A simplified model of human demographic history showing deep population structure ~1.5 Ma to ~300 ka ago shared by all present-day humans, as inferred by cobraa (red). Arrows indicate the direction of gene flow, with admixture events (double arrows) labeled by their percentage genetic contribution to the recipient population. Of the two ancestral branches A and B, A represents 80% of subsequent ancestry and features a sharp bottleneck immediately after its founding. Dashed arrows between Khoisan and other African populations reflect the fact that this divergence, the deepest among present-day human populations, has involved ongoing or intermittent gene flow 33, 37, 42. The y axis represents time in years before the present.
