Evolution in Action: Past and Present Learning Outcomes: By the end of this section, you will be able to: Identify the major contributors to evolutionary theory and their specific theoretical contributions and historical context. Explain the theory of evolution and how it applies to the understanding of human origins. Identify the key differences between Linnaean classification and phylogenetics. Define key evolutionary processes such as genetic drift, allopatric speciation, etc. Contemporary biological anthropologists utilize an evolutionary perspective . This means that the principles of evolution are used to understand how and why living organisms, including people, thrive in almost every environment on Earth. More specifically, natural selection is accepted as the guiding force that shapes why living things are the way they are. Out of all the possible variations of beings competing for the same resources on Earth, those that prospered were the ones better suited to their environments than all other competitors. The principles of evolution and natural selection will be discussed in some detail in the next few sections, but it is important to establish at this early point that this chapter relies on the foundational assumption that natural forces are the only forces directing the development of life on Earth. Early Evolutionists and the Fixity of Species Evolution is defined as change in the allele frequency within a gene pool that can lead to changes in an organism’s morphology (form and structure) over time. Evolution involves the processes of mutation , natural selection , and speciation , which will be introduced in upcoming sections. Prior to the 19th century, the prevailing idea in Western thought was that nature was fixed and static; it was made by a supreme being in the form it currently appeared, and it did not change. Within this fixed natural system, living creatures were arranged within a set order that was considered to have been decreed by God, known as the great chain of being . This order featured God at the top, angels beneath God, and then humans. Below humans were various types of animals, followed by plants and minerals. This hierarchy was significant both because it placed some creatures above others and because it distinctly separated humans from the rest of the animal world. During a period stretching from the 14th through the 18th centuries, some people began to question whether the natural world was as static as it was traditionally perceived to be. The British scientist and architect Robert Hooke is remembered as the first person in the Western world to claim not only that nature has changed over time but also that evidence of these changes remain. He hypothesized that fossils are the remains of actual plants and animals that were once alive. This conclusion was contrary to the previously accepted conclusion that fossils were nothing more than stone images. Hooke also noted that many marine fossils were located far away from any existing ocean, and he came to the then radical conclusion that Earth’s geography and physical features had experienced dramatic changes. The first person to propose a mechanism by which species could change was French naturalist Jean-Baptiste Lamarck , best known for having developed the first theory of macroevolution , a hypothesis about how the actual transformation from one species into another species could occur. Lamarck’s theory relied on the now defunct idea of the inheritance of acquired characteristics . Lamarck argued that the usefulness of a trait or organ could be ascertained based on its complexity or size. In particular, he believed that the usefulness of an organ could be judged by its size and the usefulness of a trait by its complexity. He speculated that organs and traits that help a creature to survive will become bigger and more complex over time, while those that are of little use will become smaller and simpler and eventually disappear. His classic example of this theory in action is the long neck of a giraffe. Lamarck speculated that as giraffes stretched their necks to reach the leaves at the tops of trees, their necks would grow longer, and furthermore, these longer necks would be inherited by the subsequent generations. This theory of the inheritance of acquired characteristics is also known as Lamarckian inheritance . One of the interesting things about Lamarck’s theory is that he believed that wishes, desires, wills, and needs were all sufficient to motivate change. That is, wishing for or desiring a change in one’s physical characteristics could make that change happen. There are two primary problems with Lamarckian inheritance. First, desires, wishes, and needs do not change physical characteristics without a deliberate change in behavior. Someone may wish for blue hair, but their hair color will not change without dye. The second problem is that the inheritance of acquired traits is not possible. If someone dyed their hair blue, their children would not inherit blue hair. Traits that are acquired during a lifespan are not passed on to subsequent generations. Just because Lamarck’s theory of macroevolution is not correct does not mean that it is insignificant. Lamarck recognized the importance of interactions between organisms and their environments in the evolutionary process and was the first to propose a mechanism by which evolutionary change from one species into another could actually occur. Georges Cuvier , another Frenchman and a leading scientist in the early 19th century, made numerous contributions to evolutionary thinking. He is best known for his theory of catastrophism , which he developed to explain the increasing number of fossils that were being found, some displaying impressions of creatures no longer found anywhere on Earth. Catastrophism proposes that floods, earthquakes, and other natural disasters—understood within the theory as acts of God—have been responsible for killing all the animals alive in certain places at certain times. According to Cuvier, either new animals have been created or the areas had been repopulated by animals from neighboring areas. To be consistent with emerging fossil evidence indicating that organisms had become more complex over time, Cuvier proposed that new organisms with a more modern appearance were the result of a more recent creation event. While scientists no longer adhere to catastrophism as a viable theory, Cuvier’s idea of extinction continues to be an important component of evolutionary thinking today. Another major contributor to evolutionary thinking was Scottish geologist Charles Lyell , known as the father of modern geology. He wrote a three-volume treatise, Principles of Geology (1830–1833), in which he argued that contemporary geological processes were the same as those that occurred in the past. These processes, such as wind and rain, produced the contemporary geological landscape. Mountains, lakes, and rivers were all created by these geological processes, many of them slow moving. This theory has come to be known as the principle of uniformitarianism . Lyell suggested that in order for such slow-acting forces to produce momentous change, Earth must be much older than previously suspected. Prior to Lyell’s publication, the majority of natural historians believed that the earth was less than 6,000 years old, a number arrived at through calculations made based on the Old Testament. By altering the suspected age of the earth from several thousand years to millions of years, Lyell changed the framework within which scientists viewed the geological past. Charles Darwin’s Role in Changing Views of the Natural World Charles Darwin introduced a new way of seeing the world that was both highly criticized and acclaimed in the scientific community of his time. In spite of resistance by various segments of society, his theories of natural selection became the foundation of biological science. New knowledge pertaining to genetics and molecular science has strengthened Darwin’s theories rather than weakened them. Darwin the Apprentice When he was 17 years old, well before he gained a reputation as a naturalist, scholar, and scientist, Darwin was studying to be a medical doctor at the University of Edinburgh. Like many young people, he began to question his original choice of studies, and he decided to instead learn taxidermy under John Edmonstone . John Edmonstone was born enslaved and grew up on a plantation owned by a Scottish politician in what is now Guyana in South America. Charles Waterton, the son-in-law of the plantation owner and a renowned naturalist, would visit the plantation often. He started inviting Edmonstone to accompany him on his frequent travels into the rainforest. On his travels, Edmonstone gained considerable knowledge about the flora and fauna of South America along with impressive taxidermy skills. After gaining his freedom in 1817, John Edmonstone taught taxidermy at the University of Edinburgh, where he served as a mentor to Darwin over a period of several months. It is believed that Darwin’s relationship with Edmonstone may have influenced his abolitionist views, which were later strengthened by firsthand accounts of slavery while Darwin was on his infamous voyage to the Galápagos Islands off the coast of Ecuador. At the University of Edenborough, John Edmonstone taught Darwin how to preserve birds. This is an example of Embernagra platensis , the great Pampa-finch, collected by Charles Darwin in Uruguay in May of 1833. (credit: “ Embernagra platensis platensis , Great Pampa-finch, skin. Syntype. [B 19600]” by Michelle McFarlane/Museums Victoria, CC BY 4.0) Darwin the Explorer and Scholar Charles Darwin left the University of Edinburgh and decided to pursue theology at Christ’s College, Cambridge. His studies there led to his appointment in 1831 as a naturalist on the HMS Beagle for a five-year scientific expedition around the world. During this voyage, Darwin collected, dissected, and organized various specimens, especially in the Galápagos Islands , a chain of islands off the western coast of South America. His observations in the Galápagos marked a crucial point in his thinking on evolution. He noted that the fauna and flora of the western coast of South America were similar to those he observed in the Galápagos, but still distinct enough to be considered different species. More surprisingly, the animals of each of the various islands in the Galápagos chain differed slightly from one another. Darwin observed 13 different types of finches throughout 13 different small islands. The birds on each island differed in the structure of their beaks, their body form, and the color of their feathers. Each species was specifically adapted to the specific habitats on each of the islands. Darwin used the techniques that Edmonstone taught him to preserve the Galápagos finches, which became key pieces of evidence supporting Darwin’s theory of natural selection. Charles Darwin is acknowledged as the father of the theory of natural selection. His work built upon the ideas of many other thinkers. His great contribution was in synthesizing these ideas into a coherent theory explaining the diversity of life on earth and the great changes in life over geological time. (credit: A. C. Seward, Cambridge Philosophical Society, Cambridge University Press/Wikimedia Commons, Public Domain) During his travels on the Beagle , Darwin had been thinking about artificial selection —the selective breeding of animals to produce traits that humans find useful, commonly associated with the process of domestication. Darwin understood that artificial selection provided important clues about the natural evolution of species. While on board the HMS Beagle , Darwin read a book by English economist Thomas Robert Malthus titled An Essay on the Principle of Population (1798). Darwin obtained two important points from this book. The first was that human populations, if unrestrained, will grow exponentially. This means that they will double each generation. The second point was that food resources increase much more slowly than population does. Malthus noted that the growth of human populations is kept in check by a limit of food resources, which creates a struggle for existence. The struggle for existence is not just about getting enough food but also about survival. In other words, it is