# Biological Evolution and Early Human Evidence

## It’s All in the Genes! The Foundation of Evolution

### Learning Outcomes:

By the end of this section, you will be able to:
1. Define alleles, genes, phenotypes, and genotypes.
2. Distinguish the process of mitosis from the process of meiosis.
3. Explain how Mendel’s laws of heredity affect human variation.
4. Explain how the multitude of evolutionary forces contribute to variation in the human condition.

### The Units of Life

Cells are the basic units of life in all organisms. They are the smallest entities that are capable of self-reproduction. There are two main types of cells: prokaryotic and eukaryotic cells, named for the types of organisms in which they occur. Prokaryotes are single-celled organisms, such as bacteria and archaea. Eukaryotes are more complex, multicellular organisms, such as plants and animals (including humans). One of the most important components of eukaryotic cells is the enclosed nucleus at the center of the cell; prokaryotic cells do not have this nucleus. The nucleus of a eukaryotic cell houses all of the genetic material, or DNA (deoxyribonucleic acid), that controls cellular function. Normally, the DNA forms a long string within the nucleus.
There are two main types of eukaryotic cells: somatic cells and sex cells (also known as gametes). The somatic cells make up the structural components of a body, such as the tissues, muscles, and organs. The sex cells are specifically involved in reproduction. The function of the sex cells is to unite with a sex cell from another individual to form a fertilized egg, also known as a zygote. In animals, there are two types of sex cells: ova, or eggs, and sperm.
Cell division is the process that results in the production of new cells. However, sex and somatic cells divide differently. The cellular division of somatic cells is known as mitosis, while the cellular division of sex cells is known as meiosis. Mitosis of somatic cells is sometimes referred to as simple cell division because the parent cell divides once to produce two daughter cells that are genetically identical to each other and identical to the original parent cell. During mitosis, the DNA genetic material forms structures known as chromosomes. Each daughter cell inherits an exact copy of all 46 chromosomes found within the parent cell.
Meiosis, or sex cell division, is more complicated. This type of cellular division only occurs in the testes of males and the ovaries of females. Instead of just one division, meiosis results from two cellular divisions that produce four daughter cells. In meiosis, the four daughter cells each receive half of the original genetic material from the parental cell. Thus, each daughter cell only has 23 chromosomes.
It is on the chromosomes that genes are housed. Genes are the fundamental unit of heredity. They are best understood as the sequence or ordering of the DNA material that is housed in the nucleus. The genotype is the genetic material found within an organism's cells and it is the expression of these genes that will produce the phenotype or observable trait. Sometimes, the sequencing of the DNA material produces a variation of a gene, known as an allele. An allele is defined as a similar but slightly different form of the same gene that can activate the expression of a specific trait.

