevent gases from being trapped during deep dives. Sea turtle blood can deliver oxygen efficiently to body tissues even at the pressures encountered during diving. During routine activity, green and loggerhead turtles dive for about four to five minutes, and surface to breathe for one to three seconds.
Turtles can rest or sleep underwater for several hours at a time, but submergence time is much shorter while diving for food or to escape predators. Breath-holding ability is affected by activity and stress, which is why turtles quickly drown in shrimp trawlers and other fishing gear. During the night while sleeping and to protect themselves from potential predators, the adults wedge themselves under rocks below the surface and under ledges in reefs and coastal rocks. Many green sea turtles have been observed in returning to the same sleeping location night after night.
Physiology and sensory modalities | Green sea turtle | Wikipedia | 490 | 2150513 | https://en.wikipedia.org/wiki/Green%20sea%20turtle | Biology and health sciences | Turtles | Animals |
Green sea turtles tend to have good vision, well adapted to a life at sea. The turtles can see many colors, but are most sensitive to light from violet to yellow or wavelengths of 400 to 600 nanometers. They do not see many colors in the orange to red portion of the light spectrum. On land, however, the sea turtles are nearsighted because the lenses in the eyes are spherical and adjusted to refraction underwater. Sea turtles have no external ear and only one ear bone, called the columella. With one ear bone, the turtles can hear only low frequency sounds, from 200 to 700 Hz. Sounds can also be detected through vibrations of the head, backbone, and shell. The nose of the turtle has two external openings and connects to the roof of the mouth through internal openings. The lower surface of the nasal passage has two sets of sensory cells called the Jacobson's organ. The turtle can use this organ to smell by pumping water in and out of its nose.
Since green sea turtles migrate long distances during breeding seasons, they have special adaptive systems in order to navigate. In the open ocean, the turtles navigate using wave directions, sun light, and temperatures. The sea turtles also contain an internal magnetic compass. They can detect magnetic information by using magnetic forces acting on the magnetic crystals in their brains. Through these crystals, they can sense the intensity of Earth's magnetic field and are able to make their way back to their nesting grounds or preferred feeding grounds. | Green sea turtle | Wikipedia | 301 | 2150513 | https://en.wikipedia.org/wiki/Green%20sea%20turtle | Biology and health sciences | Turtles | Animals |
Natal homing is an animal's ability to return to its birthplace in order to reproduce. Natal homing is found in all species of sea turtles and in other animals such as salmon. How these turtles are able to return to their birthplace is an interesting phenomenon. Many researchers believe that sea turtles use a process called imprinting, which is a special type of learning that occurs when turtles first hatch that allows them to recognize their native beach. There are two types of imprinting that are thought to be the reason turtles can find these beaches. The first is the chemical imprinting hypothesis. This hypothesis states that much like salmon, sea turtles are able to use olfactory cues and senses to smell their way home. However, a problem with this hypothesis is that some turtles travel thousands of miles to return to their native beaches, and the scents from that area are not likely to travel and be distinguishable from that distance.
The second hypothesis is the geomagnetic. This hypothesis states that as it hatches, a young turtle will imprint on the magnetic field of the beach they are born on. This hypothesis strongly correlates to the method which sea turtles use to navigate the earth.
In order to tolerate the constant heat loss in the water, sea turtles have the ability to shunt blood away from tissues that are tolerant of low oxygen levels toward the heart, brain, and central nervous system. Other mechanisms include basking on warm beaches and producing heat through their activity and movements of their muscles. Basking turtles sometimes look like they are crying because behind the turtles eye is the lachrymal gland which stores excess salt from the sea water, which then expels through the turtles eye. In the winter months, turtles living at higher latitudes can hibernate for a short period in the mud.
Unique characteristics and features | Green sea turtle | Wikipedia | 366 | 2150513 | https://en.wikipedia.org/wiki/Green%20sea%20turtle | Biology and health sciences | Turtles | Animals |
Green sea turtles can reach up to 40 miles per hour when swimming, making them the fastest sea turtle. The green sea turtles exhibit sex differences by their development and appearance. As adult turtles, males are easily distinguishable from the females by having a longer tail (visibly extending past the shell) and longer claws on the front flippers. The hatching time and sex of the turtles are determined by the incubation temperature of the nest. Hatchings occur more quickly in nests that are warmer than nests that are in cooler conditions. Warm nesting sites above 30 degrees Celsius favor the development of females, whereas nesting sites below 30 degrees Celsius produce males. The position of the egg in the nest also affects sex-determination. Eggs in the center tend to hatch as females due to the warmer conditions within the nest.
Green sea turtles play an essential role in the ecosystem in which they live. In the seagrass beds, the turtles feed on the seagrass by trimming only the top and leaving the roots of the plant. Through their feeding technique, the turtles help to improve the health and growth of the seagrass beds. The healthy seagrass beds that the turtles provide give habitat and feeding grounds for many species of fish and crustaceans. On the nesting beaches, the green sea turtles provide key nutrients for the ecosystem through their hatched egg shells. In their coral reef habitat, the green sea turtles have a symbiotic interaction with reef fish, including the yellow tang. The yellow tang fish swims along with the turtle and feeds on the algae, barnacles, and parasites on its shell and flippers. This species interaction provides food for the yellow tang and provides a necessary cleaning and smoothing of the turtle's shell. This cleaning helps the turtles swim by reducing the amount of drag and improves their health.
Importance to humans
Historically, the turtles' skin was tanned and used to make handbags, especially in Hawaii. Ancient Chinese considered the flesh of sea turtles a culinary delicacy, including and especially C. mydas. Particularly for this species, the turtle's fat, cartilage, and flesh, known as calipee, are sought as ingredients for making turtle soup, a popular 19th-century English and American dish. | Green sea turtle | Wikipedia | 455 | 2150513 | https://en.wikipedia.org/wiki/Green%20sea%20turtle | Biology and health sciences | Turtles | Animals |
In Java, Indonesia, sea turtle eggs were a popular delicacy. However, the turtle's flesh is regarded as ḥarām or "unclean" under Islamic law (Islam is Java's primary religion). In Bali, turtle meat was a prominent feature at ceremonial and religious feasts. Turtles were harvested in the remotest parts of the Indonesian archipelago. Bali has been importing sea turtles since the 1950s, as its own turtle supplies became depleted. The mostly Hindu Balinese do not eat the eggs, but sell them instead to local Muslims.
Commercial farms, such as the Cayman Turtle Farm in the West Indies, once bred them for commercial sale of turtle meat, turtle oil (rendered from the fat), turtle shell, and turtle leather made from the skin. The farm's initial stock was in large part from "doomed" eggs removed from nests threatened by erosion, flooding, or in chemically hostile soil. The farms held as many as 100,000 turtles at any one time. When the international markets were closed by regulations that did not allow even farm-bred turtle products to be exported internationally, the surviving farm became primarily a tourist attraction, supporting 11,000 turtles. Initially started as Mariculture Ltd., then Cayman Turtle Farm Ltd and subsequently branded Boatswain's Beach, in 2010 the farm's brandname was changed to Cayman Turtle Farm: Island Wildlife Encounter.
Sea turtles are integral to the history and culture of the Cayman Islands. When the islands were first discovered by Christopher Columbus in 1503, he named them "Las Tortugas" because of the abundance of sea turtles in the waters around the islands. Many of the earliest visitors came to the Cayman Islands to capture the turtles as a source of fresh meat during long voyages. The green turtle is a national symbol displayed as part of the coat of arms of the Cayman Islands, which also forms part of the national flag of the Cayman Islands. The country's currency uses a turtle as the watermark in its banknotes. A stylised sea turtle nicknamed "Sir Turtle" is the mascot of the national airline Cayman Airways and is part of the livery of its aircraft. | Green sea turtle | Wikipedia | 436 | 2150513 | https://en.wikipedia.org/wiki/Green%20sea%20turtle | Biology and health sciences | Turtles | Animals |
A ki'i pōhaku (petroglyph) of a green sea turtle (or honu) can be found on the Big Island of Hawaii in the Pu'u Loa lava fields. The green sea turtle has always held a special meaning for Hawaiians and this petroglyph shows its importance; it may date to when the Hawaiian Islands first became populated. The turtle symbolizes a navigator that can find his way home time after time. This symbol mirrors the real life of the green Hawaiian turtle as it will swim hundreds of miles to lay its eggs at its own place of birth. Though there are other myths as well, some Hawaiian legends say the honu were the first to guide the Polynesians to the Hawaiian Islands. Hawaiians revere the turtle and the legend of Kailua, a turtle who could take the form of a girl at will. In human form, she looked after the children playing on Punalu'u Beach.
Conservation
In recent decades, sea turtles have moved from unrestricted exploitation to global protection, with individual countries providing additional protection, although serious threats remain unabated.
