Chapter 24 Summary

Core Concepts Summary

24.1 Anatomical, molecular, and fossil evidence shows that the human lineage branches off the great apes tree.

Anatomical features indicate that primates are a monophyletic group that includes prosimians, monkeys, and apes. The apes in turn include the lesser and great apes. page 486

The great apes include orangutans, gorillas, chimpanzees, and humans. page 487

Analysis of sequence differences between humans and our closest relatives, chimpanzees, indicate that our lineage split from chimpanzees 5–7 million years ago. page 487

Lucy, an unusually complete specimen of Australopithecus afarensis, demonstrates that our ancestors were bipedal by about 3.2 million years ago. page 489

505

Hominin fossils occur only in Africa until about 2 million years ago, when Homo ergaster migrated out of Africa to colonize the Old World. page 490

24.2 Phylogenetic analysis of mitochondrial DNA and the Y chromosome shows that our species arose in Africa.

Studies of mitochondrial DNA (mtDNA) suggest that the time back to the common ancestor of modern humans is about 200,000 years, implying that modern humans (Homo sapiens) arose in Africa (the out-of-Africa theory). page 491

The mtDNA out-of-Africa pattern is supported by Y chromosome analysis, which also shows a recent African origin of modern humans. page 494

Analysis of Neanderthal DNA from 30,000-year-old material indicates that, as the ancestors of non-African humans emigrated from Africa, they interbred with the Neanderthals. page 494

Our species originated in Africa and subsequently colonized the rest of the planet, starting about 60,000 years ago. page 495

24.3 During the 5–7 million years since the most recent common ancestor of humans and chimpanzees, our lineage acquired a number of distinctive features.

The development of bipedalism involved a wholesale restructuring of anatomy. page 495

Neoteny is the process in which the timing of development is altered over evolution so that a sexually mature organism retains the physical characteristics of the juvenile form; humans are neotenous, exhibiting many traits as adults that chimpanzees exhibit as juveniles. page 496

There are many possible selective factors that explain the evolution of our large brain, including tool use, social living, and language. page 497

FOXP2, a transcription factor involved in brain development, may be important in language, as mutations in the gene that encodes FOXP2 are implicated in speech pathologies. page 498

24.4 Human history has had an important impact on patterns of genetic variation in our species.

Because our ancestors left Africa very recently in evolutionary terms, there has been little chance for genetic differences to accumulate among geographically separated populations. page 499

Humans have very little genetic variation, with only about 1 in every 1000 base pairs varying among individuals. page 499

Most of the variation among humans occurs within populations. As much as 85% of the total amount of genetic variation in humans can be found within a single population. Only about 7% of human genetic variation segregates between groups that are commonly called races. page 500

Some racial differences, such as skin color and resistance to malaria, have probably arisen by natural selection. page 500

Other racial differences have probably arisen by sexual selection. page 501

24.5 Culture is a potent force for change in modern humans.

Cultural evolution and biological evolution may interact, as in the case of the evolution of lactose tolerance in regions where cattle were domesticated. page 502

Other animals possess simple versions of culture, language, and even consciousness, but the capabilities of our species in all three are truly exceptional. page 503

Self-Assessment

  1. Describe the evidence suggesting that chimpanzees are the closest living relatives of humans.

    Self-Assessment 1 Answer

    Using a phylogenic approach based on anatomical characteristics, scientists found that either chimpanzees or gorillas were the closest living relatives of humans. Later studies of hybrids of heat-denatured human and chimpanzee DNA provided evidence that human and chimpanzee DNA was more similar than human and gorilla DNA. These hybridization studies indicated that our genome differs by about 1% from that of chimpanzees.

  2. Explain the out-of-Africa theory of human origins and how studies of mitochondrial DNA and the Y chromosome support it.

    Self-Assessment 2 Answer

    The out-of-Africa theory of human origins suggests that modern-day Homo sapiens evolved from a descendant of Homo ergaster in Africa about 200,000 years ago. Mitochondrial DNA and the Y chromosome do not undergo recombination, so the only genetic variation arises from mutation. By studying the variation of these two types of genetic material from different populations of people throughout the world, scientists were able to calibrate the rate at which mutations occur, build a phylogenetic tree showing the relationships between humans and that they had a common ancestor, and determine the timeline of modern-day Homo sapien occurrence.

  3. List four anatomical differences between chimpanzees and humans, and explain how these changes facilitated walking upright.

    Self-Assessment 3 Answer

    (1) The human spine is S-shaped so that weight is distributed right over the pelvis; (2) the human foot is narrower, has a more developed heel, and a larger big toe that facilitates a springier step; (3) the foramen magnum is repositioned in humans so that the skull is balanced directly on top of the vertebral column; and (4) the human pelvis is reconfigured for an upright posture, with the internal organs sitting within it like a basin. The legs are longer and their anatomy altered, so the legs are directly under the body, facilitating a longer stride.

  4. Given the high genetic similarity of humans and chimpanzees, how can we account for the differences we see between the two species?

    Self-Assessment 4 Answer

    Differences we see between humans and chimpanzees can be accounted for by differential gene expression. Basically, the same genes are expressed in both organisms, but at different times and expression levels throughout development.

  5. Describe three possible selective factors underlying the evolution of large brains in our ancestors.

    Self-Assessment 5 Answer

    Three possible selective factors underlying the evolution of large brains in human ancestors are: (1) tool use (manual dexterity requires a complex nervous organization); (2) social living (groups require coordination, communication, and a means of integrating and acting upon the information conveyed); and (3) language (communication requires a complex nervous system).

  6. Explain how differences among different human populations arose by natural and sexual selection.

    Self-Assessment 6 Answer

    Differences among human populations arose by natural and sexual selection. Natural selection can be seen in the evolution of skin color, due to ultraviolet radiation and vitamin D, and body shape and size due to the ability to dissipate body heat. These are traits that aid survival in the appropriate environments. Sexual selection, which favors traits that promote sexual access to the other sex, can be seen in the predominance of different facial features in different races due presumably to choices made by ancestral humans when selecting mates.

  7. Provide arguments for and against the idea that culture, language, and consciousness are uniquely human.

    Self-Assessment 7 Answer

    The idea that culture, language, and consciousness are uniquely human can be supported by the observation that humans do all three of these things to a higher level of sophistication than any other organism. However, many other organisms have simpler versions of culture, language, and consciousness, so the idea that all three are uniquely human is incorrect.