about an individual’s ability to both find enough food and not become another organism’s food. This simple concept, the struggle for existence, provided Darwin with a mechanism for how evolution could occur. Darwin realized that individuals with favorable characteristics for living in an environment are the ones that will survive to the age at which they reproduce, while those with less favorable variations will not. This mechanism for “selecting for” certain traits and features is known as the theory of natural selection . Darwin concluded from his observations that when a group of animals of the same species are geologically separated, they develop into separate species. This evolutionary process is commonly referred to as allopatric speciation (or geographic speciation) and is based on the principles that related species share a common ancestor and that species change over time. Darwin did not originate the idea of evolution. Many of the ideas used by Darwin in his theory of natural selection were developed by other thinkers. Darwin was also not the only person thinking about natural selection. Another British natural historian, Alfred Russel Wallace , developed the same idea at roughly the same time, entirely independently of Darwin. Whereas Darwin developed his ideas based on his travels to the Galápagos, Wallace’s thinking was influenced by his own travels through the Malay Archipelago between Indochina and Australia. Wallace outlined his theory of evolution by natural selection in a letter written to Darwin while he was in Malaysia. As Darwin had not yet published his own work, Wallace and Darwin jointly presented papers introducing the theory of natural selection. In 1859, Darwin finally published his book On the Origin of Species , some 20 years after his voyage on the HMS Beagle . Understanding Darwin’s Theory of Natural Selection The theory of natural selection has five main components: All organisms are capable of producing offspring faster than the food supply increases. All organisms show variation. There is a fierce struggle for existence, and those with the most suitable variations are most likely to survive and reproduce. Variations, or traits, are passed on to offspring (inherited). Small changes in every generation lead to major changes over long periods of time. A popular but often-misunderstood concept related to natural selection is the term survival of the fittest . Survival of the fittest does not necessarily mean that the biggest and fastest survive; instead, it refers to those who are most evolutionarily fit. This means that an organism has traits that are sufficient for survival and will be passed on to future generations. The term survival of the fittest was not even introduced by Darwin; rather, it was first used by English philosopher, anthropologist, and sociologist Herbert Spencer , who promoted the now discredited ideology of social Darwinism . Social Darwinism applied the concept of Darwin’s biological evolution to human societies, proposing that human culture was progressing toward the “perfect human.” Spencer’s writings became integrally related to the 19th-century rise of scientific racism and European colonialism. This peppered moth is well camouflaged on the trunk of this tree. A darker colored moth would more easily be seen and eaten and would thus be less likely to pass on its genes to offspring. Natural selection relies upon the ability of natural variations to increase an individual’s chances of reproduction. (credit: Ben Sale/Wikimedia Commons, CC BY 2.0) Examples of Darwin’s theory of natural selection can be found throughout the natural world. Perhaps one of the best known is the color change observed in peppered moths in England during the 19th century. Before the Industrial Revolution, peppered moths in England were a light grey color, well camouflaged on tree branches and less likely to be eaten by birds. Occasionally, through the process of mutation, black moths would appear in the population, but these were usually quickly eaten because they were more visible against light-colored bark. When soot from coal factories began to cover the bark of the trees, the black moths became better camouflaged and the white moths were now more visible. Consequently, the black moths were the ones to survive to reproduce, while the white ones were eaten. In a few decades, all the peppered moths in the cities were black. The process was termed industrial melanism . As coal usage decreased and the bark of the trees once again became lighter in color, white moths again dominated the urban areas. Examples of natural selection in modern times are numerous. Pesticide resistance in insects is a classic example. Pesticide resistance refers to the decreasing susceptibility of a pest population to a pesticide that previously was effective at controlling it. Pest species evolve pesticide resistance via natural selection, with the most resistant individuals surviving to pass on their ability to resist the pesticide to their offspring. Another good example is the rise of “superbugs,” bacteria that have become increasingly resistant to antibiotics. The Processes of Evolution Mutation is the creative force of evolution and represents the first stage of the evolutionary process. Mutation is defined as an alteration in a genetic sequence that results in a variant form. For a mutation to have evolutionary significance, it must occur in the sex cells (sperm and ova). This is because only genetic information that is in the sex cells is passed on from generation to generation. Mutations in non-sex chromosomes will not be passed on from one generation to the next. Whereas other evolutionary forces can modify existing genetic material, only mutation can produce new genetic material. One of the most interesting things about mutations is the fact that they are random. There is no way of predicting when a specific mutation will occur; all scientists can do is estimate the probability of a mutation occurring. Mutations do not necessarily appear when they are needed. The conventional view is that mutations are harmful, but this is not always true. Some mutations are harmful, some are advantageous, and some are neutral. Advantageous mutations lead to changes that improve an individual’s survival and/or chances of reproduction. The mutation that confers resistance to insecticide in mosquitos led to changes that improved their survival. Likewise, the mutation for black coloration in peppered moths led to increased survival during the Industrial Revolution. Neutral mutations have no effect on survival or reproduction. And some mutations are in fact quite harmful and do negatively affect certain individuals’ survival and reproduction. Mutations generally occur spontaneously in response to conditions in the body or in the environment. The exact cause of a mutation cannot usually be determined, and the rate of mutation is very difficult to determine. This is because mutations that are neutral or do not lead to obvious changes often go unnoticed. The probability of a mutation at any given gene is between 1 in 10,000 and 1 in 100,000. While the probability that a specific point in an individual’s genetic material will have a mutation is clearly very low, the probability that the totality of an individual’s genetic material will have at least one mutation is much higher. The point is that while rare, mutation is also common. For example, although many mosquitoes have adapted to insecticides through a mutation that confers some resistance to the chemicals, if the mutation had not already been present in the population, the mosquitoes would have died out. The need for a specific mutation had no effect on whether the mutation appeared or not. There is currently a controversial pilot program in Florida aimed at dealing with mosquitoes against which insecticide sprays have increasingly become ineffective. The first genetically modified mosquitoes were released in the Florida Keys in May of 2021. The genetically altered mosquitoes produce female offspring that die in the larval stage, preventing them from growing to adulthood, in which they can then bite and spread disease. Genetic science currently has the power to use mutations to control or even wipe out an entire species. Genetic engineering has the potential to benefit humanity, but it will undoubtedly also raise ethical questions and controversy. Genetically modified mosquitoes are currently being bred that will die in the larval stage, thus greatly reducing the mosquito population. (attribution: Rice University, OpenStax, under CC BY 4.0 license) Genetic Drift Genetic drift is defined as the effect of random chance on a population, notably the way in which it determines whether an individual survives and reproduces or dies. Imagine that you stick your hand into a bucket filled with Halloween candy. What is the probability you will withdraw a Snickers bar? The composition of Halloween candy in your bucket will be affected by the proportion of people handing out Snickers bars compared to other candy. If each bucket of Halloween candy were a population, then one could say that genetic drift—random chance—was affecting the composition of the candy in your Halloween bucket. An important point about genetic drift is that it is directly and inversely related to population size. The smaller the population, the larger the influence of genetic drift; the larger the population, the smaller the influence of genetic drift. In a large population, say 100,000, removing a couple of individuals will have a truly miniscule effect on the population. Note that in early human evolution, however, population sizes were small, so the effect of genetic drift may have been substantial. Gene Flow Gene flow is another important evolutionary force, involving the exchange of genetic material between populations and geographic regions. Without gene flow, there would be no diversity—and without diversity, a species is at higher risk of extinction. Gene flow can be seen in the process of pollination, in which bees or butterflies carry and transfer pollen from one area to another. Anytime a gene is introduced to a new population where it did not exist before, that is gene flow. The process of pollination is a good example of gene flow. In this case, bees and butterflies transfer genetic material, in the form of pollen, from one flower to another. (credit: “Honey Bee on a Dandelion, Sandy, Bedfordshire” by Orangeaurochs/flickr, CC BY 2.0) Speciation Speciation is the rise of a new species in response to an environmental change or pressure. Allopatric speciation, mentioned previously, is the most common form of speciation event. During allopatric speciation, a species diverges when two populations become isolated from one another and continue to evolve. This isolation is created by geographic barriers such as mountains, rivers, or oceans. A good example of allopatric speciation is the different species of squirrel found on the two sides of the Grand Canyon. Descended from a common ancestor, the squirrels became reproductively isolated from one another by the Grand Canyon, eventually resulting in different species. An example of allopatric speciation is the different species of squirrels that inhabit the Grand Canyon. The squirrel on the left is a Harris antelope squirrel and the one on the right is a white-tailed antelope squirrel. They look similar but are different species. (credit: left, “Harris Antelope Squirrel” by Saguaro National Park/flickr, CC BY 2.0; right, “White-Tailed Antelope Squirrel” by Renee Grayson/flickr, CC BY 2.0) Sympatric speciation involves species that are descended from a common ancestor and remain in one location without a geographic barrier. A good example is the East African cichlid fish, which experience reproduction isolation due not to a physical barrier but to females’ selection of mates with certain coloration. The amount of light that reaches different levels and depths of the lake impacts how colors in the males appear to the females. The East African cichlid fish are also a good example of adaptive radiation . Adaptive radiation is seen when one or more species give rise to many new species in a relatively short time. Research shows that an explosion of about 250 very diverse species of cichlids in Lake Tanganyika occurred in less than 10 million years (Takahashi