Chapter 23
  Summary   Questions   Media Resources

23.1 The evolutionary path to humans starts with the advent of primates.

 Prehensile (grasping) fingers and toes and binocular vision were distinct adaptations that allowed early primates to be successful in their particular environments.
 Mainly diurnal (day-active) anthropoids and mainly nocturnal (night-active) prosimians diverged about 40 million years ago. Anthropoids include monkeys, apes, and humans, and all exhibit complex social interactions and enlarged brains.
 The hominoids evolved from anthropoid ancestors about 25 million years ago. Hominoids consist of the apes (gibbons, orangutans, gorillas, and chimpanzees) and upright-walking hominids (human beings and their direct ancestors).

1. Which characteristics were selected for in the earliest primates to allow them to become successful in their environment?
2. How do monkeys differ from prosimians?
3. How are apes distinguished from monkeys?
4. What is the best explanation for why humans and chimpanzees are so similar genetically?

Evolution of Primates

Primate Evolutionary Tree
Ape and Hominid Skeletons

23.2 The first hominids to evolve were australopithecines.

 Early hominids belonging to the genus Australopithecus were ancestral to humans. They exhibited bipedalism (walking upright on two feet) and lived in Africa over 4 million years ago.

5. When did the first hominids appear? What were they called? What distinguished them from the apes?

23.3 The genus Homo evolved in Africa.

 Hominids with an enlarged brain and the ability to use tools belong to the genus Homo. Species of early Homo appeared in Africa about 2 million years ago and became extinct about 1.5 million years ago.
 Homo erectus appeared in Africa at least 1.5 million years ago and had a much larger brain than early species of Homo. Homo erectus also walked erect and presumably was able to talk. Within a million years, Homo erectus migrated from Africa to Europe and Asia.

6. Why is there some doubt in the scientific community that Homo habilis was a true human?
7. How did Homo erectus differ from Homo habilis?

Human Evolution
The First American

23.4 Modern humans evolved quite recently.

 The modern species of Homo appeared about 600,000 years ago in Africa and then migrated from there to Europe and Asia.
 The Neanderthals appeared in Europe about 130,000 years ago. They made diverse tools and showed evidence of symbolic thinking.
 Studies of mitochondrial DNA suggest (but do not yet prove) that all of today's human races originated from Africa.
 Categorization of humans into races does not adequately reflect patterns of genetic differentiation among people in different parts of the world.

8. The greatest number of different mitochondrial DNA sequences in humans occurs in Africa. What does this tell us about human evolution?
9. How did Cro-Magnons differ from Neanderthals? Is there any evidence that they coexisted with Neanderthals? If so, where and when?
10. Are the commonly recognized human races equivalent to subspecies of other plant and animal species?

Hominid History

  Scientists on Science
  How Scientists Think
  Student Papers

  Bioethics Case Studies
  General Biology Weblinks

Essential Study Partner
Multiple Choice Quiz