About a month ago I posted quite a bit about Neandertal introgression into modern humans. That is, the uptake of Neandertal alleles are a few specific adaptively salient loci even while ancestry remains predominantly African. Now John Hawks and Gregory Cochran have a new paper out, Dynamics of Adaptive Introgression from Archaic to Modern Humans, which synthesizes the first of the new results. The paper is open access, but I'll throw out the money shot:
We suggest that adaptive introgression of alleles from archaic humans may be one of the central mechanisms leading to the "human revolution." The behavioral characteristics of modern humans, including the employment of symbolic culture and sophisticated technologies, followed the attainment of modern human anatomical features by a considerable delay (Klein and Edgar 2002). The notion that a single small population of incipient modern humans had the perfect genetic combination for ultimate success seems quite improbable. Instead, the long coevolution of modern anatomy and behavior in contact with archaic humans, even as those archaic populations appeared to diminish, provided a rich source of adaptations for the expanding modern population. With current genomic techniques, we are beginning to find these archaic genes. We expect that they will prove central to the story of modern human origins.
What Hawks and Cochran propose is that introgression of adaptive alleles into the Out-of-Africa substratum is the key to humanity as we understand it.