Most of the history of the human species is in Africa, which is why I wrote a Substack trying to outline what I think are various alternative models about what’s happened. But the last 100,000 years has been defined by the migration out of Africa for many. The above chart is a simplified representation of what I think is a good model for what happened. For a more complex and thorough treatment, check out Genetics and Material Culture Support Repeated Expansions into Paleolithic Eurasia from a Population Hub Out of Africa.
Also, new PNAS paper, The role of genetic selection and climatic factors in the dispersal of anatomically modern humans out of Africa:
The evolutionarily recent dispersal of anatomically modern humans (AMH) out of Africa (OoA) and across Eurasia provides a unique opportunity to examine the impacts of genetic selection as humans adapted to multiple new environments. Analysis of ancient Eurasian genomic datasets (~1,000 to 45,000 y old) reveals signatures of strong selection, including at least 57 hard sweeps after the initial AMH movement OoA, which have been obscured in modern populations by extensive admixture during the Holocene. The spatiotemporal patterns of these hard sweeps provide a means to reconstruct early AMH population dispersals OoA. We identify a previously unsuspected extended period of genetic adaptation lasting ~30,000 y, potentially in the Arabian Peninsula area, prior to a major Neandertal genetic introgression and subsequent rapid dispersal across Eurasia as far as Australia. Consistent functional targets of selection initiated during this period, which we term the Arabian Standstill, include loci involved in the regulation of fat storage, neural development, skin physiology, and cilia function. Similar adaptive signatures are also evident in introgressed archaic hominin loci and modern Arctic human groups, and we suggest that this signal represents selection for cold adaptation. Surprisingly, many of the candidate selected loci across these groups appear to directly interact and coordinately regulate biological processes, with a number associated with major modern diseases including the ciliopathies, metabolic syndrome, and neurodegenerative disorders. This expands the potential for ancestral human adaptation to directly impact modern diseases, providing a platform for evolutionary medicine.
This “stand still” out of Africa has been identified for a few decades now in the genomic data. I’m not sure people necessarily believe it…but it’s clear that non-Africans are very homogeneous and closely related to each other compared to various diverse African groups. That indicates a long period of isolation and homogeneity. The “Basal Eurasians” were part of this.
But earlier “modern” or “para-stem” populations may not have been. The Altai Neanderthal samples that date to 100,000 years ago have some “modern” ancestry. How? The best assumption right now is that these were very early migrants out of the African ur-heimat that mixed with Neanderthals in their eastern range, likely coming up through East Asia? Possibly mixed with Denisovans?
The point here is that some elements of the “weakly structured model” probably applies to Eurasia too. Africa is a big continent and good habitat for humans. Pulses out of this continent probably happened all the time. It’s just that the last Eurasian expansion about 50,000 years ago erased everything earlier (or absorbed it). Though I still think if we get ancient DNA from Laos it may turn out that 1% or something of the ancestry of Andamanese may date to a population that diverged 85,000 years ago from the main out-of-Africa group, before their isolation in the Near East (since the divergence is not that great, I don’t think the models would have good power to detect this proportion).