Analysis suggests genetic mixing occurred in Africa around 35,000 years ago. According to a study published in the Proceedings of the National Academy of Sciences, up to 2 percent of the genomes of some modern African populations may originally come from a closely related species.
     Palaeontologists have long wondered whether modern humans came from a single, isolated population or a mix of various species. Last year, an analysis comparing the Neanderthal genome sequence to that of modern Homo sapiens showed that some interbreeding did take place between the two species in Europe some time between 80,000 and 30,000 years ago.
     It has been a mystery whether similar mixing took place among Homo species even earlier, before the populations that became modern humans left Africa. To find out, evolutionary biologist Michael Hammer at the University of Arizona in Tucson (USA) and his colleagues studied DNA from two African hunter-gatherer groups, the Biaka Pygmies and the San, as well as from a West African agricultural population known as the Mandenka.
     Each of these groups is descended from populations that are thought to have remained in Africa, meaning they would have avoided the genetic bottleneck effect that usually occurs with migration. This means the groups show particularly high genetic diversity, which makes their genomes more likely to have retained evidence of ancient genetic mixing.
     Because direct comparison to archaic specimens wasn't possible, the authors used computer models to simulate how infiltration from different populations might have affected patterns of variation within modern genomes. On chromosomes 4, 13 and 18, the researchers found genetic regions that were more divergent on average than known modern sequences at the same locations, hinting at a different origin. Hammer and his colleagues say these sequences must have come from a now-extinct member of the Homo genus that broke away from the modern human lineage around 700,000 years ago.
     Hammer says this disproves the conventional view that we are descended from a single population that arose in Africa and replaced all other Homo species without interbreeding. "We need to modify the standard model of human origins," he concluded.

Edited from Nature News, Popular Archaeology, EurekAlert! (5 September 2011)