CHAPTER 20 Summary

• Humans are 99.9% genetically identical to one another regardless of geographic origin. Biologically distinct human races do not exist.
• All humans are members of a single biological species, Homo sapiens, which evolved relatively recently—just 200,000 years ago.
• Physical features shared by people within populations reflect adaptations to specific environments.
• Alleles can be harmful, beneficial, or neutral in their effect on survival and reproduction.
• Skin color most likely evolved in response to environmental UV levels, an example of evolution by natural selection. Alleles for darker skin conferred an advantage in sunnier environments, while alleles for lighter skin conferred an advantage in regions that receive weak sunlight.
• Skin color represents an evolutionary trade-off between the need for vitamin D, which requires adequate sunlight for its production, and the need for folate, which is destroyed by too much sunlight.
• Fossil evidence shows that humans and apes descended from a common ancestor and that walking upright preceded development of a big brain. There were many species that could walk upright before Homo sapiens appeared.
• Fossil and DNA evidence shows that anatomically modern humans first emerged in Africa, approximately 200,000 years ago, and subsequently spread to other continents, beginning about 70,000 years ago.
• All modern-day humans can trace a portion of their genetic ancestry back to a single woman, Mitochondrial Eve, who lived 200,000 to 150,000 years ago in Africa.
• Humans evolved from apelike primate ancestors who likely had light skin. Darker skin emerged in tandem with loss of body hair as our hominid ancestors ventured into the hot savannah, while lighter skin emerged as humans migrated farther north.

• MORE TO EXPLORE

  • Jablonski, N. G., and Chaplin, G. (2010) Human skin pigmentation as an adaptation to UV radiation. Proceedings of the National Academy of Sciences 107: 8962–8968.
  • Cann R. L, et al. (1987) Mitochondrial DNA and human evolution. Nature 325: 31–36.
  • Cann, R. L, and Wilson, A. C. (2003) The recent African genesis of humans. Scientific American 13(2):54–61.
  • Groleau, R. (2002) Tracing ancestry with mtDNA. NOVA Online: http://www.pbs.org/wgbh/nova/neanderthals/mtdna.html.
  • White, T. D., et al. (2009) Ardipithecus ramidus and the paleobiology of early hominids. Science 326(5949):64, 75–86.
  • Stringer, C., and McKie, R. (1996) African Exodus: The Origins of Modern Humanity. New York: Henry Holt.