Did warfare drive out-of-Africa migration?

Roving bands of men might have waged history's first traceable war against the ancestors of all Europeans, Asians and other non-Africans, some 60,000 years ago. A new analysis of DNA variations in contemporary humans indicates that non-Africans descend from a population that contained far more males than females. This is potential evidence for conquests of the first people who embarked out of Africa, says Alon Keinan, a geneticist at Harvard Medical School in Boston. It might be that they killed some males, stole the females, and kept on moving, he says. A steady trickle of peaceful wandering men could have accomplished the same genetic effect – but if prehistoric migrations worked anything like Viking conquests, or the discovery of the New World, male migrants did not go looking for peace and love. Keinan's theory rests on comparisons of more than 100,000 genetic differences, peppered across the genomes of African, Asian and European men. In populations where males pair equally with females, on average they will have three X-chromosomes for every four of a non-sex chromosome called an autosome. This is because women have two Xs and men just one. If more men than women pass on their DNA over time, the female contribution to the gene pool falls, resulting in less X-chromosomes. "You have many more grandfathers than grandmothers," Keinan says. This skew exists in peoples from all parts of the world except Africa, Keinan and colleague David Reich have found. The ancestors of Europeans and Asians left Africa sometime between 60,000 and 100,000 years ago. Keinan's team speculate that males from Africa, who may have settled in the Arabian Peninsula, Egypt, or elsewhere, attacked the first "out of Africa" population. "It sounds plausible to me," says Martin Richards, a geneticist who studies human history at the University of Leeds. "We don't know the route people used to get out of Africa," says Chris Stringer, an anthropologist at the Natural History Museum in London. But he thinks the Nile valley is a strong contender for the path of the first migrants – and perhaps their later adversaries. However, the chance of finding archaeological evidence for these migrants is slim. "You're looking for a population that was there only a short period of time, perhaps only 10 generations, so the physical impact of that population in that environment wouldn't be enough to detect," Reich says. Their analysis also challenges a study published earlier in 2008, which found that all humans descend from fewer numbers of males than females. The researchers suggested that polygyny, where few men procreate with many women, accounts for this result. "It's possible, in principle, that both are true in some level," says Reich. Polygyny that occurred over the last million years of human evolution could have left an imprint in our genomes, says Michael Hammer, a geneticist at the University of Arizona, who led that study. Reich and Keinan, on the other hand, focused their analysis on the period when anatomically modern humans left Africa. "We'll have to figure out this issue in future work," Reich says.

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