### Gregor Mendel and the Laws of Heredity

The true nature of inheritance was not really understood until the beginning of the 20th century, when the 19th-century work of Gregor Mendel, a Catholic priest from Slovakia, was rediscovered. While in college, Gregor Mendel was introduced to cell theory, which states that all organisms are composed of cells and that cells are the fundamental unit of all living things. Cell theory raised many questions in Mendel’s mind, including whether both parents contribute equally to the cells in their offspring. In 1854, Mendel began a series of experiments with pea plants to help resolve this question and better understand how traits are inherited from generation to generation.
The first stage of Mendel’s experiments was identifying plants that breed true, meaning that each parent only produces one kind of offspring when self-crossed. A self-cross is essentially a self-mating; some plants, such as peas, have both male and female parts and can self-fertilize. Not all self-crosses are the same as the parent plant, however. For example, self-crossed pea plants that have yellow pods sometimes produce offspring with yellow pods and sometimes produce offspring with green pods. Mendel continued to selectively breed only those pea plants that produced offspring that were the same as the parents. He called them purebreds and referred to them as the P1 generation. It took him more than two years to establish plants that always bred true.
Then Mendel selected seven traits of his pea plants that each had two distinct phenotypes, or observable expressions of the trait. For example, seed shape can be either round or wrinkled, while pod color can be either yellow or green. Over the next eight years, Mendel studied the mating and resulting traits of more than 28,000 plants. Mendel’s first round of experiments used his purebred pea plants to create what is known as a monohybrid cross. A monohybrid cross is a mating between two purebred individuals who differ in a single characteristic. In Mendel’s monohybrid crosses, the parent pea plants differed from one another in terms of whether the pods of the parental pea plants were yellow or green or whether the seeds of the parental pea plant were wrinkled or round.
In his first monohybrid crosses, Mendel mated a purebred yellow pea plant with a purebred green pea plant. He found that all the offspring resulting from this monohybrid cross were yellow, even though when the green peas self-crossed, all their offspring were green. In other words, all the hybrid offspring were yellow in color. A hybrid plant is one in whose parents differ in a term of a specific characteristic, such as pod color or seed shape. The trait that was expressed (yellow) Mendel referred to as dominant, and the trait that disappeared (green) he referred to as recessive. Mendel’s next set of experiments involved mating two hybrid plants—in other words, those that resulted from the monohybrid cross. In these experiments, he found that the recessive traits reappeared in a ratio of three dominant to one recessive.
Mendel’s experiments suggested two very important facts. First, Mendel noted that various expressions of a trait (such as pea color) were controlled by discrete units that occur in pairs and that offspring inherited one unit of each pair from each parent. This observation became Mendel’s first law of inheritance, the law of segregation, which states that the two alleles for each trait segregate, or separate, during the formation of gametes (eggs and sperm) and that during the reproductive process, the alleles combine at random with other alleles. Today, we know that the process of meiosis—division of sex cells—explains Mendel’s law of segregation. Each of the seven traits identified by Mendel is controlled by a pair of genes in the plant, one on each chromosome. During the reproductive cycle, the chromosomes separate from one another so that each gamete has only one allele for each trait. During fertilization, the alleles combine, and the two-gene state is restored.
After Mendel established his first law of inheritance, he extended his studies to more complex situations. He began performing experiments with two set of traits, using dihybrid crosses. A dihybrid cross is a cross between individuals who differ with respect to two gene pairs—for example, a cross between a plant with a round yellow pea and a plant with a wrinkled green pea. Because yellow and round are both dominant traits and wrinkled and green are both recessive, all the offspring resulting from the first-generation mating were 100% yellow and round. The green color and the wrinkled pea shape had disappeared. However, these recessive traits reappeared in a ratio of three dominant to one recessive when two round yellow individuals from the first-generation dihybrid cross were mated. The green color and the wrinkled pea shape had not truly disappeared. In the second generation of the dihybrid cross, Mendel found that 9/16 of the offspring were round and yellow, 3/16 were wrinkled and yellow, 3/16 were round and green, and 1/16 were wrinkled and green. The results of these dihybrid crosses indicate that the two characteristics—pea color and pea shape—segregate independently. The expression of one trait is not influenced by the expression of the other trait. This is known as the law of independent assortment, which is Mendel’s second law of inheritance. There is nothing to dictate that round peas will be yellow or that wrinkled peas will be green. The alleles that code for different traits sort independently of one another during sex cell division (meiosis).

### Mendelian Inheritance in Humans

Mendel’s laws of inheritance also apply to humans. Indeed, the principles of segregation and independent assortment account for the transmission of certain human traits. Human blood type is one of the most familiar Mendelian traits. Blood type has three phenotypes—A, B, and O—based on three alleles of a single gene. If only the A allele or both the A and O alleles are present, the phenotype is A. If only the B allele or both the B and O alleles are present, the phenotype is B. If both A and B are present, the phenotype is AB. If neither A nor B is present, the phenotype is O. Note that O is recessive to both A and B, while A and B are codominant. Codominance means that instead of one allele masking the other, the products of both alleles are observed. Additional examples of Mendelian traits, or those controlled by a single gene, include Huntington’s disease, widow’s peak, cystic fibrosis, sickle cell anemia, Tay-Sachs disease, hemophilia, and red-green color blindness. OMIM, or Online Mendelian Inheritance in Man, hosts an online database of almost 5,000 Mendelian human traits.
It is important to note that the majority of human traits are not controlled by a single pair of genes. More often, a single gene can have multiple effects. Even more commonly, multiple genes are needed to produce a single effect. These are referred to as polygenic traits. Most human traits are polygenic, not Mendelian. A good way of determining if a trait is polygenic is to assess whether the trait can be measured. Traits that can be measured, such as height or weight, are polygenic. Also, traits that have a wide range or lots of variability and can be affected by environmental factors are probably polygenic. The survival of a species depends on genetic diversity and variation. If there is a reduction of a gene pool due to geographic isolation or other environmental factors than a species is at risk of extinction.