and Koblmüller 2011). Other research suggests that the common ancestor was the result of a hybrid swarm from two different locations, as seen in . (Meier et al. 2017). There are more than 250 different species of East African cichlid fish, all traceable to two common ancestors. The process through which a great number of species arises from a common ancestor within a relatively short period of time is known as adaptive radiation. (credit: “1471-2148-5-17-3” by Phylogeny Figures/flickr, CC BY 2.0) In peripatric speciation , members of the same population are separated and over time evolve as separate species. Ring speciation is considered by some to be a type of peripatric speciation. Ring speciation occurs when several species coexist for a time in a region near one end of a geographic barrier. When part of the population migrates away from the original population (or gene pool) to the other side of the barrier, reproductive isolation results. Reproductive isolation is strongest for that part of the population that is farthest away from the original population. When too much variation has occurred between two groups, they will no longer interbreed, and as a result, speciation—the development of two separate species—can occur. While fairly rare, ring speciation is believed to explain the different species of the California salamander genus Ensatina . This map shows the range of different species of the California salamander genus Ensatina , believed to have developed through the process of ring speciation. In ring speciation, reproductive isolation leads to the development of new species from a common ancestor, due to separation caused by distance and/or a physical barrier. (credit: Thomas J. Devitt, Stuart J. E. Baird, and Craig Moritz/Wikimedia Commons, CC BY 2.0) Gradualism vs. Punctuated Evolution Biological anthropologists are interested not only in how a species is best defined but also in how often and by what means new species are developed. The traditional view of evolution assumes that morphological, behavioral, and genetic changes occur gradually and accumulate in a single unbroken and unbranching line; this view of evolution is known as gradualism . If this perspective is correct, scientists would expect to find numerous fossils exhibiting evidence that they are slowly and gradually transitioning into new and distinct species. However, while fossils are rare, fossils showing evidence of transitional forms are even rarer. While the dearth of transitional fossils is often attributed to the incompleteness of the fossil record, it has caused some biological anthropologists to question if evolution is truly gradual. What can be observed in the fossil record are static populations that are interrupted by sudden bursts of change. This phenomenon of long periods of stasis, or no change, followed by quick periods of change is known as punctuated equilibrium . Instead of a gradual accumulation of small changes, punctuated equilibrium suggests that rapid changes due to a variety of environmental factors, including climate change, are characteristic of the formation of new species. The fossil data for a large number of organisms show just this—long periods of stasis followed by rapid and massive change. The scarcity of intermediary forms in the fossil record has led some to conclude that punctuated equilibrium is the dominant theory. However, the fact that intermediary forms do exist suggests that gradualism is also an important factor in the evolution process. One research study found that 30 to 35 percent of speciation events occurred as the result of a sudden event or change, while the remainder showed evidence of gradualism (Phillips 2006). In both the gradual and punctuated models, speciation takes the form of branches through time rather than a linear progression. Evolution is neither linear nor progressive, but rather a branching process—a tree of life containing both areas of divergence and points of a shared common ancestry. The Tree of Life: Showing Evolutionary Relationships This sketch made by Charles Darwin illustrates his attempts to think through the branches of evolutionary relationships. (credit: Charles Darwin/Wikimedia Commons, Public Domain) During Darwin’s time, evolutionary relationships had to be determined largely by structural morphologies and physical characteristics. Molecular science had not yet been developed. The binomial nomenclature discussed earlier not only allowed distinction between species but also provided clues to evolutionary relationships. For example, which of the below species of butterfly would be the most distantly related? Danaus gilippus Danaus genutia Limenitis archippus Danaus plexippus Danaus petilia The answer, of course, would be Limenitis archippus , the viceroy butterfly, which is a mimic of the monarch butterfly ( Danaus plexippus ). The first part of the viceroy’s name, Limenitis , is the genus. The fact that it is different from the others shows that it is more distantly related. Species can sometimes be difficult to identify by physical characteristics alone. The two butterflies in this image are examples of two different species, one a monarch and the other a viceroy butterfly. What differences can you see? (credit: left, “ Today’s Mass Extinction and Holocene-Anthropocene Thermal Maximum” by khteWisconsin/flickr, Public Domain; right, “A Viceroy Butterfly” by Benny Mazur/flickr, CC BY 2.0) It is important to note that the Linnaean classification system has limits. Sometimes, species can be difficult to identify by physical characteristics alone. Species that exhibit mimicry and larval forms in different stages of development can take on the appearance of other organisms, resulting in errors in classification. Can you tell which of the butterflies in is the monarch? Close examination reveals that the markings on the wings are a bit different. The monarch is on the left, and the monarch mimic, the viceroy, is on the right. Likewise, in , you can see how it might be difficult to correctly classify barnacles, crabs, and limpets based on physical appearances. One may be tempted to classify the barnacle and the limpet as being closely related due to the conical shells that they share, when in actuality, the barnacle is more closely related to the crab, as they are both crustaceans. The conical shells of the barnacle and the limpet are similar adaptations in response to similar environmental pressures, not evidence that they are closely related or share a common ancestor. Classifying species based on physical similarities alone can lead to false conclusions. Although barnacles and limpets look much more like one another than they do the crab on the left, barnacles are actually more closely related to the crab. (credit: left, “DSC_5206” by Sally Wyatt/flickr, CC BY 2.0; top right, “Barnacles” by Mo Riza/flickr, CC BY 2.0; bottom right, “Limpet Family at Sunny Cove” by Tim Green/flickr, CC BY 2.0) Structural Morphologies as Evidence of Relationship Structural similarities may be derived traits ( homologous structures ), inherited from a common ancestor, or they may have developed independently ( analogous structures ). An example of a homologous structure is the grasping hand found in both humans and chimpanzees, which suggests that humans and chimpanzees share a common ancestor that also had a grasping hand. Analogous structures are seen in the wing of a butterfly and the wing of a bat. While both wings serve a similar function, these two organisms likely developed their wings independently and do not necessarily share a common ancestor. Identifying homologies is essential for creating hierarchies of phylogenetic relationships because homology indicates that shared features are due to common descent. However, homologies can be difficult to identify in nature, and they are easy to confuse with analogous traits. The structural similarities visible in these various species are homologous, meaning that the similarities are the result of these animals sharing a common ancestor. (attribution: Rice University, OpenStax, under CC BY 4.0 license) Cladistics , or the use of cladograms , is a method of visually distinguishing between homologous ancestral and derived characteristics. Ancestral characteristics are found in the common ancestor of the species being classified, whereas derived characteristics are only found in the groups in question. An ancestral characteristic that humans share with common ancestors is opposable thumbs. In contrast, a derived trait that is only found in modern humans is the chin. By exclusively looking at derived characteristics, biological anthropologists can develop a clearer understanding of the relationships between the groups being studied. The Molecular Tree of Life and Phylogenetics The emergence of genetic and molecular science has provided additional tools and lines of evidence to verify evolutionary relationships. The phylogenetic tree is a model used by modern taxonomists to reveal the complexity and diversity of life and its many branches. Phylogenetic trees show how species and other taxon groups evolved from a series of common ancestors. They are based on both physical and genetic evidence. Phylogenetic trees illustrate how old species are believed to be and their degree of relatedness to one another. This particular tree pertains to primate species. (credit: Kosigrim/Wikimedia Commons, Public Domain) allopatric speciation speciation that occurs when two populations of the same species become isolated from each other due to a change in the environment, such as geographic isolation. analogous structures anatomical similarities between two species that suggest not a common ancestor but rather similar environmental adaptations. ancestral characteristics homologous structures or traits that may also be found in the common ancestor of the species being classified. artificial selection the process of deliberately breeding certain specimens of plants or animals to encourage desired traits. catastrophism the theory that changes in Earth’s fauna and flora were caused by supernatural catastrophic forces rather than evolution. cladistics the classification of organisms based on branchings of descendent lineages from a common ancestor derived characteristics physical traits that are present in related organisms but absent from their last common ancestor. They are often associated with a speciation event. evolution changes that appear in a species over time. Evolution is dependent on genetic variation and natural selection to pass on beneficial traits that will increase survival of the species. gene flow alteration of the frequencies of alleles in a population that results from interbreeding with organisms from another population. genetic drift random changes in the frequencies of alleles in a gene pool. gradualism the idea that species evolve slowly and continuously over long periods of time. great chain of being a concept detailing a hierarchical structure of all matter and life. homologous structures similar anatomical structures that appear in different species and suggest a common ancestor. industrial melanism the prevalence of dark-colored varieties of animals (for example, peppered moths) in industrial areas where they are better camouflaged against predators than paler forms. inheritance of acquired characteristics the disproved idea that an organism can pass on to its offspring physical characteristics that it has acquired during its lifetime. mutation a change in the structure of a gene that results in a variant form that may be transmitted to subsequent generations. natural selection the process by which a species that is able to adapt and to pass on beneficial traits to its offspring ensures survival of the species; first formally introduced by Charles Darwin. punctuated equilibrium a hypothesis holding that the evolution of species proceeds in a characteristic pattern of relative stability for long periods of time interspersed with much shorter periods during which many species become extinct and new species emerge. survival of the fittest the theory that the most evolutionarily fit members of a species will pass on their traits to later generations. sympatric speciation speciation without a geographic barrier. uniformitarianism the concept that Earth’s surface was shaped in the past by slow-moving geological processes.