All populations are considered "threatened". | Green sea turtle | Wikipedia | 236 | 2150513 | https://en.wikipedia.org/wiki/Green%20sea%20turtle | Biology and health sciences | Turtles | Animals |
Threats
Human action presents both intentional and unintentional threats to the species' survival. Intentional threats include continued hunting, poaching and egg harvesting. More dangerous are unintentional threats, including boat strikes, fishermen's nets that lack turtle excluder devices, pollution and habitat destruction. Chemical pollution may create tumors; effluent from harbors near nesting sites may create disturbances; and light pollution may disorient hatchlings. With chemical pollution present, there is a development of tar balls that is often eaten by green sea turtles in a confusion of their food. Tar balls cause the green sea turtle to ingest toxins that can block their guts and cause swelling of the tissue, displacing the liver and intestines. Habitat loss usually occurs due to human development of nesting areas. Beach-front construction, land "reclamation" and increased tourism are examples of such development. An infectious tumor-causing disease, fibropapillomatosis, is also a problem in some populations. The disease kills a sizeable fraction of those it infects, though some individuals seem to resist the disease. In addition, at least in the Southwestern Atlantic (Río de la Plata, Uruguay), exotic invasive species such as the rapa whelk Rapana venosa, were reported massively bio-fouling immature green turtles, reducing buoyancy, increasing drag, and causing severe injuries to the carapace. Because of these threats, many populations are in a vulnerable state. | Green sea turtle | Wikipedia | 305 | 2150513 | https://en.wikipedia.org/wiki/Green%20sea%20turtle | Biology and health sciences | Turtles | Animals |
Pacific green turtles' foraging habitats are poorly understood and mostly unknown. These foraging grounds are most likely along the coast of Baja California, Mexico and southern California, in which these turtles have a high risk of incidental capture by coastal fisheries. The main mortality factor for these turtles is the shrimp trawlers in Mexico, in which many of these turtles go undocumented. The only foraging area that has been identified is San Diego Bay, but it is heavily polluted with metals and PCBs. These contaminants have a negative effect on the ocean environment, and have been shown to cause lesions and sometimes mortality. Green turtles also are threatened by entanglement and ingestion of plastic. In San Diego Bay, an adult green turtle was found dead with monofilament netting tightly packed in its esophagus. In addition there are indications that global climate change is affecting the ability of green turtle populations in Australia to produce males due to their temperature-dependent sex determination and the rising temperatures in the northern Great Barrier Reef region. Construction of new thermal power stations can raise local water temperature, which is also said to be a threat.
Green sea turtles are the most commonly traded species along Java's south coast and are sold in the form of whole, stuffed animals or turtle oil, locally known as "minyak bulus".
The geographer James J. Parsons' book titled The Green Turtle and Man played a special role in the conservation movement to save the species from extinction.
Global initiatives
The International Union for Conservation of Nature (IUCN) has repeatedly listed green sea turtles in its Red List under differing criteria. In 1982, they officially classified it as an endangered species. The 1986, 1988, 1990, 1994, and the landmark 1996 edition of the IUCN Red List, retained the listing.
In 2001, Nicholas Mrosovsky filed a delisting petition, claiming some green turtle populations were large, stable and in some cases, increasing. At the time, the species was listed under the strict EN A1abd criteria. The IUCN Standards and Petitions Subcommittee ruled that visual counts of nesting females could not be considered "direct observation" and thus downgraded the species' status to EN A1bd—retaining the turtle's endangered status. | Green sea turtle | Wikipedia | 454 | 2150513 | https://en.wikipedia.org/wiki/Green%20sea%20turtle | Biology and health sciences | Turtles | Animals |
In 2004, the IUCN reclassified C. mydas as endangered under the EN A2bd criteria, which essentially states the wild populations face a high risk of extinction because of several factors. These factors include a probable population reduction of more than 50% over the past decade as estimated from abundance indices and by projecting exploitation levels.
On 3 May 2007, C. mydas was listed on Appendix I of the Convention on International Trade in Endangered Species (CITES) as a member of the family Cheloniidae. The species was originally listed on Appendix II in 1975. The entire family was moved to Appendix I in 1977, with the exception of the Australian population of C. mydas. In 1981, the Australian population joined the rest. The Appendix I listing prohibits commercial international trade in the species (including parts and derivatives). The Zoological Society of London has listed the reptile as an EDGE species.
The Mediterranean population is listed as critically endangered. The eastern Pacific, Hawaiian and Southern California subpopulations are designated threatened. Specific Mexican subpopulations are listed as endangered. The Florida population is listed as endangered. The World Wide Fund for Nature has labeled populations in Pakistan as "rare and declining".
Since 1999, the Florida Aquarium has led extensive sea turtle rehabilitation efforts and visitor and community education & conservation platforms to advance sea turtle protection. Over a 20-year period, the aquarium received 200 sea turtles, and while not all could be released due to the nature of their injuries or illnesses, 180 were successfully released. In 2019, they opened a state-of-the-art Sea Turtle Rehabilitation Center in Apollo Beach, Florida. In the first year, The Florida Aquarium Animal Response Team managed the care of 21 sea turtles, initiated new foraging-readiness testing for release candidates in deep-dive tank, and released 14 animals. In 2020, they also initiated a study to better understand how micro-plastics are impacting the sea turtles in their care. In 2016, Florida enacted extensive protection measures. Florida statutes (F.A.C. Rule 68E-1) restrict the take, possession, disturbance, mutilation, destruction, selling, transference, molestation, and harassment of marine turtles, nests or eggs. Protection is also afforded to marine turtle habitat. A specific authorization from commission staff is required to conduct scientific, conservation, or educational activities that directly involve marine turtles in or collected from Florida, their nests, hatchlings or parts thereof, regardless of applicant's possession of any federal permit. | Green sea turtle | Wikipedia | 504 | 2150513 | https://en.wikipedia.org/wiki/Green%20sea%20turtle | Biology and health sciences | Turtles | Animals |
In the State of Hawaii, specifically on the Island of Hawai'i (Hawaii County), state representative Faye Hanohano, a Native Hawaiian rights activist, pressed for a measure to delist C. mydas from protected status so that Native Hawaiians could legally harvest the turtles and possibly their eggs as well. The bill, HCR14, was largely overlooked by the media since at that point it was only a local issue. While the bill was passed in the United States House of Representatives, the United States Senate's Committee on Energy and Environment refused to hear it, which meant that the bill did not go on to be heard by the Senate.
Country-specific initiatives
In addition to management by global entities such as the IUCN and CITES, specific countries around the world have undertaken conservation efforts.
The Indonesian island of Bali has traditional uses that were considered sustainable, but have been questioned considering greater demand from the larger and wealthier human population. The harvest was the most intensive in the world. In 1999, Indonesia restricted turtle trade and consumption because of the decreasing population and threat of a tourist boycott. It rejected a request made by Bali Governor I Made Mangku Pastika in November 2009 to set a quota of 1,000 turtles to be killed in Hindu religious ceremonies. While conservationists respect the need for turtles in rituals, they wanted a smaller quota.
Multiple protected areas of the Philippines have significant green sea turtle nesting and feeding sites. The most notable is Turtle Islands Wildlife Sanctuary, an UNESCO tentative site which encompasses an entire municipality and one of Southeast Asia's most important green sea turtle nesting sites. Other notable sites include the UNESCO tentative site of El Nido-Taytay Management Resource Protected Area and the UNESCO World Heritage Site of Tubbataha Reefs Natural Park. The species is protected under Republic Act 9147 or the Wildlife Resources Conservation and Protection Act, while the sites where they live and nest are protected under the National Integrated Protected Areas System Act.
Ecotourism is one initiative in Sabah, Malaysia. The island of Pulau Selingan is home to a turtle hatchery. Staff people place some of the eggs laid each night in a hatchery to protect them from predators. Incubation takes around sixty days. When the eggs hatch, tourists assist in the release of the baby turtles into the sea. | Green sea turtle | Wikipedia | 466 | 2150513 | https://en.wikipedia.org/wiki/Green%20sea%20turtle | Biology and health sciences | Turtles | Animals |
The Hawaiian subpopulation has made a remarkable comeback and is now one focus of ecotourism and has become something of a state mascot. Students of Hawaii Preparatory Academy on the Big Island have tagged thousands of specimens since the early 1990s.
In the United Kingdom the species is protected by a Biodiversity Action Plan, due to excess harvesting and marine pollution. The Pakistani-branch of the World Wide Fund for Nature has been initiating projects for secure turtle hatching since the 1980s. However, the population has continued to decline.
In the Atlantic, conservation initiatives have centered around Caribbean nesting sites. The Tortuguero nesting beaches in Costa Rica have been the subject of egg-collection limits since the 1950s. The Tortuguero National Park was formally established in 1976, in part, to protect that region's nesting grounds. On Ascension Island, which contains some of the most important nesting beaches, an active conservation program has been implemented. Karumbé has been monitoring foraging and developmental areas of juvenile green turtles in Uruguay since 1999.
In Mozambique, there are a number of initiatives to protect sea turtles. In the Primeiras e Segundas, WWF Mozambique has established a turtle tagging and protection program. The archipelago is a vital nesting area for green turtles, including Ilha do Fogo where Fire Island Conservation manage a turtle monitoring programme, and at Celdeira Island, where several nesting females have been tagged. | Green sea turtle | Wikipedia | 281 | 2150513 | https://en.wikipedia.org/wiki/Green%20sea%20turtle | Biology and health sciences | Turtles | Animals |
Cayman Turtle Farm located in Grand Cayman in the northwest Caribbean Sea is the first farm to have achieved the second generation of green sea turtles bred, laid, hatched, and raised in captivity. Since its beginning in 1968, the farm has released over 31,000 turtles into the wild, and each year more captive-bred turtles are released into the Caribbean Sea from beaches around the island of Grand Cayman. Captive-bred turtles released from the farm as hatchlings or yearlings with "living tags," have now begun to return to nest on Grand Cayman as adults. On February 19, 2012 the farm released the first 2nd-generation captive-bred green sea turtle equipped with a Position Tracking Transponder, or PTT (also known as a satellite tag). In addition, the farm provides turtle meat products to the local population for whom turtle has been part of the traditional cuisine for centuries. In so doing, the farm curtails the incentive to take turtles from the wild, which over the years in addition to the Cayman Turtle Farm's release of captive-bred turtles has enabled an increase in the number of turtles sighted in the waters around the island of Grand Cayman and nesting on its beaches.
