Humans migrated out of Africa in at least three major waves and later emigrants probably interbred with the archaic people they encountered, according to the results of a new genetic study. The findings, announced today in the journal Nature, challenge a popular view of human evolution that holds that anatomically modern humans replaced the archaic inhabitants of Europe and Asia.

Previous genetic analyses aimed at testing the various hypotheses of modern human origins have tended to focus on a single DNA region, such as maternally inherited mitochondrial DNA. And in many cases the results of those studies have been interpreted as support for the so-called Out of Africa replacement model. In the new study, however, Alan Templeton of Washington University analyzed gene trees representing a number of different genome regions to reconstruct the evolutionary history of our species. His results support a scenario in which the human lineage initially left Africa around 1.7 million years ago and subsequently expanded at least twice after that--at around 600,000 years ago and again at roughly 95,000 years ago. Importantly, the work indicates that the anatomically modern people who left Africa most recently made love, not war, with the archaic Eurasians they encountered. Had that migration instead involved replacement, Templeton says, the genetic signatures of the earlier expansion and those of recurrent gene flow between populations would have been wiped away.

"Humans expanded again and again out of Africa," Templeton concludes, "but these expansions resulted in interbreeding, not replacement, and thereby strengthened the genetic ties between human populations throughout the world."

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