Social Sciences


What Is a Primate? Learning Outcomes: By the end of this section, you will be able to: Define primate. Describe the relationship between primate behavior and environment. Identify and classify the key taxonomic groups of primates. What Is a Primate? Orangutans, the only great ape from Asia, are one of many living primate species. Others include lemurs, monkeys, gibbons, and human beings. (credit: Dawn Armfield/Wikimedia Commons, Public Domain) Primates —including human beings—are characterized by a number of distinct physical features that distinguish them from other mammals. These include opposable thumbs and (in nonhuman primates) opposable big toes; the presence of five digits (fingers or toes) on the appendages; flat nails instead of curved claws; pads at the tips of the fingers made up of deposits of fat and nerves; reduced reliance on sense of smell and a relatively small snout; depth perception; binocular vision (being able to see one image with both eyes); a relatively slow reproductive rate; relatively large brain size; and postorbital bars (bony rings that completely surround the eyes). The hands of this bonobo, including its opposable thumbs, look very similar to human hands. Opposable thumbs or toes are a primate trait shared by no other group of mammals. (credit: “Bonobo Plankendaal” by Marie van Dieren/flickr, CC BY 2.0) The first four traits enhance dexterity and enable primates to use their hands and feet differently from other mammals. Other traits on this list represent a shift in emphasis among the sense organs between primates and other mammals. Primates are characterized by a greater emphasis on vision and a reduced reliance on smell relative to other mammals. Primate Behavioral Variation Anthropologists regularly ask, “What makes us human?” Comparative studies of humans with nonhuman primates help answer this question. Comparing the behavior of nonhuman primates and the behavior of human beings helps anthropologists identify what culture is and develop operational definitions for it. Without the comparative perspective provided by primatology, anthropologists would be missing an important piece of the puzzle of what makes humans human. Without primatology, anthropologists would not be able to fully understand humankind. Studying nonhuman primates in their environment is key to understanding variations in behavior and can shed light on humanity’s ancient past. Primatologists are studying the chimpanzees at Gombe National Park in Tanzania, where they live in the rainforest. The behavior of chimpanzees that live in the tropical regions of Africa is quite different from the behavior of chimpanzees that live in the savanna at Fongoli in Senegal, in West Africa. Gombe chimps hunt red colobus monkeys without the use of tools, just catching them with their hands, while the Fongoli chimpanzees hunt galagos (also known as bush babies) using sticks that they adapt and used as spears (Pruetz, J.D, et al, 2015). The two environments also show differences in gender roles with both males and females in the Fongoli savannah group involved in hunting while only male chimpanzees hunt in the rainforests. Studying how these nonhuman primates both make and use tools is critical for understanding how humans’ fossil ancestors may have used and constructed tools. An important question that primatologists and biological anthropologists seek to answer is the question, do nonhuman primates have culture? Whenever we see an exchange of ideas where one individual is involved in teaching another and when that knowledge is passed on to others in a group is according to anthropologists, a form of culture. We see this happen in chimpanzee groups where older chimpanzees teach the young how to use sticks to termite-fish, the process of extracting termites from a termite mound using a stick. This chimpanzee lives in the Gombe National Park in Tanzania. Chimps living in Gombe’s rainforest environment have developed a very different set of hunting techniques and tool use from their relatives living in the grassy savannah. (credit: “Chimp Eden Sanctuary – Mimi” by Afrika Force/flickr, CC BY 2.0) Explaining Primate Success Why primates evolved as they did and how they filled and exploited the range of ecological niches they now fill are questions that have not yet been adequately addressed. Over the last century, various hypotheses have been raised to account for the evolution of primates and their unusual anatomical characteristics. These theories include the arboreal theory, the visual predation hypothesis, and the angiosperm theory. The arboreal theory proposes that primates evolved the traits they did as an adaptation to life in the trees. Specifically, primates evolved thumbs and big toes that are perpendicular to the other digits to help them grasp onto branches. Matt Cartmill , a professor of anthropology at Boston University who spent his career trying to understand why primates evolved the way they did, has complicated this theory. Cartmill recognized that forward-facing eyes are characteristic not only of primates but also of predators such as cats and owls that prey on small animals. Thus, forward-facing eyes, grasping hands and feet, and the presence of nails instead of claws may not have arisen as adaptations to an arboreal environment. Rather, they may be adaptations that helped early primates succeed as predators. According to the visual predation hypothesis , primate features are adaptations for hunting insects and other small prey in the shrubby forest undergrowth and the lowest tiers of the forest canopy. The angiosperm theory states that the basic primate traits developed in coevolution with the rise of flowering plants, also known as angiosperms. Flowering plants provide numerous resources, including nectar, seeds, and fruits, and their appearance and diversification were accompanied by the appearance of ancestral forms of major groups of modern birds and mammals. Some argue that visual predation is not common among modern primates and that forward-facing eyes and grasping extremities may have arisen in response to the need for fine visual and tactile discrimination in order to feed on small food items, such as fruits, berries, and seeds, found among the branches and stems of flowering plants. Primate Classification and Taxonomy Scientists generally classify the order Primates into two suborders: Strepsirrhini (prosimians) and Haplorrhini (tarsiers and anthropoids). The Strepsirrhini or Prosimians The Strepsirrhini are considered to be primitive primates that evolved much earlier than other primates. This suborder includes lemurs and lorises . All the Strepsirrhini primates, or strepsirrhines, possess numerous anatomical traits that distinguish them from the Haplorrhini primates, or haplorrhines. These include a clawlike nail on the second toe, referred to as a grooming claw, and incisors in the lower jaw that are tightly packed together and protrude from the mouth, forming what is called a toothcomb. There are seven families of living strepsirrhines, and all of them are found in what anthropologists refer to as the Old World, which consists of the continents of Africa, Asia, and Europe. Five groups of living strepsirrhines are found only on the island of Madagascar off the coast of Africa. Two additional families are found in Africa and Asia. The pygmy slow loris ( Nycticebus pygmaeus ) is an example of a Strepsirrhini primate. Pygmy slow lorises be found in Vietnam, Laos, and a province of China. (credit: Lionel Mauritson/Wikimedia Commons, Public Domain) The Haplorrhini or Anthropoids The Haplorrhini are broken down into two further infraorders, Simiiformes and Tarsiiformes, and the Simiiformes are further divided into Platyrrhini and Catarrhini. The Platyrrhini , or platyrrhines, are exclusively found in the New World (specifically Central and South America) and are colloquially referred to as New World monkeys. Their name is derived from the rounded shape of their external nostrils, which open off to the sides. New World monkeys are also distinguishable by their prehensile tails that serves as an extra limb for extra support when moving in the trees. The Catarrhini , or catarrhines, are found throughout Africa and Asia. They differ from the New World primates in that they possess narrow nostrils that face downward. The Catarrhini contain two superfamilies, Cercopithecoidea and Hominoidea , and are exclusively Old World. The Cercopithecoidea contain two main groups: cheek pouch monkeys ( Cercopithecinae ) and leaf-eating monkeys ( Colobinae ). The most distinctive feature of the cercopithecoid primates is their molars, which exhibit two parallel ridges. The most distinguishing feature of the hominoids is that they do not have tails and are largely terrestrial, or ground-dwelling. Examples of Hominoidea include gibbons, chimpanzees, gorillas, orangutans, and humans. The Tarsier Puzzle The tarsier , which belongs to the family Tarsiidae, has both prosimian and anthropoid characteristics, which has made it difficult for scientists to classify. Tarsiers are currently classified within their own classification under the haplorrhines. One of the characteristics that tarsiers share with other haplorrhines, (including humans) is the inability to manufacture their own Vitamin C. They are the smallest known primate and are nocturnal, with extremely large eyes that take up much of the space in their skull. Due to their size of the eyes, the tarsier cannot rotate them; instead, it can rotate its head 360 degrees like an owl. Tarsiers are also the only primate carnivore, eating largely flying insects and sometimes small animals like bats and lizards. Tarsiers do not do well in captivity. They are extremely sensitive to noise and can become easily stressed. In fact, they can become so stressed that they die by suicide by banging their heads against tree trunks. The Philippine tarsier ( Carlito syrichta ) is found only in the southern portion of the Philippine islands. The tarsier has been challenging for scientists to classify, exhibiting both prosimian and anthropoid characteristics. (credit: “8thApril2007 – ‘Tarsier’ Monkey” by Jacky W./flickr, CC BY 2.0) angiosperm theory a hypothesis that suggests that primate origins and typical primate characteristics developed in response to the emergence of flowering plants. arboreal theory a hypothesis that proposes that primates evolved the traits they did as an adaptation to life in the trees. Catarrhini a subcategory of the primate infraorder Simiiformes that includes any primate considered an Old World monkey, an ape, or in the lineage of humans. This classification features downward-facing nostrils and a 2.1.2.3 dental formula. Cercopithecoidea a superfamily of the primate infraorder Simiiformes, subcategory Catarrhini, that consists of Old World monkeys. Haplorrhini a suborder of primates that contains tarsiers, New World monkeys, Old World monkeys, apes, and humans. Hominoidea a superfamily of the primate infraorder Simiiformes, subcategory Catarrhini, that consists of gibbons, great apes, and humanlike primates, including Homo and related fossil forms. Platyrrhini a subcategory of the primate infraorder Simiiformes that comprises New World monkeys Strepsirrhini a suborder of primates that includes lemurs, lorises, and galagos (bush babies). visual predation hypothesis a hypothesis that explains the origins of unique primate traits as adaptations for preying on insects and small animals.