In the Pacific, green sea turtles nest on the motu (islets) in the Funafuti Conservation Area, a marine conservation area covering 33 square kilometers (12.74 square miles) of reef, lagoon and motu on the western side of Funafuti atoll in Tuvalu.
On Raine Island, up to 100,000 nesting females have been observed in a season, with the cay producing 90% of the region's green turtles. However, the hatching rate declined in the 1990s, and a further decline in the population was threatened by the deaths of thousands of females as they struggled to climb the small sandy cliffs. In addition, as the shape of the island had changed over time, the spread of the beaches outwards had led to greater risk of inundation of the turtle nests. Between 2011 and 2020, a collaborative project by the Queensland Government, BHP (as corporate sponsor), the Great Barrier Reef Marine Park Authority, Great Barrier Reef Foundation, and Wuthathi and Meriam traditional owners, reshaped the island using heavy machinery in a way that gave the female turtles a smoother passage and reduced the risk of nest inundation. A sophisticated monitoring and research system, using 3D modelling, satellite technology and drones was employed, and monitoring continues. | Green sea turtle | Wikipedia | 502 | 2150513 | https://en.wikipedia.org/wiki/Green%20sea%20turtle | Biology and health sciences | Turtles | Animals |
, a project called "The Turtle Cooling Project" is being undertaken by scientists from the World Wildlife Fund Australia, University of Queensland, Deakin University and the Queensland Government. It is looking at the effect of global warming on northern green turtle breeding, in particular the effect of producing more male turtles owing to the higher temperatures. They are working in the area around Raine Island, Heron Island and Moulter Cay.
Genetics
The genome of Chelonia mydas was sequenced in 2013 to examine the development and evolution of the turtle body plan. | Green sea turtle | Wikipedia | 110 | 2150513 | https://en.wikipedia.org/wiki/Green%20sea%20turtle | Biology and health sciences | Turtles | Animals |
A circumferentor, or surveyor's compass, is an instrument used in surveying to measure horizontal angles. It was superseded by the theodolite in the early 19th century.
A circumferentor consists of a circular brass box containing a magnetic needle, which moves freely over a brass circle, or compass divided into 360 degrees. The needle is protected by a glass covering. A pair of sights is located on the North-South axis of the compass. Circumferentors were typically mounted on tripods and rotated on ball-and-socket joints.
Circumferentors were made throughout Europe, including in England, France, Italy, and Holland. By the early 19th century, Europeans preferred theodolites to circumferentors. However, the circumferentor remained in common use in mines and in wooded or uncleared areas, such as in America.
Usage
Measuring angles
To measure an angle with a circumferentor, such as angle EKG (Figure 1), place the instrument at K, with the fleur-de-lis in the card towards you. Then direct the sights, until through them you see E; and note the degree pointed at by the south end of the needle, such as 296°. Then, turn the instrument around, with the fleur-de-lis still towards you, and direct the sights to G; note the degree at which the south end of the needle point, such as 182°. Finally, subtract the lesser number, 182, from the greater, 296°; the remainder, 114°, is the number of degrees in the angle EKG.
If the remainder is more than 180 degrees, it must be subtracted from 360 degrees.
Surveying a region
To take the plot of a field, forest, park, etc., with a circumferentor, consider region ABCDEFGHK in Figure 2, an area to be surveyed.
Placing the instrument at A, the fleur-de-lis towards you, direct the sights to B; where suppose the south end of the needle cuts 191°; and the ditch, wall, or hedge, measuring with a Gunter's chain, contains 10 chains, 75 links.
Placing the instrument at B, direct the sights as before to C; the south end of the needle, e.g. will cut 279°; and the line BC contains 6 chains and 83 links. | Circumferentor | Wikipedia | 509 | 5431281 | https://en.wikipedia.org/wiki/Circumferentor | Technology | Surveying tools | null |
Then move the instrument to C; turn the sights to measure D, and measure CD as before. In the same manner, proceed to D, E, F, G, H, and lastly to K; still noting the degrees of every bearing, or angle, and the distances of every side. This will result in a table of the following form:
From this table, the field is to be plotted, or protracted.
Alternative plotting method:
An alternative way to plot the area in Figure 2 is to use several angles and only a few measurements and calculate their positions.
This could be done by starting at the center point in Figure 2 which is not labeled, but which will be referred to as "Center." Assume each point can be seen from each other point. From the "center" point, sight and record the angle to each point using the sights as described above. Then move to, and measure the distance to, one of the other points referenced, such as point B. At point B, measure the angles to all the other points. Then, move to an additional point such as point F. Again, measure the distance from the center to the point chosen (F). At that point, measure and record the angles to each of the other points as was done at point B. Chose a scale (a ratio between the size of the area to be plotted and the size of the paper on which you will draw the plot) that will allow the plot to fit on your paper and plot the angles and distances.
The advantage of this method over the first one above is that there are fewer distance measurements and any errors in angles or distances will not be cumulative; that is, if you use the first survey method, any angle that is slightly off will distort the remainder of the plot. The second method can also be used when it is not possible to measure some of the distances, for example, if there is a water barrier between two of the points. Also, if there are any inaccuracies in the measurements, they will be revealed in the plot because the points plotted from various angles will not coincide. | Circumferentor | Wikipedia | 430 | 5431281 | https://en.wikipedia.org/wiki/Circumferentor | Technology | Surveying tools | null |
Additional considerations include the number of times the circumferentor must be set up and aligned. With the first method, the instrument must be set up at each point with a compass. With the second method, the initial set up is at "center." After that, for example at point B, the instrument can be set up by aligning the sight with the reciprocal of the angle between "center" and B. Thus, any local change in the magnetic field that would affect the compass would be nullified.
Surveyor's double prism
A double prism is a device to measure right angles, consisting of two five sided prisms stacked on top of each other and a plumb-bob. It is used to stake out right angles, for example on a construction site. | Circumferentor | Wikipedia | 158 | 5431281 | https://en.wikipedia.org/wiki/Circumferentor | Technology | Surveying tools | null |
The philosophy of artificial intelligence is a branch of the philosophy of mind and the philosophy of computer science that explores artificial intelligence and its implications for knowledge and understanding of intelligence, ethics, consciousness, epistemology, and free will. Furthermore, the technology is concerned with the creation of artificial animals or artificial people (or, at least, artificial creatures; see artificial life) so the discipline is of considerable interest to philosophers. These factors contributed to the emergence of the philosophy of artificial intelligence.
The philosophy of artificial intelligence attempts to answer such questions as follows:
Can a machine act intelligently? Can it solve any problem that a person would solve by thinking?
Are human intelligence and machine intelligence the same? Is the human brain essentially a computer?
Can a machine have a mind, mental states, and consciousness in the same sense that a human being can? Can it feel how things are? (i.e. does it have qualia?)
Questions like these reflect the divergent interests of AI researchers, cognitive scientists and philosophers respectively. The scientific answers to these questions depend on the definition of "intelligence" and "consciousness" and exactly which "machines" are under discussion.
Important propositions in the philosophy of AI include some of the following:
Turing's "polite convention": If a machine behaves as intelligently as a human being, then it is as intelligent as a human being.
The Dartmouth proposal: "Every aspect of learning or any other feature of intelligence can in principle be so precisely described that a machine can be made to simulate it."
Allen Newell and Herbert A. Simon's physical symbol system hypothesis: "A physical symbol system has the necessary and sufficient means of general intelligent action."
John Searle's strong AI hypothesis: "The appropriately programmed computer with the right inputs and outputs would thereby have a mind in exactly the same sense human beings have minds."
Hobbes' mechanism: "For 'reason' ... is nothing but 'reckoning,' that is adding and subtracting, of the consequences of general names agreed upon for the 'marking' and 'signifying' of our thoughts..." | Philosophy of artificial intelligence | Wikipedia | 433 | 2958015 | https://en.wikipedia.org/wiki/Philosophy%20of%20artificial%20intelligence | Technology | Artificial intelligence concepts | null |
Can a machine display general intelligence?
Is it possible to create a machine that can solve all the problems humans solve using their intelligence? This question defines the scope of what machines could do in the future and guides the direction of AI research. It only concerns the behavior of machines and ignores the issues of interest to psychologists, cognitive scientists and philosophers, evoking the question: does it matter whether a machine is really thinking, as a person thinks, rather than just producing outcomes that appear to result from thinking?
The basic position of most AI researchers is summed up in this statement, which appeared in the proposal for the Dartmouth workshop of 1956:
"Every aspect of learning or any other feature of intelligence can in principle be so precisely described that a machine can be made to simulate it."
