Chapter Summary

Focus of the chapter:

  • Definition of hominid
  • Why hominids evolved: three theories
  • Costs and benefits of bipedalism
  • The first hominids
  • Evolution and extinction of species

The term hominid is used to describe a bipedal organism with mostly grinding dentition (nonhoning). Both traits evolved early in the evolutionary record. Hominid origin is highly debated, with no clear "missing link" agreed on by scientists.

As early as Charles Darwin, theories of hominid evolution were developed. As evidence and analytic techniques changed, so did the nature of the theory, moving from simple reliance on hunting theories to those involving the environment and social behavior.

Bipedalism (walking on two limbs) as a structural adaptation has both benefits and costs. It’s an evolutionary balance among energy requirements, locomotion, and reproduction.

The earliest hominids likely evolved 6–7 mya and are in the genus Sahelanthropus. Subsequent fossil finds have been placed into the genera Orrorin, Ardipithecus, and Australopithecus. Each is characterized by changes in anatomy and brain development.

At various points in early hominid evolution, multiple forms existed in the same locations. Some forms became extinct. Others (Australopithecus africanus) may have evolved into the genus Homo.