# Biological Evolution and Early Human Evidence

## Evolution in Action: Past and Present

### Learning Outcomes:

By the end of this section, you will be able to:
1. Identify the major contributors to evolutionary theory and their specific theoretical contributions and historical context.
2. Explain the theory of evolution and how it applies to the understanding of human origins.
3. Identify the key differences between Linnaean classification and phylogenetics.
4. 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,  (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.

### 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  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.
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 , 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:
1. All organisms are capable of producing offspring faster than the food supply increases.
2. All organisms show variation.
3. There is a fierce struggle for existence, and those with the most suitable variations are most likely to survive and reproduce.
4. Variations, or traits, are passed on to offspring (inherited).
5. 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.
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.

### 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.

### 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.
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).
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.

### 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

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?
1. Danaus gilippus
2. Danaus genutia
3. Limenitis archippus
4. Danaus plexippus
5. 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.
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.

### 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.
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.


# Biological Evolution and Early Human Evidence

## What Is a Primate?

### Learning Outcomes:

By the end of this section, you will be able to:
1. Define primate.
2. Describe the relationship between primate behavior and environment.
3. Identify and classify the key taxonomic groups of primates.

### What Is a Primate?

Primates—including human beings—are characterized by a number of distinct physical features that distinguish them from other mammals. These include
1. opposable thumbs and (in nonhuman primates) opposable big toes;
2. the presence of five digits (fingers or toes) on the appendages;
3. flat nails instead of curved claws;
4. pads at the tips of the fingers made up of deposits of fat and nerves;
5. reduced reliance on sense of smell and a relatively small snout;
6. depth perception;
7. binocular vision (being able to see one image with both eyes);
8. a relatively slow reproductive rate;
9. relatively large brain size; and
10. postorbital bars (bony rings that completely surround the eyes).
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.

### 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 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.


# Biological Evolution and Early Human Evidence

## Origin of and Classification of Primates

### Learning Outcomes:

By the end of this section, you will be able to:
1. Explain the concept of deep time.
2. Define fossils and explain some dating methods used on fossils.
3. 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 14C (pronounced “C-14”) dating. All plants and animals contain the isotope carbon-14 (14C). Plants absorb 14C from the air, and animals ingest plants containing the isotope. Because plants only absorb 14C 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 14C that remains in their cells. Carbon-14 has a known half-life of 5,730 years. This means that approximately half of the original 14C in an organism will be eliminated in 5,730 years after its death. For example, if an organism had an original 14C value of 100, then after 5,730 years, only 50 units of 14C 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 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.

### 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 . 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 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.
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.  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 , found in Greece, which has facial morphology that links it to both African apes and humans.


# Biological Evolution and Early Human Evidence

## Our Ancient Past: The Earliest Hominins

### Learning Outcomes:

By the end of this section, you will be able to:
1. Compare and contrast some early hominin species.
2. Identify some key adaptations and characteristics found in early hominins.
3. 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.
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 . Analysis of the Laetoli footprints indicates a modern striding gait.
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  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.
 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

 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).

### 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.
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

 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.

### 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.
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,  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  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.

### Paranthropus robustus

Thirteen years after Raymond Dart’s discovery, South African paleontologist and medical doctor Robert Broom discovered  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,  (OH-5)—originally classified as —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  (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.

### 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.

### 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

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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/.


# The Genus Homo and the Emergence of Us

## Introduction

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?”


# The Genus Homo and the Emergence of Us

## Defining the Genus Homo

### Learning Objectives

By the end of this section, you will be able to:
1. Describe the time periods and geological context of the genus Homo.
2. Identify some key differences between the genus Homo and Australopithecus.
3. Define some of the limitations of and challenges in the classification of hominin species in the genus Homo.
4. Explain the concept of encephalization as it relates to early hominin evolutionary development and as a tool for hominin classification.