Arguments against the basic premise must show that building a working AI system is impossible because there is some practical limit to the abilities of computers or that there is some special quality of the human mind that is necessary for intelligent behavior and yet cannot be duplicated by a machine (or by the methods of current AI research). Arguments in favor of the basic premise must show that such a system is possible.
It is also possible to sidestep the connection between the two parts of the above proposal. For instance, machine learning, beginning with Turing's infamous child machine proposal, essentially achieves the desired feature of intelligence without a precise design-time description as to how it would exactly work. The account on robot tacit knowledge eliminates the need for a precise description altogether.
The first step to answering the question is to clearly define "intelligence".
Intelligence
Turing test
Alan Turing reduced the problem of defining intelligence to a simple question about conversation. He suggests that: if a machine can answer any question posed to it, using the same words that an ordinary person would, then we may call that machine intelligent. A modern version of his experimental design would use an online chat room, where one of the participants is a real person and one of the participants is a computer program. The program passes the test if no one can tell which of the two participants is human. Turing notes that no one (except philosophers) ever asks the question "can people think?" He writes "instead of arguing continually over this point, it is usual to have a polite convention that everyone thinks". Turing's test extends this polite convention to machines:
If a machine acts as intelligently as a human being, then it is as intelligent as a human being. | Philosophy of artificial intelligence | Wikipedia | 504 | 2958015 | https://en.wikipedia.org/wiki/Philosophy%20of%20artificial%20intelligence | Technology | Artificial intelligence concepts | null |
One criticism of the Turing test is that it only measures the "humanness" of the machine's behavior, rather than the "intelligence" of the behavior. Since human behavior and intelligent behavior are not exactly the same thing, the test fails to measure intelligence. Stuart J. Russell and Peter Norvig write that "aeronautical engineering texts do not define the goal of their field as 'making machines that fly so exactly like pigeons that they can fool other pigeons'".
Intelligence as achieving goals
Twenty-first century AI research defines intelligence in terms of goal-directed behavior. It views intelligence as a set of problems that the machine is expected to solve – the more problems it can solve, and the better its solutions are, the more intelligent the program is. AI founder John McCarthy defined intelligence as "the computational part of the ability to achieve goals in the world."
Stuart Russell and Peter Norvig formalized this definition using abstract intelligent agents. An "agent" is something which perceives and acts in an environment. A "performance measure" defines what counts as success for the agent.
"If an agent acts so as to maximize the expected value of a performance measure based on past experience and knowledge then it is intelligent."
Definitions like this one try to capture the essence of intelligence. They have the advantage that, unlike the Turing test, they do not also test for unintelligent human traits such as making typing mistakes.
They have the disadvantage that they can fail to differentiate between "things that think" and "things that do not". By this definition, even a thermostat has a rudimentary intelligence.
Arguments that a machine can display general intelligence
The brain can be simulated
Hubert Dreyfus describes this argument as claiming that "if the nervous system obeys the laws of physics and chemistry, which we have every reason to suppose it does, then ... we ... ought to be able to reproduce the behavior of the nervous system with some physical device". This argument, first introduced as early as 1943 and vividly described by Hans Moravec in 1988,
is now associated with futurist Ray Kurzweil, who estimates that computer power will be sufficient for a complete brain simulation by the year 2029. A non-real-time simulation of a thalamocortical model that has the size of the human brain (1011 neurons) was performed in 2005, and it took 50 days to simulate 1 second of brain dynamics on a cluster of 27 processors. | Philosophy of artificial intelligence | Wikipedia | 507 | 2958015 | https://en.wikipedia.org/wiki/Philosophy%20of%20artificial%20intelligence | Technology | Artificial intelligence concepts | null |
Even AI's harshest critics (such as Hubert Dreyfus and John Searle) agree that a brain simulation is possible in theory.
However, Searle points out that, in principle, anything can be simulated by a computer; thus, bringing the definition to its breaking point leads to the conclusion that any process at all can technically be considered "computation". "What we wanted to know is what distinguishes the mind from thermostats and livers," he writes. Thus, merely simulating the functioning of a living brain would in itself be an admission of ignorance regarding intelligence and the nature of the mind, like trying to build a jet airliner by copying a living bird precisely, feather by feather, with no theoretical understanding of aeronautical engineering.
Human thinking is symbol processing
In 1963, Allen Newell and Herbert A. Simon proposed that "symbol manipulation" was the essence of both human and machine intelligence. They wrote:
"A physical symbol system has the necessary and sufficient means of general intelligent action."
This claim is very strong: it implies both that human thinking is a kind of symbol manipulation (because a symbol system is necessary for intelligence) and that machines can be intelligent (because a symbol system is sufficient for intelligence).
Another version of this position was described by philosopher Hubert Dreyfus, who called it "the psychological assumption":
"The mind can be viewed as a device operating on bits of information according to formal rules."
The "symbols" that Newell, Simon and Dreyfus discussed were word-like and high levelsymbols that directly correspond with objects in the world, such as <dog> and <tail>. Most AI programs written between 1956 and 1990 used this kind of symbol. Modern AI, based on statistics and mathematical optimization, does not use the high-level "symbol processing" that Newell and Simon discussed.
Arguments against symbol processing
These arguments show that human thinking does not consist (solely) of high level symbol manipulation. They do not show that artificial intelligence is impossible, only that more than symbol processing is required.
Gödelian anti-mechanist arguments | Philosophy of artificial intelligence | Wikipedia | 425 | 2958015 | https://en.wikipedia.org/wiki/Philosophy%20of%20artificial%20intelligence | Technology | Artificial intelligence concepts | null |
In 1931, Kurt Gödel proved with an incompleteness theorem that it is always possible to construct a "Gödel statement" that a given consistent formal system of logic (such as a high-level symbol manipulation program) could not prove. Despite being a true statement, the constructed Gödel statement is unprovable in the given system. (The truth of the constructed Gödel statement is contingent on the consistency of the given system; applying the same process to a subtly inconsistent system will appear to succeed, but will actually yield a false "Gödel statement" instead.) More speculatively, Gödel conjectured that the human mind can eventually correctly determine the truth or falsity of any well-grounded mathematical statement (including any possible Gödel statement), and that therefore the human mind's power is not reducible to a mechanism. Philosopher John Lucas (since 1961) and Roger Penrose (since 1989) have championed this philosophical anti-mechanist argument.
Gödelian anti-mechanist arguments tend to rely on the innocuous-seeming claim that a system of human mathematicians (or some idealization of human mathematicians) is both consistent (completely free of error) and believes fully in its own consistency (and can make all logical inferences that follow from its own consistency, including belief in its Gödel statement) . This is probably impossible for a Turing machine to do (see Halting problem); therefore, the Gödelian concludes that human reasoning is too powerful to be captured by a Turing machine, and by extension, any digital mechanical device.
However, the modern consensus in the scientific and mathematical community is that actual human reasoning is inconsistent; that any consistent "idealized version" H of human reasoning would logically be forced to adopt a healthy but counter-intuitive open-minded skepticism about the consistency of H (otherwise H is provably inconsistent); and that Gödel's theorems do not lead to any valid argument that humans have mathematical reasoning capabilities beyond what a machine could ever duplicate. This consensus that Gödelian anti-mechanist arguments are doomed to failure is laid out strongly in Artificial Intelligence: "any attempt to utilize (Gödel's incompleteness results) to attack the computationalist thesis is bound to be illegitimate, since these results are quite consistent with the computationalist thesis." | Philosophy of artificial intelligence | Wikipedia | 473 | 2958015 | https://en.wikipedia.org/wiki/Philosophy%20of%20artificial%20intelligence | Technology | Artificial intelligence concepts | null |
Stuart Russell and Peter Norvig agree that Gödel's argument does not consider the nature of real-world human reasoning. It applies to what can theoretically be proved, given an infinite amount of memory and time. In practice, real machines (including humans) have finite resources and will have difficulty proving many theorems. It is not necessary to be able to prove everything in order to be an intelligent person.
Less formally, Douglas Hofstadter, in his Pulitzer Prize winning book Gödel, Escher, Bach: An Eternal Golden Braid, states that these "Gödel-statements" always refer to the system itself, drawing an analogy to the way the Epimenides paradox uses statements that refer to themselves, such as "this statement is false" or "I am lying". But, of course, the Epimenides paradox applies to anything that makes statements, whether it is a machine or a human, even Lucas himself. Consider:
Lucas can't assert the truth of this statement.
This statement is true but cannot be asserted by Lucas. This shows that Lucas himself is subject to the same limits that he describes for machines, as are all people, and so Lucas's argument is pointless.
After concluding that human reasoning is non-computable, Penrose went on to controversially speculate that some kind of hypothetical non-computable processes involving the collapse of quantum mechanical states give humans a special advantage over existing computers. Existing quantum computers are only capable of reducing the complexity of Turing computable tasks and are still restricted to tasks within the scope of Turing machines. . By Penrose and Lucas's arguments, the fact that quantum computers are only able to complete Turing computable tasks implies that they cannot be sufficient for emulating the human mind. Therefore, Penrose seeks for some other process involving new physics, for instance quantum gravity which might manifest new physics at the scale of the Planck mass via spontaneous quantum collapse of the wave function. These states, he suggested, occur both within neurons and also spanning more than one neuron. However, other scientists point out that there is no plausible organic mechanism in the brain for harnessing any sort of quantum computation, and furthermore that the timescale of quantum decoherence seems too fast to influence neuron firing.
