hlw_ch24

CHAPTER 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. The great apes include orangutans, gorillas, chimpanzees, and humans.
Analysis of sequence differences between humans and our closest relatives, chimpanzees, indicate that our lineage split from chimpanzees 5-7 million years ago.
Lucy, an unusually complete specimen of Australopithecus afarensis, demonstrates that our ancestors were bipedal by about 3.2 million years ago.
Human lineage fossils occur only in Africa until about 2 million years ago, when Homo ergaster migrated out of Africa to colonize the Old World.

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).
The mtDNA out-of-Africa pattern is supported by Y-chromosome analysis, which also shows a recent African origin of modern humans.
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.
Our species originated in Africa and subsequently colonized the rest of the planet, starting about 60,000 years ago.

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.
Neoteny is the acquisition of sexual maturity in an otherwise juvenile state; humans are neotenous, exhibiting many traits as adults that chimpanzees exhibit as juveniles.
There are many possible selective factors that explain the evolution of our large brain, including tool use, social living, and language.
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.

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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.
Humans have very little genetic variation, with only about 1 in every 1000 base pairs varying among individuals.
Most of the variation segregates 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 races.
Some racial differences, such as skin color and resistance to malaria, have probably arisen via natural selection.
Other racial differences have probably arisen via sexual selection.

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

Self-Assessment Question 1

Describe what evidence suggests that chimpanzees are the closest living relatives of humans.

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Model Answer:

Using a phylogenic approach based on anatomical characteristics, scientists found that either chimpanzees or gorillas were the closest living relatives of humans. Chimpanzees were identified as being humans’ closest living relatives when molecular techniques were used to compare the genetic material of chimpanzees and humans. Scientists found that our genomes differ by about 1% from that of chimpanzees.

Self-Assessment Question 2

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

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Model 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 time line of modern-day Homo sapien occurrence.

Self-Assessment Question 3

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

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Model Answer:

Four anatomical differences between chimpanzees and humans that allow humans to walk upright are: (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.

Self-Assessment Question 4

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

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Model 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.

Self-Assessment Question 5

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

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Model 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).

Self-Assessment Question 6

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

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Model Answer:

Differences among human populations arose by natural and sexual selection. Natural selection can be seen through 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. Sexual selection can be seen through the predominance of different facial features in different races due to humans’ choice when selecting mates.

Self-Assessment Question 7

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

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Model 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.