### Putting Homo into Context

Before learning about the hominin species that make up the category genus Homo, it will be helpful to become familiar with the key archaeological time periods with which Homo is associated. The species and cultural developments mentioned below will be explored in greater detail in the sections that follow.
1. Lower Paleolithic (from roughly 3 million years ago to approximately 300,000 BCE): This period includes H. habilis and the Oldowan tool industry, followed by H. ergaster and the Acheulean tool industry.
2. Middle Paleolithic (approximately 300,000–40,000 BCE): This period includes continued use of Acheulean tools by H. heidelbergensis, followed by H. neanderthalensis and the Mousterian tool industry.
3. Upper Paleolithic (approximately 43,000–26,000 BCE): The Upper Paleolithic saw the emergence of cave art like that found in the famous Chauvet Cave in France (), Venus figurines (), and an increased use of bone and antler in tools and jewelry. The most recent ice age occurred during this time, with glaciers covering huge parts of the planet. The emergence of Paleoindians and the use of Clovis points, which were used to kill large game such as mastodons and mammoths, occurred near the end of this time period.
4. Neolithic (Agricultural Age) (8,000–3,000 BCE): New innovations appear during the agricultural age, or “Neolithic revolution,” as H. sapiens set up permanent settlements. Humans begin to transition from being hunters and gatherers to growing grow crops, owning land, and domesticating animals.