Dreyfus: the primacy of implicit skills | Philosophy of artificial intelligence | Wikipedia | 475 | 2958015 | https://en.wikipedia.org/wiki/Philosophy%20of%20artificial%20intelligence | Technology | Artificial intelligence concepts | null |
Hubert Dreyfus argued that human intelligence and expertise depended primarily on fast intuitive judgements rather than step-by-step symbolic manipulation, and argued that these skills would never be captured in formal rules.
Dreyfus's argument had been anticipated by Turing in his 1950 paper Computing machinery and intelligence, where he had classified this as the "argument from the informality of behavior." Turing argued in response that, just because we do not know the rules that govern a complex behavior, this does not mean that no such rules exist. He wrote: "we cannot so easily convince ourselves of the absence of complete laws of behaviour ... The only way we know of for finding such laws is scientific observation, and we certainly know of no circumstances under which we could say, 'We have searched enough. There are no such laws.'"
Russell and Norvig point out that, in the years since Dreyfus published his critique, progress has been made towards discovering the "rules" that govern unconscious reasoning. The situated movement in robotics research attempts to capture our unconscious skills at perception and attention. Computational intelligence paradigms, such as neural nets, evolutionary algorithms and so on are mostly directed at simulated unconscious reasoning and learning. Statistical approaches to AI can make predictions which approach the accuracy of human intuitive guesses. Research into commonsense knowledge has focused on reproducing the "background" or context of knowledge. In fact, AI research in general has moved away from high level symbol manipulation, towards new models that are intended to capture more of our intuitive reasoning.
Cognitive science and psychology eventually came to agree with Dreyfus' description of human expertise. Daniel Kahnemann and others developed a similar theory where they identified two "systems" that humans use to solve problems, which he called "System 1" (fast intuitive judgements) and "System 2" (slow deliberate step by step thinking).
Although Dreyfus' views have been vindicated in many ways, the work in cognitive science and in AI was in response to specific problems in those fields and was not directly influenced by Dreyfus. Historian and AI researcher Daniel Crevier wrote that "time has proven the accuracy and perceptiveness of some of Dreyfus's comments. Had he formulated them less aggressively, constructive actions they suggested might have been taken much earlier."
Can a machine have a mind, consciousness, and mental states? | Philosophy of artificial intelligence | Wikipedia | 485 | 2958015 | https://en.wikipedia.org/wiki/Philosophy%20of%20artificial%20intelligence | Technology | Artificial intelligence concepts | null |
This is a philosophical question, related to the problem of other minds and the hard problem of consciousness. The question revolves around a position defined by John Searle as "strong AI":
A physical symbol system can have a mind and mental states.
Searle distinguished this position from what he called "weak AI":
A physical symbol system can act intelligently.
Searle introduced the terms to isolate strong AI from weak AI so he could focus on what he thought was the more interesting and debatable issue. He argued that even if we assume that we had a computer program that acted exactly like a human mind, there would still be a difficult philosophical question that needed to be answered.
Neither of Searle's two positions are of great concern to AI research, since they do not directly answer the question "can a machine display general intelligence?" (unless it can also be shown that consciousness is necessary for intelligence). Turing wrote "I do not wish to give the impression that I think there is no mystery about consciousness… [b]ut I do not think these mysteries necessarily need to be solved before we can answer the question [of whether machines can think]." Russell and Norvig agree: "Most AI researchers take the weak AI hypothesis for granted, and don't care about the strong AI hypothesis."
There are a few researchers who believe that consciousness is an essential element in intelligence, such as Igor Aleksander, Stan Franklin, Ron Sun, and Pentti Haikonen, although their definition of "consciousness" strays very close to "intelligence". (See artificial consciousness.)
Before we can answer this question, we must be clear what we mean by "minds", "mental states" and "consciousness".
Consciousness, minds, mental states, meaning | Philosophy of artificial intelligence | Wikipedia | 361 | 2958015 | https://en.wikipedia.org/wiki/Philosophy%20of%20artificial%20intelligence | Technology | Artificial intelligence concepts | null |
The words "mind" and "consciousness" are used by different communities in different ways. Some new age thinkers, for example, use the word "consciousness" to describe something similar to Bergson's "élan vital": an invisible, energetic fluid that permeates life and especially the mind. Science fiction writers use the word to describe some essential property that makes us human: a machine or alien that is "conscious" will be presented as a fully human character, with intelligence, desires, will, insight, pride and so on. (Science fiction writers also use the words "sentience", "sapience", "self-awareness" or "ghost"—as in the Ghost in the Shell manga and anime series—to describe this essential human property). For others , the words "mind" or "consciousness" are used as a kind of secular synonym for the soul.
For philosophers, neuroscientists and cognitive scientists, the words are used in a way that is both more precise and more mundane: they refer to the familiar, everyday experience of having a "thought in your head", like a perception, a dream, an intention or a plan, and to the way we see something, know something, mean something or understand something. "It's not hard to give a commonsense definition of consciousness" observes philosopher John Searle. What is mysterious and fascinating is not so much what it is but how it is: how does a lump of fatty tissue and electricity give rise to this (familiar) experience of perceiving, meaning or thinking?
Philosophers call this the hard problem of consciousness. It is the latest version of a classic problem in the philosophy of mind called the "mind-body problem". A related problem is the problem of meaning or understanding (which philosophers call "intentionality"): what is the connection between our thoughts and what we are thinking about (i.e. objects and situations out in the world)? A third issue is the problem of experience (or "phenomenology"): If two people see the same thing, do they have the same experience? Or are there things "inside their head" (called "qualia") that can be different from person to person? | Philosophy of artificial intelligence | Wikipedia | 459 | 2958015 | https://en.wikipedia.org/wiki/Philosophy%20of%20artificial%20intelligence | Technology | Artificial intelligence concepts | null |
Neurobiologists believe all these problems will be solved as we begin to identify the neural correlates of consciousness: the actual relationship between the machinery in our heads and its collective properties; such as the mind, experience and understanding. Some of the harshest critics of artificial intelligence agree that the brain is just a machine, and that consciousness and intelligence are the result of physical processes in the brain. The difficult philosophical question is this: can a computer program, running on a digital machine that shuffles the binary digits of zero and one, duplicate the ability of the neurons to create minds, with mental states (like understanding or perceiving), and ultimately, the experience of consciousness?
Arguments that a computer cannot have a mind and mental states
Searle's Chinese room
John Searle asks us to consider a thought experiment: suppose we have written a computer program that passes the Turing test and demonstrates general intelligent action. Suppose, specifically that the program can converse in fluent Chinese. Write the program on 3x5 cards and give them to an ordinary person who does not speak Chinese. Lock the person into a room and have him follow the instructions on the cards. He will copy out Chinese characters and pass them in and out of the room through a slot. From the outside, it will appear that the Chinese room contains a fully intelligent person who speaks Chinese. The question is this: is there anyone (or anything) in the room that understands Chinese? That is, is there anything that has the mental state of understanding, or which has conscious awareness of what is being discussed in Chinese? The man is clearly not aware. The room cannot be aware. The cards certainly are not aware. Searle concludes that the Chinese room, or any other physical symbol system, cannot have a mind.
Searle goes on to argue that actual mental states and consciousness require (yet to be described) "actual physical-chemical properties of actual human brains." He argues there are special "causal properties" of brains and neurons that gives rise to minds: in his words "brains cause minds."
Related arguments: Leibniz' mill, Davis's telephone exchange, Block's Chinese nation and Blockhead | Philosophy of artificial intelligence | Wikipedia | 440 | 2958015 | https://en.wikipedia.org/wiki/Philosophy%20of%20artificial%20intelligence | Technology | Artificial intelligence concepts | null |
Gottfried Leibniz made essentially the same argument as Searle in 1714, using the thought experiment of expanding the brain until it was the size of a mill. In 1974, Lawrence Davis imagined duplicating the brain using telephone lines and offices staffed by people, and in 1978 Ned Block envisioned the entire population of China involved in such a brain simulation. This thought experiment is called "the Chinese Nation" or "the Chinese Gym". Ned Block also proposed his Blockhead argument, which is a version of the Chinese room in which the program has been re-factored into a simple set of rules of the form "see this, do that", removing all mystery from the program.
Responses to the Chinese room | Philosophy of artificial intelligence | Wikipedia | 144 | 2958015 | https://en.wikipedia.org/wiki/Philosophy%20of%20artificial%20intelligence | Technology | Artificial intelligence concepts | null |
Responses to the Chinese room emphasize several different points.
The systems reply and the virtual mind reply: This reply argues that the system, including the man, the program, the room, and the cards, is what understands Chinese. Searle claims that the man in the room is the only thing which could possibly "have a mind" or "understand", but others disagree, arguing that it is possible for there to be two minds in the same physical place, similar to the way a computer can simultaneously "be" two machines at once: one physical (like a Macintosh) and one "virtual" (like a word processor).
Speed, power and complexity replies: Several critics point out that the man in the room would probably take millions of years to respond to a simple question, and would require "filing cabinets" of astronomical proportions. This brings the clarity of Searle's intuition into doubt.
Robot reply: To truly understand, some believe the Chinese Room needs eyes and hands. Hans Moravec writes: "If we could graft a robot to a reasoning program, we wouldn't need a person to provide the meaning anymore: it would come from the physical world."
