QUESTION: What caused the migration of early humans out of Africa? Was there sufficient climatic change to make it better to leave than take the risk of trying to adapt to some sort of environmental change?
– Shaun McArdle
ANSWER: Based on genetic and fossil evidence, it is widely accepted that humans originated in Africa. Many researchers recognize two major phases of dispersal. The first, Out of Africa 1, began almost 2 million years ago with Homo erectus, the first truly upright-walking human ancestor. The second, Out of Africa 2, began about 100,000 years ago with Homo sapiens, which evolved in Africa between the two phases of dispersal and eventually replaced archaic humans.
Other researchers consider this view simplistic. They generally agree about the early dispersal but propose that there were multiple later dispersals, some of which may have been from Europe and Asia back to Africa. One reason for the uncertainty is that human fossils are rare compared with stone artifacts, and the cultural traits implied by the tools cannot be reliably tied to biological characteristics of the populations that created them.
What caused humans to disperse is not known. Some explanations focus on unique features of human culture. Other explanations, acknowledging that other successful species also disperse, focus on environmental changes.
One hypothesis is that the first dispersal was caused by one group of humans out-competing another. The hypothesis is based on the discovery that two technologically distinct populations of humans existed at the time, and only the less advanced toolmakers dispersed from Africa, perhaps because they were at a disadvantage on their shared range. A hypothesis based on advances in toolmaking and competition between groups has also been proposed to explain the second dispersal.
Oscillations from wet to dry conditions occurred around the times of the dispersals. The climatic changes were accompanied by dispersal of other large animals, which may have been followed and exploited by human populations.
Whatever the reason the dispersals began, a factor that probably facilitated them by enhancing humans' survival is the reduction in zoonotic diseases – diseases, such as sleeping sickness, that rely on animals for transmission but also affect humans. Zoonotic diseases are especially prevalent in Africa.
Sherry Seethaler is a UCSD science writer and educator. Send scientific questions to her at Quest, The San Diego Union-Tribune, P.O. Box 120191, San Diego, CA 92112-0191. Or e-mail sseethaler@ucsd.edu. Please include your name, city of residence and phone number.