### The Challenge of Defining the Genus Homo

The previous chapter introduced the australopithecines, who were diverse in their physical characteristics (gracile and robust), with large jaws and teeth and small brain size. A key characteristic shared by both the australopithecines and the genus Homo is bipedalism. The transition to bipedalism is linked with various anatomical changes, including longer legs, changes in spinal curvature, and the development of arches in the feet to conserve energy and increase balance when walking.
What criteria other than bipedalism might be used to classify a species under the genus Homo? Many anthropologists have attempted to establish specific criteria to use in determining a classification of Homo. Paleoanthropologists Mary Leakey, Louis Leakey, and John Napier, as well as primatologist Phillip Tobias, were among the first to extensively study the fossils of , considered to be one of the earliest species in the genus Homo.  had a brain size of around 661–700 cc, which was larger than the australopithecines’, with hands that were capable of the dexterity needed for making tools, due to bone structure changes and a repositioning of the thumb, which allowed for better grip.
The type specimen OH 7 of H. habilis dated between 2 and 1.7 MYA and was found in 1960 at Olduvai Gorge by Jonathan and Mary Leakey. It was described by Louis Leakey in 1964. Type specimen refers to a specimen that serves as the standard for the taxon or classification group for that species. OH 7 is the identification or accession number of this specific specimen and stands for “Olduvai Hominid #7.” The specimen consisted of a partial juvenile skull, hand, and foot bones. It possessed teeth that were much smaller than those of any australopithecine and was possibly in coexistence with the robust australopithecines (Paranthropus). Based on an endocranial cast (an imprint of the interior of the brain case), it was determined that H. habilis may have possessed what is called a Broca’s area in the brain. Broca’s area, which includes two Brodmann areas (referred to as 44 and 45), is located in the middle of the left cerebral cortex of the brain and is especially important for speech development (). Some scientists have suggested that H. habilis started to develop the neural networks necessary for human speech, while others argue that H. habilis probably already had speech.
The postcranial features (skeletal structures in the body other than the skull) of Homo habilis are not as well established, as is the case for many other early hominin fossils. This can be problematic, as many hominin species coexisted with overlapping traits. Likewise, it can be problematic to have postcranial material and not the cranium or skull. The skull often serves as a diagnostic tool when postcranial materials do not provide enough evidence or provide confusing evidence.
Based on their research on H. habilis, Mary Leakey, Louis Leakey, and John Napier proposed the following criteria for classifying Homo: a brain size over 600 cc; a round, globular skull; tool use; reduced prognathism (protrusion of the jaw) and smaller jaws and mandibles; humanlike postcranial features; and feet that are fully adapted for walking (Leakey, Tobias, and Napier 1964). While this list established specific and fairly comprehensive guidance, the diversity of traits and the ways in which they overlapped didn’t always line up with the criteria.
H. habilis has been at the center of several debates regarding their taxonomic position and relationship with other early archaic Homo species. For example, H. habilis was initially believed to have been a direct human ancestor through the lineage of  and then modern humans. This viewpoint is now debated and has resulted in a scientific divide between those supporting H. habilis and those suggesting another Homo species, , as being the ancestor of H. erectus. H. rudolfensis is an archaic Homo dated to about 2 MYA, which coexisted with other Homo species during that time period. A cranium was discovered in 1972 along Lake Turkana in Kenya by Bernard Ngeneo, a local Kenyan. The specimen was later described by paleoanthropologist Richard Leakey. There is a lot that is not known about this species; scientists are missing postcranial materials, and as of yet no tools have been found. There are hypotheses that propose that H. rudolfensis might be a H. habilis male, exhibiting a larger cranium than that seen in a female H. habilis. Others suggest it is a completely different species. Another controversy centers on tool use. While Homo habilis was long regarded as the earliest hominin to use stone tools, it has been determined, based on evidence of cutmarks, that at least one australopithecine (A. garhi) used stone tools before H. habilis, at around 2.6 MYA (Semaw et al. 1997).
While there are still questions as to the phylogenetic relationship of H. habilis and H. rudolfensis, there is general agreement that Homo did evolve from Australopithecus. The timing and placement of the split between Australopithecus and Homo, however, is still debated. H. habilis was determined to not be an Australopithecus due to its smaller teeth, a humanlike foot, and hand bones that suggested an ability to manipulate objects with precision.
One of the main considerations in classifying H. habilis as a Homo and not an Australopithecus was its cranial capacity, which is a measurement that indicates brain size. With some exceptions, cranial capacity can serve as an indicator of where a hominin fossil might belong in the hominin phylogenetic tree. Encephalization refers to a progressive increase in brain size over time. In human evolution, we can observe encephalization beginning with Homo habilis and progressing more rapidly through H. erectus. Encephalization correlates with an increase in behavioral, cognitive, and cultural complexity. Cognitive developments correspond with our ability to construct and form ideas, including the ability think in and communicate via symbolic and abstract language, such as that used in storytelling, ritual, and art. There are always exceptions, however, such as the island-dwelling, small-brained H. floresiensis, who will be introduced later in this chapter. In spite of having a very small brain, H. floresiensis made and used tools and built fires. This discovery has challenged what we thought we knew about the correlation of brain size and cognitive development in human evolution.
The encephalization quotient (EQ) can serve as a good indicator (with some exceptions) of classification within the genus Homo. The encephalization quotient is a calculation arrived at by comparing the ratio between actual brain size (determined with either a mass or volume calculation) and expected brain size. Body size is a factor in these measurements as expected brain size reflects the relationship between brain and body size for a given taxonomic group (Jerison 1973). The larger the brain weight relative to the overall body weight, the more likely that the brain was used for more complex cognitive tasks. Harry J. Jerison (1973) was the first to develop EQ measurements. The formula he used for calculating EQ in birds and mammals is brain mass/0.12 × (body mass)0.66. Other formulas have also been proposed, such as EQ = brain mass (11.22 × body mass 0.76) (Martin 1981). While EQ is a strong tool for studying brain size in early hominins, there are always potential margins of error when dealing with fragmentary fossils, and increasingly alternative forms of measurements are being proposed. One study proposes that EQ should no longer be used as a tool in calculating brain size in primates and other vertebrate species, based on the premise that cognitive performance does not depend on body size and so body size should not be included in the formula (Schaik et al. 2021). Other theories consider the number of cortical neurons and neural connections as most important when considering cognitive ability (Roth and Dicke 2012). According to this approach, the density of the cortex is more associated with intelligence than is brain size. These alternate approaches would perhaps better explain those exceptions in the fossil records, such as H. floresiensis. Other interesting research is looking at potential levels of cognition and memory as it relates to levels of tool complexity (Read and van der Leeuw 2008).
In spite of these criticisms, many see EQ measurements as providing fairly consistent results. Modern humans (Homo sapiens) have an EQ of roughly 6.0–7.0 (meaning that their brain mass is six to seven times greater than what one would expect to find in a comparable mammal of the same body size). H. erectus has an EQ of 4.0, while for an australopithecine EQ is around 2.5 to 3.0 (Fuente 2012, 227).  shows increases in average brain sizes for various species over time.