Brain simulator reply: What if the program simulates the sequence of nerve firings at the synapses of an actual brain of an actual Chinese speaker? The man in the room would be simulating an actual brain. This is a variation on the "systems reply" that appears more plausible because "the system" now clearly operates like a human brain, which strengthens the intuition that there is something besides the man in the room that could understand Chinese.
Other minds reply and the epiphenomena reply: Several people have noted that Searle's argument is just a version of the problem of other minds, applied to machines. Since it is difficult to decide if people are "actually" thinking, we should not be surprised that it is difficult to answer the same question about machines. | Philosophy of artificial intelligence | Wikipedia | 398 | 2958015 | https://en.wikipedia.org/wiki/Philosophy%20of%20artificial%20intelligence | Technology | Artificial intelligence concepts | null |
A related question is whether "consciousness" (as Searle understands it) exists. Searle argues that the experience of consciousness cannot be detected by examining the behavior of a machine, a human being or any other animal. Daniel Dennett points out that natural selection cannot preserve a feature of an animal that has no effect on the behavior of the animal, and thus consciousness (as Searle understands it) cannot be produced by natural selection. Therefore, either natural selection did not produce consciousness, or "strong AI" is correct in that consciousness can be detected by suitably designed Turing test.
Is thinking a kind of computation?
The computational theory of mind or "computationalism" claims that the relationship between mind and brain is similar (if not identical) to the relationship between a running program (software) and a computer (hardware). The idea has philosophical roots in Hobbes (who claimed reasoning was "nothing more than reckoning"), Leibniz (who attempted to create a logical calculus of all human ideas), Hume (who thought perception could be reduced to "atomic impressions") and even Kant (who analyzed all experience as controlled by formal rules). The latest version is associated with philosophers Hilary Putnam and Jerry Fodor.
This question bears on our earlier questions: if the human brain is a kind of computer then computers can be both intelligent and conscious, answering both the practical and philosophical questions of AI. In terms of the practical question of AI ("Can a machine display general intelligence?"), some versions of computationalism make the claim that (as Hobbes wrote):
Reasoning is nothing but reckoning.
In other words, our intelligence derives from a form of calculation, similar to arithmetic. This is the physical symbol system hypothesis discussed above, and it implies that artificial intelligence is possible. In terms of the philosophical question of AI ("Can a machine have mind, mental states and consciousness?"), most versions of computationalism claim that (as Stevan Harnad characterizes it):
Mental states are just implementations of (the right) computer programs.
This is John Searle's "strong AI" discussed above, and it is the real target of the Chinese room argument (according to Harnad).
Other related questions
Can a machine have emotions? | Philosophy of artificial intelligence | Wikipedia | 464 | 2958015 | https://en.wikipedia.org/wiki/Philosophy%20of%20artificial%20intelligence | Technology | Artificial intelligence concepts | null |
If "emotions" are defined only in terms of their effect on behavior or on how they function inside an organism, then emotions can be viewed as a mechanism that an intelligent agent uses to maximize the utility of its actions. Given this definition of emotion, Hans Moravec believes that "robots in general will be quite emotional about being nice people". Fear is a source of urgency. Empathy is a necessary component of good human computer interaction. He says robots "will try to please you in an apparently selfless manner because it will get a thrill out of this positive reinforcement. You can interpret this as a kind of love." Daniel Crevier writes "Moravec's point is that emotions are just devices for channeling behavior in a direction beneficial to the survival of one's species."
Can a machine be self-aware?
"Self-awareness", as noted above, is sometimes used by science fiction writers as a name for the essential human property that makes a character fully human. Turing strips away all other properties of human beings and reduces the question to "can a machine be the subject of its own thought?" Can it think about itself? Viewed in this way, a program can be written that can report on its own internal states, such as a debugger.
Can a machine be original or creative?
Turing reduces this to the question of whether a machine can "take us by surprise" and argues that this is obviously true, as any programmer can attest. He notes that, with enough storage capacity, a computer can behave in an astronomical number of different ways. It must be possible, even trivial, for a computer that can represent ideas to combine them in new ways. (Douglas Lenat's Automated Mathematician, as one example, combined ideas to discover new mathematical truths.) Kaplan and Haenlein suggest that machines can display scientific creativity, while it seems likely that humans will have the upper hand where artistic creativity is concerned.
In 2009, scientists at Aberystwyth University in Wales and the U.K's University of Cambridge designed a robot called Adam that they believe to be the first machine to independently come up with new scientific findings. Also in 2009, researchers at Cornell developed Eureqa, a computer program that extrapolates formulas to fit the data inputted, such as finding the laws of motion from a pendulum's motion.
Can a machine be benevolent or hostile? | Philosophy of artificial intelligence | Wikipedia | 490 | 2958015 | https://en.wikipedia.org/wiki/Philosophy%20of%20artificial%20intelligence | Technology | Artificial intelligence concepts | null |
This question (like many others in the philosophy of artificial intelligence) can be presented in two forms. "Hostility" can be defined in terms function or behavior, in which case "hostile" becomes synonymous with "dangerous". Or it can be defined in terms of intent: can a machine "deliberately" set out to do harm? The latter is the question "can a machine have conscious states?" (such as intentions) in another form.
The question of whether highly intelligent and completely autonomous machines would be dangerous has been examined in detail by futurists (such as the Machine Intelligence Research Institute). The obvious element of drama has also made the subject popular in science fiction, which has considered many differently possible scenarios where intelligent machines pose a threat to mankind; see Artificial intelligence in fiction.
One issue is that machines may acquire the autonomy and intelligence required to be dangerous very quickly. Vernor Vinge has suggested that over just a few years, computers will suddenly become thousands or millions of times more intelligent than humans. He calls this "the Singularity". He suggests that it may be somewhat or possibly very dangerous for humans. This is discussed by a philosophy called Singularitarianism.
In 2009, academics and technical experts attended a conference to discuss the potential impact of robots and computers and the impact of the hypothetical possibility that they could become self-sufficient and able to make their own decisions. They discussed the possibility and the extent to which computers and robots might be able to acquire any level of autonomy, and to what degree they could use such abilities to possibly pose any threat or hazard. They noted that some machines have acquired various forms of semi-autonomy, including being able to find power sources on their own and being able to independently choose targets to attack with weapons. They also noted that some computer viruses can evade elimination and have achieved "cockroach intelligence". They noted that self-awareness as depicted in science-fiction is probably unlikely, but that there were other potential hazards and pitfalls.
Some experts and academics have questioned the use of robots for military combat, especially when such robots are given some degree of autonomous functions. The US Navy has funded a report which indicates that as military robots become more complex, there should be greater attention to implications of their ability to make autonomous decisions.
The President of the Association for the Advancement of Artificial Intelligence has commissioned a study to look at this issue. They point to programs like the Language Acquisition Device which can emulate human interaction. | Philosophy of artificial intelligence | Wikipedia | 497 | 2958015 | https://en.wikipedia.org/wiki/Philosophy%20of%20artificial%20intelligence | Technology | Artificial intelligence concepts | null |
Some have suggested a need to build "Friendly AI", a term coined by Eliezer Yudkowsky, meaning that the advances which are already occurring with AI should also include an effort to make AI intrinsically friendly and humane.
Can a machine imitate all human characteristics?
Turing said "It is customary ... to offer a grain of comfort, in the form of a statement that some peculiarly human characteristic could never be imitated by a machine. ... I cannot offer any such comfort, for I believe that no such bounds can be set."
Turing noted that there are many arguments of the form "a machine will never do X", where X can be many things, such as:
Be kind, resourceful, beautiful, friendly, have initiative, have a sense of humor, tell right from wrong, make mistakes, fall in love, enjoy strawberries and cream, make someone fall in love with it, learn from experience, use words properly, be the subject of its own thought, have as much diversity of behaviour as a man, do something really new.
Turing argues that these objections are often based on naive assumptions about the versatility of machines or are "disguised forms of the argument from consciousness". Writing a program that exhibits one of these behaviors "will not make much of an impression." All of these arguments are tangential to the basic premise of AI, unless it can be shown that one of these traits is essential for general intelligence.
Can a machine have a soul?
Finally, those who believe in the existence of a soul may argue that "Thinking is a function of man's immortal soul." Alan Turing called this "the theological objection". He writes:
In attempting to construct such machines we should not be irreverently usurping His power of creating souls, any more than we are in the procreation of children: rather we are, in either case, instruments of His will providing mansions for the souls that He creates.The discussion on the topic has been reignited as a result of recent claims made by Google's LaMDA artificial intelligence system that it is sentient and had a "soul".
LaMDA (Language Model for Dialogue Applications) is an artificial intelligence system that creates chatbots—AI robots designed to communicate with humans—by gathering vast amounts of text from the internet and using algorithms to respond to queries in the most fluid and natural way possible. | Philosophy of artificial intelligence | Wikipedia | 492 | 2958015 | https://en.wikipedia.org/wiki/Philosophy%20of%20artificial%20intelligence | Technology | Artificial intelligence concepts | null |
The transcripts of conversations between scientists and LaMDA reveal that the AI system excels at this, providing answers to challenging topics about the nature of emotions, generating Aesop-style fables on the moment, and even describing its alleged fears. Pretty much all philosophers doubt LaMDA's sentience.