Social Sciences


Origin of and Classification of Primates Learning Outcomes: By the end of this section, you will be able to: Explain the concept of deep time. Define fossils and explain some dating methods used on fossils. Identify some of the key characteristics of early primate fossils, including their respective time periods. Understanding Concepts of Time Geologists divide deep history into time periods known as eras. Eras are generally based on the fossil life forms observed. The oldest of the geological eras is the Eoarchean , which began approximately four billion years ago. The majority of the fossil evidence that we have for primate evolution comes from the Cenozoic era —the current geological era, dating from 65 million year ago (MYA) to the present. The Cenozoic era is divided into a series of epochs. Each epoch is associated with specific forms of primates that evolved during that time period. Fossils and Dating Methods Biological anthropologists primarily, although not exclusively, study fossil artifacts. A fossil is any remainder of a plant or animal that has been preserved in the earth. Upon the death of an organism, its body slowly decomposes until all that remains are the teeth and the bones or a mere impression of the organism’s original form. Under most conditions, teeth and bones and impressions eventually deteriorate, too. However, occasionally conditions are favorable for preservation. Examples of favorable materials for fossil formation include volcanic ash, limestone, and mineralized groundwater. Scientists do not have fossils of everything that lived in the past, and in some cases, remains from only a few individuals of a species have been found. The fossil record is very incomplete. Robert Martin, a curator at the Field Museum of Natural History in Chicago, estimates that there have been more than 6,000 primate species, while the remains of only 3 percent have been found. Fossils are very rare, but they are extremely informative about human biological evolution. Making Sense of Fossils An important part of understanding fossils is determining how old they might be and putting them in chronological order. In order to use a primate fossil to reconstruct the evolutionary history of primates, anthropologists must first be able to estimate approximately how old that specific fossil is. For some time, relative dating methods were the only methods available for dating fossils. Relative dating calculates the approximate age of a fossil in comparison to other fossil specimens. The last half century has seen important advances in absolute dating, including techniques that have made possible the dating of the earliest phases of primate evolution. Absolute dating calculates the actual biological age of a fossil in years within a range of years. Relative Dating Techniques Stratigraphy is the best-known and most commonly used method of relative dating. Stratigraphy is based on the observation that soil is deposited in successive layers, or strata. The oldest layers of soil (and any artifacts or fossils within them) will appear beneath more recent layers of soil (and any artifacts or fossils within them). In addition to using the location of layers of soil to date fossils deposited within these layers, biological anthropologists also sometimes make use of other items consistently found in a specific layer of the soil. These items are referred to as indicator artifacts because they help indicate the relative age of fossils and other artifacts. The best indicator artifacts are those that have a wide geographic distribution, are presence for a short period of geological time, and/or are from a species that underwent rapid evolutionary change. Different indicator artifacts have been used to ascertain relative age in different areas of the world. In Africa, elephants, pigs, and horses have been used to establish relative dates of different geological strata. The stratigraphy at Olduvai Gorge in East Africa, for example, was established based on fossil pigs. The various species of pig in successive strata are different and distinct, allowing paleoanthropologists to distinguish the strata based on the pig species found within them. Once the stratigraphy of an area is established, the relative ages of two different fossils in different sites can be determined by the associated indicator artifacts. If a site has been disturbed, stratigraphy will not be a satisfactory way to determine relative age. In such a situation, it may be possible to use absolute dating methods to estimate the age of fossils found together in a disturbed site. Absolute Dating Techniques Many absolute dating methods are based on the rate of decay of a radioactive isotope. A radioactive isotope is a chemical element that dissipates excess energy by spontaneously emitting radiation. These emissions happen at known and stable rates. Once the rate of decay of a radioactive isotope is established, the age of a specimen containing that isotope can be estimated within a range of possible error. C-14 The best-known method for determining the absolute age of fossils is carbon-14 or 14 C (pronounced “C-14”) dating. All plants and animals contain the isotope carbon-14 ( 14 C). Plants absorb 14 C from the air, and animals ingest plants containing the isotope. Because plants only absorb 14 C when they are alive and animals only consume plants when they are alive, scientists can determine how long ago an animal or plant died based on the amount of 14 C that remains in their cells. Carbon-14 has a known half-life of 5,730 years. This means that approximately half of the original 14 C in an organism will be eliminated in 5,730 years after its death. For example, if an organism had an original 14 C value of 100, then after 5,730 years, only 50 units of 14 C would be present. Thermoluminescence Another absolute dating technique that is frequently used by paleoanthropologists is thermoluminescence dating . Thermoluminescence dating requires that either the fossils to be dated or the sediments that the fossils are within have been exposed to a high-temperature event, such as a volcanic explosion. During such a high-temperature event, all the radioactive elements within the material are released. Consequently, the amount of radioactive elements that have accumulated in the artifact since the time of the high-temperature event can be used to calculate the artifact’s age. Primates of the Paleocene Epoch The Paleocene epoch began approximately 65 MYA and ended about 54 MYA. It is the most poorly understood epoch of the Cenozoic era, as it is the time period with the fewest fossils to represent it. However, this epoch is considered important to primate evolution because it offers the first unequivocal record of the earliest primates. Evidence of the most primitive primate yet identified was found in the U.S. state of Montana, in a geological deposit that was dated to the earliest part of the Paleocene. This creature is known as Purgatorius . Purgatorius is similar to extinct and living primates – and distinct from other mammals – in the presence of an elongated last lower molar and an enlarged upper central incisor (resulting in what one could think of as “Bugs Bunny teeth”). These two characteristics, which are shared by all living primates today, suggest that Purgatorius may be the common ancestor of later primates. Purgatorius unio may be the common ancestor of all later primate. Remains of Purgatorius unio have been found in deposits dated to be about 63 million years old. (attribution: Rice University, OpenStax, under CC BY 4.0 license) Primates of the Eocene Epoch The Eocene epoch , which began approximately 54 MYA and ended about 34 MYA, is marked by the disappearance of Purgatorius and the first appearance of primates that more closely resemble modern-day primates, especially in the fact that they possess postorbital bars composed entirely of bone. A postorbital bar is a bony ring surrounding the entirety of the eye orbit. This contrasts with other mammals whose postorbital bars are part bone and part cartilage. Some fossil specimens also possess a toothcomb and/or a grooming claw, characteristics that are exclusively found in strepsirrhine primates today. Other anatomical characteristics that are significant would be the ankle bones which researchers believe played a key role in the evolutionary success of primates. The evolution of primates during the Eocene was tremendous. It has been hypothesized that there were four times as many strepsirrhine primates during the Eocene than there are living primates today. Fossil primates in Eocene deposits are common in North America and Europe and are becoming known in Asia and Africa. However, there are currently no known fossil primates from the Eocene in South America or Antarctica. Primates of the Oligocene Epoch The Oligocene epoch , which began approximately 34 MYA and ended about 22 MYA, marks the appearance of the first fossil monkeys. The earliest unambiguous haplorrhine fossils were found at the Fayum , an archaeological site about 60 miles from Cairo, Egypt, that today represents part of the Sahara. The Fayum primates are divided into two main groups: Parapithecoidea and Propliopithecoidea . Based on their teeth, these primates are believed to be the earliest New World and Old World monkeys, respectively. Teeth are generally described according to a dental formula that indicates the number of each type of teeth in each quadrant of the jaw. An organism with a 2.1.2.3 dental formula has two incisors, one canine, two premolars, and three molars in each quadrant of their upper and lower jaws. Based on the presence of a third premolar, a trait found in all New World monkeys, it is probable that Propliopithecus represents the earliest New World monkeys, even though they first evolved in Africa. Likewise, it is probable the propliopithecoids represent