Hot off the presses: A structured coalescent model reveals deep ancestral structure shared by all modern humans in Nature: Genetics. Most of the paper is about the mathematics and modeling used to arrive at the deep structure conclusion and where the results of each piece points. But for the layman there is the following.
we present evidence for an extended period of structure in the history of all modern humans, in which two ancestral populations that diverged ~1.5 million years ago came together in an admixture event ~300 thousand years ago, in a ratio of ~80:20%.
In the past we had a vague picture of there being very few human lineages that were essentially distinct. That was the model of physical anthropology I was taught in the 70's. Some ongoing interbreeding events between populations was considered possible but unusual. We now think quite differently, that given the timescales involved, interbreeding was always likely eventually, and the genetic river that empties into the sea that is modern mankind is more like a delta than a single current. Analogous to this:
These coexisting populations diverged and recombined, intermarrying with far-distant cousins. Two groups that had separated 1.5 million years ago interbred more than a million years later, and this was the combo that led to us. One group's genes outnumbered the other's about 4:1.
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