Views on the role of philosophy
Some scholars argue that the AI community's dismissal of philosophy is detrimental. In the Stanford Encyclopedia of Philosophy, some philosophers argue that the role of philosophy in AI is underappreciated. Physicist David Deutsch argues that without an understanding of philosophy or its concepts, AI development would suffer from a lack of progress.
Conferences and literature
The main conference series on the issue is "Philosophy and Theory of AI" (PT-AI), run by Vincent C. Müller.
The main bibliography on the subject, with several sub-sections, is on PhilPapers.
A recent survey for Philosophy of AI is Müller (2023). | Philosophy of artificial intelligence | Wikipedia | 196 | 2958015 | https://en.wikipedia.org/wiki/Philosophy%20of%20artificial%20intelligence | Technology | Artificial intelligence concepts | null |
Acacia, commonly known as wattles or acacias, is a genus of about of shrubs and trees in the subfamily Mimosoideae of the pea family Fabaceae. Initially, it comprised a group of plant species native to Africa, South America, and Australasia, but is now reserved for species mainly from Australia, with others from New Guinea, Southeast Asia, and the Indian Ocean. The genus name is Neo-Latin, borrowed from the Greek (), a term used in antiquity to describe a preparation extracted from Vachellia nilotica, the original type species.
A number of species of Acacia have been introduced to various parts of the world, and two million hectares of commercial plantations have been established.
Description
Plants in the genus Acacia are shrubs or trees with bipinnate leaves, the mature leaves sometimes reduced to phyllodes or rarely absent. There are 2 small stipules at the base of the leaf, but sometimes fall off as the leaf matures. The flowers are borne in spikes or cylindrical heads, sometimes singly, in pairs or in racemes in the axils of leaves or phyllodes, sometimes in panicles on the ends of branches. Each spike or cylindrical head has many small golden-yellow to pale creamy-white flowers, each with 4 or 5 sepals and petals, more than 10 stamens, and a thread-like style that is longer than the stamens. The fruit is a variably-shaped pod, sometimes flat or cylindrical, containing seeds with a fleshy aril on the end.
Taxonomy
The genus was first validly named in 1754 by Philip Miller in The Gardeners Dictionary. In 1913 Nathaniel Lord Britton and Addison Brown selected Mimosa scorpioides (≡ Acacia scorpioides () = Acacia nilotica () ), a species from Africa, as the lectotype of the name.
Etymology
The genus name comes from Neo-Latin; Gaspard Bauhin in his book Pinax (1623) writes it coming from Pedanius Dioscorides who uses the name akakia for species Vachellia nilotica, the original type species growing in Egypt, from akakis meaning "point". | Acacia | Wikipedia | 453 | 2959226 | https://en.wikipedia.org/wiki/Acacia | Biology and health sciences | Fabales | Plants |
The origin of "wattle" may be an Old Teutonic word meaning "to weave". From around 700 AD, was used in Old English to refer to the flexible woody vines, branches, and sticks which were interwoven to form walls, roofs, and fences. Since about 1810 it has been used as the common name for the Australian legume trees and shrubs that can provide these branches.
History
Genus Acacia was considered to contain some leading to 1986. That year, Leslie Pedley questioned the monophyletic nature of the genus, and proposed a split into three genera: Acacia sensu stricto (161 species), Senegalia (231 species) and Racosperma (960 species), the last name first proposed in 1829 by Carl Friedrich Philipp von Martius as the name of a section in Acacia, but raised to generic rank in 1835. In 2003, Pedley published a paper with 834 new combinations in Racosperma for species, most of which were formerly placed in Acacia. All but 10 of these species are native to Australasia, where it constitutes the largest plant genus.
In the early 2000s, it had become evident that the genus as it stood was not monophyletic and that several divergent lineages needed to be placed in separate genera. It turned out that one lineage comprising over 900 species mainly native to Australia, New Guinea, and Indonesia was not closely related to the much smaller group of African lineage that contained A. nilotica – the type species. This meant that the Australasian lineage (by far the most prolific in number of species) would need to be renamed. Pedley's proposed name of Racosperma for this group had received little acclaim in the botanical community. Australian botanists proposed a less disruptive solution, setting a different type species for Acacia (A. penninervis) and allowing this largest number of species to remain in Acacia, resulting in the two pan-tropical lineages being renamed Vachellia and Senegalia, and the two endemic American lineages renamed Acaciella and Mariosousa. | Acacia | Wikipedia | 426 | 2959226 | https://en.wikipedia.org/wiki/Acacia | Biology and health sciences | Fabales | Plants |
In 2003, Anthony Orchard and Bruce Maslin filed a proposal to conserve the name Acacia with a different type, in order to retain the Australasian group of species in the genus Acacia. Following a controversial decision to choose a new type for Acacia in 2005, the Australian component of Acacia s.l. now retains the name Acacia. At the 2011 International Botanical Congress held in Melbourne, Australia, the decision to use the name Acacia, rather than the proposed Racosperma for this genus, was upheld. Other Acacia s.l. taxa continue to be called Acacia by those who choose to consider the entire group as one genus.
The Australian species of the genus Paraserianthes s.l. (namely P. lophantha) are deemed its closest relatives. The nearest relatives of Acacia and Paraserianthes s.l. in turn include the Australian and South East Asian genera Archidendron, Archidendropsis, Pararchidendron and Wallaceodendron, all of the tribe Ingeae.
Species
The names of more than 1,080 species of Acacia, mostly native to Australia, have been accepted by Plants of the World Online as at January 2025.
Fossil record
An Acacia-like long fossil seed pod has been described from the Eocene of the Paris Basin. Acacia-like fossil pods under the name Leguminocarpon are known from late Oligocene deposits at different sites in Hungary. Seed pod fossils of †Acacia parschlugiana and †Acacia cyclosperma are known from Tertiary deposits in Switzerland. †Acacia colchica has been described from the Miocene of West Georgia. Pliocene fossil pollen of an Acacia sp. has been described from West Georgia (including Abkhazia). The oldest fossil Acacia pollen in Australia are recorded as being from the late Oligocene epoch, 25 million years ago.
Distribution and habitat
Species of Acacia occurs in all Australian states and territories, and on its nearby islands. About 20 species occur naturally outside Australia and also occur in Australia. One species (Acacia koa) is native to Hawaii and one (Acacia heterophylla) is native to Mauritius and Réunion in the Indian Ocean. | Acacia | Wikipedia | 446 | 2959226 | https://en.wikipedia.org/wiki/Acacia | Biology and health sciences | Fabales | Plants |
They are present in all terrestrial habitats, including alpine settings, rainforests, woodlands, grasslands, coastal dunes and deserts. In drier woodlands or forests they are an important component of the understory. Elsewhere they may be dominant, as in the Brigalow Belt, Myall woodlands and the eremaean Mulga woodlands.
In Australia, Acacia forest is the second most common forest type after eucalypt forest, covering or 8% of total forest area. Acacia is also the nation's largest genus of flowering plants with almost found.
Ecology
Acacia is a common food source and host plant for butterflies of the genus Jalmenus. The imperial hairstreak, Jalmenus evagoras, feeds on at least 25 acacia species. Many reptiles feed on the sap, such as the native house gecko in Australia. The sap is also consumed by bugs (Hemiptera), such as Hackerobrachys viridiventris and Sextius virescens.
Toxicity
Some species of acacia contain psychoactive alkaloids, and some contain potassium fluoroacetate.
Uses
The seed pods, flowers, and young leaves are generally edible either raw or cooked.
Aboriginal Australians have traditionally harvested the seeds of some species, to be ground into flour and eaten as a paste or baked into a cake. Wattleseeds contain as much as 25% more protein than common cereals, and they store well for long periods due to the hard seed coats. In addition to consuming the edible seed and gum, Aboriginal people also employed the timber for implements, weapons, fuel and musical instruments. A number of species, most notably Acacia mangium (hickory wattle), A. mearnsii (black wattle) and A. saligna (coojong), are economically important and are widely planted globally for wood products, tannin, firewood and fodder. A. melanoxylon (blackwood) and A. aneura (mulga) supply some of the most attractive timbers in the genus. Black wattle bark supported the tanning industries of several countries, and may supply tannins for production of waterproof adhesives.
In Vietnam, Acacia is used in plantations of non-native species that are regularly clear-cut for paper or timber uses.
Wattle bark collected in Australia in the 19th century was exported to Europe where it was used in the tanning process. One ton of wattle or mimosa bark contained about of pure tannin. | Acacia | Wikipedia | 511 | 2959226 | https://en.wikipedia.org/wiki/Acacia | Biology and health sciences | Fabales | Plants |
The gum of some species may be used as a substitute for gum arabic, known as Australian gum or wattle gum.
Cultivation
Some species of acacia – notably Acacia baileyana, A. dealbata and A. pravissima – are cultivated as ornamental garden plants. The 1889 publication Useful Native Plants of Australia describes various uses for eating. | Acacia | Wikipedia | 69 | 2959226 | https://en.wikipedia.org/wiki/Acacia | Biology and health sciences | Fabales | Plants |
The epidermis (from the Greek ἐπιδερμίς, meaning "over-skin") is a single layer of cells that covers the leaves, flowers, roots and stems of plants. It forms a boundary between the plant and the external environment. The epidermis serves several functions: it protects against water loss, regulates gas exchange, secretes metabolic compounds, and (especially in roots) absorbs water and mineral nutrients. The epidermis of most leaves shows dorsoventral anatomy: the upper (adaxial) and lower (abaxial) surfaces have somewhat different construction and may serve different functions. Woody stems and some other stem structures such as potato tubers produce a secondary covering called the periderm that replaces the epidermis as the protective covering.