the earliest catarrhine primate, as they are the first fossil monkeys that possess a dental formula of 2.1.2.3 found in catarrhine primates. Miocene Apes The Miocene epoch contains fossil evidence of some of the earliest apes such as Proconsul africanus africanus which lived in Africa from 23 to 14 mya. The earliest Miocene ape, found in Africa, is Proconsul . Unlike modern-day apes, the Proconsul lacked long, curved digits, suggesting that they were able to hang from branches but more often moved about on all four of their limbs. Proconsul also lacked a tail, which is why they are considered apes and not monkeys. Like all Old World monkeys and apes, including humans, their teeth show a pattern of 2.1.2.3. Another well-known ape from the Miocene is Sivapithecus . Sivapithecus fossils are very common throughout Asia, with a particularly large number having been found in Turkey. Like modern-day humans, they exhibit very thick dental enamel, suggesting that these apes regularly ate very hard foods. The most intriguing aspect of Sivapithecus morphology is that the skulls show a tremendous resemblance to the living orangutan in features such as its tall nasal openings and high eye sockets. Sivapithecus is one the earliest known ape species. Fossil remains exhibit the tall nasal openings and high eye sockets currently visible in orangutans. (credit: “ Sivapithecus indicus (Fossil Ape) (Dhok Pathan Formation, Upper Miocene; Potwar Plateau, Pakistan)” by James St. John/flickr, CC BY 2.0) While it is known that orangutans probably evolved from a Sivapithecus -type ape, there are no clear candidates for the ancestors of modern African great apes. There have only been two fossils found that clearly and unequivocally belong to the ancestors of modern African apes. Samburupithecus is a large late Miocene ape found in northern Kenya. It is known to resemble modern African apes. It differs from other Miocene fossils in having molar teeth that are elongated in a direction from the front of the mouth toward the back, instead of from cheek to tongue. Another fossil from the late Miocene (9–10 MYA) that is sometimes identified as an ancestor of modern African apes is Ouranopithecus , found in Greece, which has facial morphology that links it to both African apes and humans. fossils any remains, impression, or traces of living things from a former geologic age. Parapithecoidea a superfamily of primates from the early Oligocene that is believed to represent the earliest New World monkeys, though they first evolved in Africa. Proconsul a genus of ape from the early Miocene. Propliopithecoidea a superfamily of primates from the early Oligocene that is related to Old World monkeys and is believed to represent the earliest catarrhine primate. Purgatorius genus of the earliest primate or proto-primate. stratigraphy a branch of geology dealing with the classification, nomenclature, correlation, and interpretation of stratified rocks.

Social Sciences


Our Ancient Past: The Earliest Hominins Learning Outcomes: By the end of this section, you will be able to: Compare and contrast some early hominin species. Identify some key adaptations and characteristics found in early hominins. Identify key adaptations and derived traits that emerged in changing environments. Walking on Two Feet The term hominin refers to all species considered to be in direct lineage to humans, which include the genera Homo, Australopithecus , Paranthropus , and Ardipithecus . Hominids refers to all modern and extinct great apes, which include humans, gorillas, chimpanzees, and orangutans and their ancestors. These terms have been understood to represent different things over the years, but the definitions provided here are the most current. While all hominins may differ in varying ways from one another, they all share one anatomical behavioral complex: bipedal locomotion. Scientists can hypothesize about how a creature moved by analyzing several aspects of its morphology. Brachiators , animals that move by swinging from branch to branch, generally have long arms, while leapers , animals that propel their bodies through the force of their lower limbs, have relatively long legs. Arboreal primates have arms and legs of equal length. In bipedal locomotion , one leg is called the stance leg, and the other is called the step leg. While the stance leg is on the ground, the step leg is off the ground and striding forward. During normal walking, both feet are on the ground only about 25 percent of the time. As speed of locomotion increases, the percentage of time that both feet are on the ground decreases. As a result, for most of the time that bipedal organisms are moving, their body is balanced on only one of their legs (the stance leg). To ensure that bipedal organisms do not fall over while balanced on their stance leg, they have undergone many anatomical changes since the earliest hominin ancestors. One of the most important anatomical changes that facilitate successful bipedalism is the angling of the femur (upper leg bone) inward at what is referred to as a valgus angle , which positions the knees and feet under the center of the pelvis. Bipedal hominins have also evolved spinal curves that make it possible for the hips to balance the weight of the upper body. The evolution of the arch in the foot as well as the realignment of the big toe so that it is parallel to the other toes is also instrumental in transmitting weight during the step phase of bipedal locomotion. In humans, the femur bones angle inward. This adaptation, known as the valgus angle, makes bipedal locomotion (walking upright) more comfortable and more efficient. (attribution: Rice University, OpenStax, under CC BY 4.0 license) The most important evidence of early hominin bipedalism is provided by the work of English paleoanthropologist Mary Leakey . In the 1980s, Mary Leakey discovered a 75-foot trail of footprints made by three bipedal individuals who had crossed a thick bed of wet volcanic ash about 3.5 MYA. These footprints were found in East Africa at the site of Laetoli . Based on the date and the location, it is probable that these footprints were made by Australopithecus afarensis . Analysis of the Laetoli footprints indicates a modern striding gait. These replicas of the 3.6-million-year-old hominin footprints found in Tanzania by Mary Leakey are on display at the National Museum of Nature and Science in Tokyo, Japan. (credit: “ Australopithecus afarensis Fossil Hominid Footprints (Pliocene, 3.6–3.7 Ma; Laetoli Area, Northern Tanzania, Eastern Africa)” by James St. John/flickr, CC BY 2.0) The evolution of hominin bipedalism required complex anatomical reorganization. For natural selection to produce such a tremendous amount of change, the benefits of these changes must have been great. There have been dozens of hypotheses for these changes, ranging from freeing hands to carry tools, food, or offspring to increasing energy efficiency or thermoregulation (the ability to maintain the body’s temperature) by exposing more of the body’s surface. None of the hypotheses are testable, making it truly challenging to understand why humanity’s ancestors made such a huge behavioral shift. The next sections explore some of the key discoveries of early hominin fossils in which anthropologists see some of the earliest indications of the adaptation of bipedalism in the human story. Miocene Hominids The first hominid fossils appear in the late Miocene , 10 to 5 MYA. Sometime between 7 MYA and 4 MYA, hominids moved out of the trees and began to adapt more fully to a ground-based living niche. Unfortunately, the fossil evidence from this time period is extremely sparse, but new finds continue to be discovered. A complete cranium of Sahelanthropus tchadensis was found in 2002 by French paleoanthropologist Michel Brunet and his team in Chad in West Africa. Sahelanthropus is a fossil ape that lived approximately 7 MYA and is claimed by some researchers to be the last common ancestor of humans and chimpanzees. Genetic studies indicate that humans and chimpanzees diverged from one another sometime between 5 MYA and 7 MYA, so this species lived right at the time of the divergence. The cranial capacity is a mere 350 cubic centimeters (cc), which is equivalent to a chimpanzee; the modern human cranial capacity is approximately 1,400 cc. Sahelanthropus also has a very large brow ridge (the large bone above the eyes), and the location of the foramen magnum , the opening at the base of the skull where the spinal column enters the skull, suggests that its head was not held over its spine and thus it was not bipedal. Orrorin tugenensis was found in Kenya in 2001 by geologist Martin Pickford of the Collège de France and paleontologist Brigitte Senut of France’s National Museum of Natural History. Orrorin tugenensis was dated to approximately 6 MYA. Orrorin was proposed to be a hominin due to anatomical traits that suggest bipedalism. For example, the femoral head (the big, rounded ball at the top of the leg bone that connects the leg to the hip) is much larger than in quadrupedal apes, suggesting the femur was being used to support the weight of the upper body. The muscles attached to the femur also suggest bipedal movement. Another feature that suggests that Orrorin is truly a hominin is the teeth, which exhibit thick dental enamel and small, square molars, much like modern humans. Pliocene Hominins The Pliocene epoch extended from 5 MYA to 1.8 MYA. Fossils from the Pliocene show evidence of the evolution of hominins that are clearly bipedal. They also show evidence of clear, albeit primitive, cultural behavior. Climatically, the Pliocene was colder than the preceding Miocene, which resulted in changing sea levels and an increase in ice at the poles, opening up some previously inaccessible areas. During this period, North and South America became connected through the Isthmus