Behavioral and Biological Origins of Modern Humans

by Dr. Richard G. Klein
Stanford University

Narrative Index

Two theories dominate current thinking on the origin of modern humans. The first, often known as the "out -of-Africa" theory, posits that from at least one million years ago, human populations followed different evolutionary trajectories on different continents, culminating by 100,000 years ago in the emergence of a least three continentally distinct human populations. In the Far East, there were the still poorly documented nonmodern people who probably represent an evolved end product of Homo erectus; in Europe, there were the equally nonmodern Neanderthals; and in Africa, there were modern or near-modern humans. In its most extreme form, the "out-of-Africa" theory posits that modern people expanded from Africa, beginning 60,000-50,000 years ago, and replaced the Neanderthals and equally archaic east Asians without gene exchange or inter breeding. In its less extreme form, "out-of-Africa" allows for some gene flow between expanding moderns and resident archaic populations.

The primary alternative to "out-of-Africa" is the theory of multiregional evolution, which postulates that modern humans originated essentially everywhere--in Africa, but also in Europe and Asia--where non-modern humans had lived previously. Proponents of the multiregional model agree that widely dispersed human populations tended to diverge morphologically from the time they first colonized Eurasia(one million years ago or before), but they argue that continuous gene flow ensured the rapid spread of highly adaptive novelties (like larger brains) and thereby kept all human populations on the same fundamental evolutionary track toward modern people.

It is still possible to argue about these theories, but burgeoning fossil, archeological, and genetic evidence now supports the "out-of-Africa" theory far more strongly.