Description
The epidermis is the outermost cell layer of the primary plant body. In some older works the cells of the leaf epidermis have been regarded as specialized parenchyma cells, but the established modern preference has long been to classify the epidermis as dermal tissue, whereas parenchyma is classified as ground tissue. The epidermis is the main component of the dermal tissue system of leaves (diagrammed below), and also stems, roots, flowers, fruits, and seeds; it is usually transparent (epidermal cells have fewer chloroplasts or lack them completely, except for the guard cells.)
The cells of the epidermis are structurally and functionally variable. Most plants have an epidermis that is a single cell layer thick. Some plants like Ficus elastica and Peperomia, which have a periclinal cellular division within the protoderm of the leaves, have an epidermis with multiple cell layers. Epidermal cells are tightly linked to each other and provide mechanical strength and protection to the plant. Particularly, wavy pavement cells are suggested to play a pivotal role in preventing or guiding cracks in the epidermis. The walls of the epidermal cells of the above-ground parts of plants contain cutin, and are covered with a cuticle. The cuticle reduces water loss to the atmosphere, it is sometimes covered with wax in smooth sheets, granules, plates, tubes, or filaments. The wax layers give some plants a whitish or bluish surface color. Surface wax acts as a moisture barrier and protects the plant from intense sunlight and wind. | Epidermis (botany) | Wikipedia | 499 | 2959657 | https://en.wikipedia.org/wiki/Epidermis%20%28botany%29 | Biology and health sciences | Plant tissues | null |
The epidermal tissue includes several differentiated cell types: epidermal cells, guard cells, subsidiary cells, and epidermal hairs (trichomes). The epidermal cells are the most numerous, largest, and least specialized. These are typically more elongated in the leaves of monocots than in those of dicots.
Trichomes or hairs grow out from the epidermis in many species. In the root epidermis, epidermal hairs termed root hairs are common and are specialized for the absorption of water and mineral nutrients.
In plants with secondary growth, the epidermis of roots and stems is usually replaced by a periderm through the action of a cork cambium.
Stoma complex
The leaf and stem epidermis is covered with pores called stomata (sing; stoma), part of a stoma complex consisting of a pore surrounded on each side by chloroplast-containing guard cells, and two to four subsidiary cells that lack chloroplasts. The stomata complex regulates the exchange of gases and water vapor between the outside air and the interior of the leaf. Typically, the stomata are more numerous over the abaxial (lower) epidermis of the leaf than the (adaxial) upper epidermis. An exception is floating leaves where most or all stomata are on the upper surface. Vertical leaves, such as those of many grasses, often have roughly equal numbers of stomata on both surfaces.
The stoma is bounded by two guard cells. The guard cells differ from the epidermal cells in the following aspects: | Epidermis (botany) | Wikipedia | 336 | 2959657 | https://en.wikipedia.org/wiki/Epidermis%20%28botany%29 | Biology and health sciences | Plant tissues | null |
The guard cells are bean-shaped in surface view, while the epidermal cells are irregular in shape
The guard cells contain chloroplasts, so they can manufacture food by photosynthesis (The epidermal cells of terrestrial plants do not contain chloroplasts)
Guard cells are the only epidermal cells that can make sugar. According to one theory, in sunlight, the concentration of potassium ions (K+) increases in the guard cells. This, together with the sugars formed, lowers the water potential in the guard cells. As a result, water from other cells enters the guard cells by osmosis so they swell and become turgid. Because the guard cells have a thicker cellulose wall on one side of the cell, i.e. the side around the stomatal pore, the swollen guard cells become curved and pull the stomata open.
At night, the sugar is used up and water leaves the guard cells, so they become flaccid and the stomatal pore closes. In this way, they reduce the amount of water vapor escaping from the leaf.
Cell differentiation in the epidermis
The plant epidermis consists of three main cell types: pavement cells, guard cells and their subsidiary cells that surround the stomata and trichomes, otherwise known as leaf hairs. The epidermis of petals also form a variation of trichomes called conical cells.
Trichomes develop at a distinct phase during leaf development, under the control of two major trichome specification genes: TTG and GL1. The process may be controlled by the plant hormones gibberellins, and even if not completely controlled, gibberellins certainly have an effect on the development of the leaf hairs. GL1 causes endoreplication, the replication of DNA without subsequent cell division as well as cell expansion. GL1 turns on the expression of a second gene for trichome formation, GL2, which controls the final stages of trichome formation causing the cellular outgrowth.
Arabidopsis thaliana uses the products of inhibitory genes to control the patterning of trichomes, such as TTG and TRY. The products of these genes will diffuse into the lateral cells, preventing them from forming trichomes and in the case of TRY promoting the formation of pavement cells. | Epidermis (botany) | Wikipedia | 483 | 2959657 | https://en.wikipedia.org/wiki/Epidermis%20%28botany%29 | Biology and health sciences | Plant tissues | null |
Expression of the gene MIXTA, or its analogue in other species, later in the process of cellular differentiation will cause the formation of conical cells over trichomes. MIXTA is a transcription factor.
Stomatal patterning is a much more controlled process, as the stoma affects the plant's water retention and respiration capabilities. As a consequence of these important functions, differentiation of cells to form stomata is also subject to environmental conditions to a much greater degree than other epidermal cell types.
Stomata are pores in the plant epidermis that are surrounded by two guard cells, which control the opening and closing of the aperture. These guard cells are in turn surrounded by subsidiary cells which provide a supporting role for the guard cells.
Stomata begin as stomatal meristemoids. The process differs between dicots and monocots. Spacing is thought to be essentially random in dicots though mutants do show it is under some form of genetic control, but it is more controlled in monocots, where stomata arise from specific asymmetric divisions of protoderm cells. The smaller of the two cells produced becomes the guard mother cells. Adjacent epidermal cells will also divide asymmetrically to form the subsidiary cells.
Because stomata play such an important role in the plants' survival, collecting information on their differentiation is difficult by the traditional means of genetic manipulation, as stomatal mutants tend to be unable to survive. Thus the control of the process is not well understood. Some genes have been identified. TMM is thought to control the timing of stomatal initiation specification and FLP is thought to be involved in preventing the further division of the guard cells once they are formed.
Environmental conditions affect the development of stomata, in particular, their density on the leaf surface. It is thought that plant hormones, such as ethylene and cytokines, control the stomatal developmental response to the environmental conditions. Accumulation of these hormones appears to cause increased stomatal density such as when the plants are kept in closed environments. | Epidermis (botany) | Wikipedia | 428 | 2959657 | https://en.wikipedia.org/wiki/Epidermis%20%28botany%29 | Biology and health sciences | Plant tissues | null |
Liquid oxygen, sometimes abbreviated as LOX or LOXygen, is a clear cyan liquid form of dioxygen . It was used as the oxidizer in the first liquid-fueled rocket invented in 1926 by Robert H. Goddard, an application which is ongoing.
Physical properties
Liquid oxygen has a clear cyan color and is strongly paramagnetic: it can be suspended between the poles of a powerful horseshoe magnet. Liquid oxygen has a density of , slightly denser than liquid water, and is cryogenic with a freezing point of and a boiling point of at . Liquid oxygen has an expansion ratio of 1:861 and because of this, it is used in some commercial and military aircraft as a transportable source of breathing oxygen.
Because of its cryogenic nature, liquid oxygen can cause the materials it touches to become extremely brittle. Liquid oxygen is also a very powerful oxidizing agent: organic materials will burn rapidly and energetically in liquid oxygen. Further, if soaked in liquid oxygen, some materials such as coal briquettes, carbon black, etc., can detonate unpredictably from sources of ignition such as flames, sparks or impact from light blows. Petrochemicals, including asphalt, often exhibit this behavior.
The tetraoxygen molecule (O4) was predicted in 1924 by Gilbert N. Lewis, who proposed it to explain why liquid oxygen defied Curie's law. Modern computer simulations indicate that, although there are no stable O4 molecules in liquid oxygen, O2 molecules do tend to associate in pairs with antiparallel spins, forming transient O4 units.
Liquid nitrogen has a lower boiling point at −196 °C (77 K) than oxygen's −183 °C (90 K), and vessels containing liquid nitrogen can condense oxygen from air: when most of the nitrogen has evaporated from such a vessel, there is a risk that liquid oxygen remaining can react violently with organic material. Conversely, liquid nitrogen or liquid air can be oxygen-enriched by letting it stand in open air; atmospheric oxygen dissolves in it, while nitrogen evaporates preferentially.
The surface tension of liquid oxygen at its normal pressure boiling point is .
Uses
In commerce, liquid oxygen is classified as an industrial gas and is widely used for industrial and medical purposes. Liquid oxygen is obtained from the oxygen found naturally in air by fractional distillation in a cryogenic air separation plant. | Liquid oxygen | Wikipedia | 497 | 314402 | https://en.wikipedia.org/wiki/Liquid%20oxygen | Physical sciences | Group 16 | Chemistry |
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