of Panama, and a land bridge across the Bering Strait appeared between Alaska and Siberia. Ardipithecus ramidus Ardipithecus ramidus was found in Ethiopia in 1992 by American paleoanthropologist Tim White and was dated to about 4.4 MYA. This is the first discovered hominin species to be dated to the Pliocene era. Based on the forward position of the foramen magnum, it can be concluded that Ardipithecus was bipedal. Also, the upper arm bones are very small, suggesting that the arms were not used to support weight during locomotion. Ardipithecus possesses numerous traits, such as thin dental enamel, evidence of a reduced canine, and an opposable big toe. As a result of the latter trait, many believe that Ardipithecus was bipedal on the ground and quadrupedal in the trees. This hypothesis is supported by the fact that the fossil bones were found in relatively heavily forested environments. The reduced canine is a derived trait appearing even earlier than A. ramidus and is not what we would typically see in African ape males who have large intimidating canines. Current hypotheses suggest that over time smaller canines became dominant when there became less need to show aggression along with a female preference for males with milder temperaments (Suwa, G., et al. 2021). These skeletal remains have been identified as Ardipithecus , the first hominin species discovered that has been dated to the Pliocene Era. (credit: Sailko/Wikimedia Commons, CC BY 3.0) The Robust and Gracile Australopithecines The next few sections will examine various australopithecine species that had diverse physical characteristics related to morphology of the teeth and skull. Based on these characteristics, paleoanthropologists classified these species into gracile and robust forms, as illustrated in . Gracile species had a more pronounced projection of the jaw ( prognathism ), less flared cheeks with no sagittal crest, and smaller teeth and jaws. The sagittal crest in the robust australopithecines accommodated large temporalis jaw muscles for chewing tough plant materials. Australopithecine species are classified as either robust or gracile. A defining feature of the robust species is the sagittal crest visible on the Paranthropus boisei skull on the left. Gracile species, such as A. afarensis , on the right, display more pronounced projection of the face (prognathism). (credit: left, Rama/Wikimedia Commons, Public Domain; right, “ Australopithecus afarensis Fossil Hominid (Pliocene, Eastern Africa) 1” by James St. John/flickr, CC BY 2.0) Species considered to be the gracile include Australopithecus anamensis , A. afarensis , A. africanus , A . garhi , and A. sediba . The robust australopithecines (classified under the genus Paranthropus ) include Paranthropus robustus , P. boisei , and P. aethiopicus . The gracile species emerged around 4 MYA and disappeared 2 MYA, while robust species continued to exist for another million years. The next sections will first take a look at some of the gracile forms of australopithecine, followed by the robust forms. Australopithecus africanus Australopithecus africanus was the first australopithecine discovered, in 1924, and was described by Australian anatomist and anthropologist Raymond Dart , who found the fossil in a box of fossils sent to him by lime quarry workers at a site called Taung in South Africa. The most notable specimen in the box was a skull from a child, which Dart had to chip away from the stone it was embedded in. It took Dart four years to separate the teeth. The skull is now known as the Taung skull or Taung child. Dart argued that the Taung child represents “an extinct race of apes intermediate between living anthropoids and man” (Wayman 2011). He noted that the skull was long and narrow, not rounded as in modern humans, and its brain averaged a mere 422 cc, equivalent to a chimpanzee. However, the Taung child did not possess brow ridges, had circular orbits, and had minimal prognathism as well as small canines and no diastema (space in the jaw for large canines to be positioned when the mouth closes). These latter traits are all analogous to modern humans. Most importantly, Dart noted that the forward position of the foramen magnum indicated that the skull was poised on top of the vertebral column, suggesting bipedalism and an upright posture. This partial skull is from a specimen known as the Taung child. The species, Australopithecus africanus , displays traits that resemble modern humans in some ways but not others. (credit: Daderot/Wikimedia Commons, Public Domain) Australopithecus afarensis In 1973, a good portion of a skeleton (about 40 percent) was found in the Afar region of Ethiopia by American paleoanthropologist Donald Johanson . He called the skeleton Lucy , after a Beatles song. It was dated to around 3.75–2.8 MYA and was determined to be a member of the species Australopithecus afarensis . Like all fossils recently discovered, Lucy was given an identification or accession number, KNM-AL-288. The KNM acronym stands for the Kenya National Museum, where the fossil is housed, and AL stands for the Afar locality where the fossil was found. Since then, more specimens of this species have been found in Kenya, Tanzania, and Ethiopia, all in East Africa. This child stands next to a recreated skeleton of A. afarensis . The long arms and long, curved fingers and toes of A. afarensis are apparent. (credit: “ Australopithecus afarensis Fossil Hominid (Lucy Skeleton) (Hadar Formation, Pliocene, 3.2 Ma; Hadar Area, Afar Triangle, Northern Ethiopia, Eastern Africa) 2” by James St. John/flickr, CC BY 2.0) Australopithecus afarensis is dated from 3.9 to 2.9 MYA with an endocranial capacity of around 400 cc, which is approximately the same as a common chimpanzee. There are two morphological features that provide evidence that A. afarenis moved more like a great ape than a human. First, it had arms that were substantially longer than modern humans’. Long arms are generally found in animals that hang from branches, suggesting that A. afarensis also exhibited this behavior. Also, A. afarensis possesses finger and toe bones that are long and curved, another characteristic of animals that hang from branches. However, there is one important morphological feature of A. afarensis that suggests that this species may have moved somewhat like modern humans. The shape of A. afarensis ’s pelvis (hip bones) looks substantially more like a modern human’s than it does an ape’s. Instead of the hip bones being long and narrow, they are short and wide. Most paleoanthropologists believe that this change in pelvic shape indicates that A. afarensis moved like modern humans do, on two legs. While A. afarensis may have locomoted bipedally, the morphological differences between A. afarensis and modern humans suggest they did not move in exactly the same way. Current consensus is that A. afarensis was both tree dwelling and bipedal. Other anatomical evidence of bipedalism includes a more anterior position of the foramen magnum and the angle of the femoral head and neck. Australopithecus garhi Also found in Ethiopia, Australopithecus garhi is dated to approximately 2.5 MYA. Its cranial capacity is slightly greater than A. afarensis , at 450 cc. Australopithecus garhi has incisors that are larger than those of any of the known australopithecines or Homo . The function of the large incisors is not yet known. The most exciting aspect of A. garhi is that it provides evidence of the earliest use of stone tools by a hominin. Specifically, A. garhi fossils were found with fossil bones of ruminants, such as antelopes, that displayed numerous cut marks. Cut marks are made on bones by the process of removing meat from the bones with stone or metal tools. Based on this finding, biological anthropologists have hypothesized that A. garhi used some type of stone tool for butchering. Australopithecus sediba In 2008, the clavicle bone of Australopithecus sediba was discovered by Matthew Berger , the nine-year-old son of American paleontologist Lee Berger, in Malapa, South Africa. Further excavation in a cave feature uncovered two partial skeletons, one of an adult female and the other a younger juvenile. A. sediba is considered an important species because it appears in the fossil record around the time of the first emergence of the genus Homo around 2 mya. The classification of A. sediba was initially difficult to determine, due to its complex overlapping features, which include humanlike spine, pelvis, hands, and teeth and a chimpanzee-like foot. This combination of traits suggests both tree climbing and bipedal adaptations. After studying the characteristics collectively, anthropologists classified A. sediba as a species of Australopithecus . It is considered a direct ancestor of Homo erectus and Homo ergaster , which are discussed in Chapter 5, The Genus Homo and the Emergence of Us . It is believed that A. sediba could be a descendent of A. africanus , which suggests the species may be a dead end within the lineage to humans. Its classification and relationship with the genus Homo will likely remain highly debated. These bones are from Australopithecus sediba , which displays a humanlike spine and pelvis but a chimpanzee-like foot. (credit: Phiston/Wikimedia Commons, CC BY 3.0) Paranthropus robustus Thirteen years after Raymond Dart’s discovery, South African paleontologist and medical doctor Robert Broom discovered Paranthropus robustus at a site called Kromdraai in South Africa . The most obvious difference between Dart’s and Broom’s respective fossils, A. africanus and P. robustus , is that the morphology of Broom’s fossil is much larger. Its features include a sagittal crest and a flared zygomatic arch for the attachment of a large temporalis muscle for chewing a diet reliant on hard nuts and seeds. This interpretation was further supported by scanning electron microscopy (SEM), which was used to evaluate the markings etched into the teeth. As the teeth increased in size the incisors and canines shrank, giving Paranthropus a flatter face with less projection of the jaw. There are some who argue that depending on the environment and locale, some Paranthropus may have been omnivores, with varied diets similar to those of H. ergaster . (Lee-Thorp, Thackeray, and van der Merwe 2000). Paranthropus boisei Following in Broom’s footsteps, other scientists began searching for fossils in East Africa. Beginning in 1931, Kenyan and British paleoanthropologist Louis Leakey and his wife, Mary Leakey , worked in what is known as the Eastern Rift Valley , which is a 1,200-mile trough extending through Ethiopia, Kenya, and Tanzania. They searched for almost 30 years before they found their first hominin fossil, Paranthropus boisei (OH-5)—originally classified as Zinjanthropus boisei —in 1959. It is often referred to as the hyper-robust hominin because of its mohawk of bone on the top of the skull. Other features include a low or absent forehead, a flat face, large jaws, and large attachment sites over the entire skull for chewing muscles. Paranthropus aethiopicus We have little knowledge about Paranthropus aethiopicus (shown in ), which has been dated to about 2.5 MYA and is referred to as the “black skull.” It is believed that this species falls somewhere between the robust and gracile australopithecines, having characteristics of both. The species was discovered in Ethiopia in 1967 by a French expedition team headed by Camille Arambourg and Yves Coppens. Much remains to be learned about Paranthropus aethiopicus , which has characteristics of both the robust and gracile australopithecines. (credit: “ Paranthropus aethiopicus (Fossil Hominid) (Nachukui Formation, Upper Pliocene, 2.5 Ma; Lomekwi, Lake Turkana Area, Kenya) 3” by James St. John/flickr, CC BY 2.0) Landmarks and Questions While the fossils discovered up to this point have provided a small window into the story of humanity’s past, they have also simultaneously raised numerous questions. Questions related to phylogenetic relationships and points of divergence are challenges for paleoanthropologists, who have only fragmentary fossil evidence to build hypotheses around. Nevertheless, the discoveries that have been made represent important landmarks in anthropologists’ understanding, providing clues that will lead to the next steps in the human journey. Pedestrian Survey Conduct a pedestrian survey to try to locate fossils near where you live (trilobites in New York, ammonites in Texas, shark teeth near riverbeds, arrowheads). Think about where you would most likely find a fossil and why. Try to extract one without destroying the environment around it, which provides important context. Try to figure out what kind of fossil it is by doing some Internet research. Why do you think that this fossil was preserved? What information would make the search for fossils easier? Summary Biological anthropology strives to understand how humans interact and behave in the present, how humans evolved biologically, and how humanity’s ancient ancestors lived in diverse climates and environments. The anthropological approach to exploring these questions is grounded in evolutionary theory. Charles Darwin was one of the first to propose a mechanism by which evolution occurred, which he called natural selection. Natural selection is based on the premise that those with more favorable characteristics survive and reproduce at greater rates than those without them. Natural selection depends on the evolutionary processes of mutation, speciation, gene flow, and genetic drift. Darwin’s theory did not address how these favorable characteristics could be inherited. Gregor Mendel’s experiments on peas addressed this very question. Mendel’s work resulted in two very important observations. He observed that the two alleles for each trait separate during the formation of the sex cells and that the probability of having one trait does not affect the probability that an individual will have another trait. Carolus Linnaeus is best known for creating the classification system that taxonomists use today, which is based on physical similarities and differences. Phylogenetics is a hypothesis about how species are related to one another and to a common ancestor. Today, biological anthropologists apply taxonomies and phylogenies to the current nonhuman primate and hominin fossil record. It is in the Miocene that the first fossil apes, such as Proconsul , are seen. The first evidence of hominin-like fossils appears by the end of the Miocene. A large number of morphological changes observed in early hominins suggest considerable environmental and climatic change. During the Pliocene epoch, extending from 5 to 1.8 MYA, the evolution of hominins that were clearly bipedal is evident in the fossil record, as is evidence of cultures that used stone tools. The path is now ready for the next group in humanity’s evolutionary history to enter the scene. Critical Thinking Questions Bibliography Aiello, Leslie C. 1986. “The Relationships of the Tarsiiformes: A Review of the Case for the Haplorhini.” In Major Topics in Primate and Human Evolution , edited by Bernard Wood, Lawrence Martin, and Peter Andrews, 47–65. Cambridge: Cambridge University Press. Blaxland, Beth. 2020. “Hominid and Hominin—What’s the Difference?” Australian Museum. February 10, 2020. https://australian.museum/learn/science/human-evolution/hominid-and-hominin-whats-the-difference/. Crompton, Robin. 1989. “Mechanisms for Speciation in Galago and Tarsius .” Human Evolution 4 (2): 105–116. Dasgupta, Shreya. 2019. “Super Variable California Salamander Is ‘an Evolutionist’s Dream.’” Mongabay. March 18, 2019. https://news.mongabay.com/2019/03/super-variable-california-salamander-is-an-evolutionists-dream/. David, A. Rosalie, and Michael R. Zimmerman. 2010. “Cancer: An Old Disease, a New Disease or Something in Between?” Nature Reviews Cancer 10:728–733. https://doi.org/10.1038/nrc2914. Dunbar, Robin I. M. 1988. Primate Social Systems . Ithaca, NY: Cornell University Press. Fleagle, John G. 1999. Primate Adaptation and Evolution . 2nd ed. San Diego: Academic Press. Fuentes, Agustín. 2019. Biological Anthropology: Concepts and Connections . 3rd ed. New York: McGraw-Hill Education. Groves, Colin. 2001. Primate Taxonomy . Washington, DC: Smithsonian Institution Press. Hill, W. C. Osman. 1955. Primates: Comparative Anatomy and Taxonomy . Vol. 2, Haplorhini: Tarsioidea . New York: Interscience. Lee-Thorp, Julia, J. Francis Thackerary, and Nikolaas van der Merwe. 2000. “The Hunters and the Hunted Revisited.” Journal of Human Evolution 39 (6): 565–576. https://doi.org/10.1006/jhev.2000.0436. McKee, Jeffrey K., Frank E. Poirier, and W. Scott McGraw. 2005. Understanding Human Evolution . 5th ed. Upper Saddle River, NJ: Pearson Prentice Hall. Meier, Joana I., David A. Marques, Salome Mwaiko, Catherine E. Wagner, Laurent Excoffier, and Ole Seehausen. 2017. “Ancient Hybridization Fuels Rapid Cichlid Fish Adaptive Radiations.” Nature Communications 8. https://doi.org/10.1038/ncomms14363. Phillips, Melissa Lee. 2006. “Genetic evidence for punctuated equilibrium.” The Scientist , October 5, 2006. https://www.the-scientist.com/daily-news/genetic-evidence-for-punctuated-equilibrium-47144. Pruetz, J.D., et. al, P. (2015). New evidence on the tool-assisted hunting exhibited by chimpanzees (Pan troglodytes verus) in a savannah habitat at Fongoli. R. Soc. open sci.2140507140507http://doi.org/10.1098/rsos.140507 Stringer, Chris, and Peter Andrews. 2011. The Complete World of Human Evolution . 2nd ed. London: Thames & Hudson. Canine sexual dimorphism in Ardipithecus ramidus was nearly human-like. Proceedings of the National Academy of Sciences , 118 (49) e2116630118; DOI: 10.1073/pnas.2116630118 Takahashi, Tetsumi, and Stephan Koblmüller. 2011. “The Adaptive Radiation of Cichlid Fish in Lake Tanganyika: A Morphological Perspective.” International Journal of Evolutionary Biology 2011. https://doi.org/10.4061/2011/620754. Wayman, Erin. 2011. “How Africa Became the Cradle of Humankind.” Smithsonian , October 17, 2011. https://www.smithsonianmag.com/science-nature/how-africa-became-the-cradle-of-humankind-108875040/. diastema a space or gap between the canines and the other teeth that allows for the upper and lower teeth to bite together. foramen magnum the opening at the base of the skull where the spinal column and nerves enter to reach the brain. The position of the foramen magnum can be used to determine if a species was bipedal. hominin the group representing modern humans, extinct human species, and all of humanity’s immediate ancestors, including the genera Homo , Australopithecus , Paranthropus , and Ardipithecus . hominid the group representing all modern and extinct great apes, including humans, chimpanzees, gorillas, orangutans, and all their immediate ancestors. prognathism projection of the face, as seen in many nonhuman primates and early hominins.

Social Sciences


Introduction Liang Bua Cave on the island of Flores in Indonesia. A potentially new species of the genus Homo, Homo floresiensis was discovered in this cave in 2003. (credit: “Flores: Ruteng to Bajawa” by Bryn Pinzgauer/flickr, CC BY 2.0) (credit: “Flores” by Ryan Somma/flickr, CC BY 2.0) Our human story continues with the rise of the genus Homo which at one time represented at least 8 different species in our human lineage – with only H. sapiens surviving. The genus Homo displays some of the most diverse and complex examples of both australopithecine and Homo characteristics, which has made the classification of species in this genus challenging. In this chapter we take a look at how paleoanthropologists have defined Homo and at attempts to answer the question, “What does a species of the genus Homo look like?”

Social Sciences