Reevaluating “multiple origins” for modern humans

Following up on the post below, The Deep Origins Of East African Hunter-Gatherers, as well as some discussions on Twitter, I think I want to do some clarification about where I think we are now. My thoughts shouldn’t be a surprise if you have read everything I’ve said, but I may not have put them all together in one place.

Around the turn of the century, nearly twenty years ago, the consensus had definitively turned against a “multiregional” origin of modern humans, toward one where an “out of Africa” migration ~50,000 years ago was paramount. Many people took the “paramount” part and simply asserted that we are all Africans descended from a population that flourished in the east of the continent about 50,000 years ago. There was a lot of circumstantial evidence to support this, at least spottily, from both archaeology and genetics. There were also problems and lacunae in both fields. But the data was spotty enough that the extreme position was defensible.

We now have a lot more information and need to update our model. First, most people agree that indigenous Eurasian hominins, Neanderthals and Denisovans, contributed to the ancestry of people outside of Sub-Saharan Africa. Additionally, it’s been evident for a long time now that the massive population bottleneck that is present in all non-African populations dating to ~50,000 years ago is far less evident in Sub-Saharan African genomes.

Finally, it’s pretty clear that humans with modern morphology were present within Africa for hundreds of thousands of years before the movement out of Africa.

Therefore, a new reevaluation of the old model that is converging is a possibility is that multi-regionalism was operative within Africa for hundreds of thousands of years, followed by a massive expansion on the northeast edge of Africa that resulted in most of the ancestry of other human groups outside of the continent, with some assimilation (e.g., Neanderthal). This is a far more complicated model than the older one, but sometimes the truth is more complicated than simplicity.

But I think we’ll probably need to make further modifications, and that’s because gene flow is not always unidirectional. Specifically, the Y chromosomal work, in particular, is strongly indicative of migration of lineages more typical of Eurasians expanding within Africa within the last 50,000 years. And, as a commenter on this weblog has pointed out, even the “deep lineages” within Africa, Y haplogroups A and B, show signs of massive expansion within the last 50,000 years.

This may mean that a population liminal to Africa and Southwest Asia underwent a very rapid expansion ~50,000 years ago. The replacement of indigenous lineages was far more thorough outside of Africa, with 5% or less assimilation in most places. But, it probably impacted Africa as well. Though a larger fraction of diverged modern ancestry persisted in Africans than Eurasian hominin ancestry in non-Africans. In other words, the high genetic diversity of Africans today, and particular groups like the Khoisan, is due to the mixture between an ancient migration from the same population that was the source of “out of Africa” in Eurasia and Oceania, and disparate deeply structured lineages within Africa, that date back 200-400 thousand years ago.

Additionally, I think some earlier “modern” lineages were assimilated in eastern Asia with the latest migration out of Africa. And, some of the ancestry within Africa probably predates the origin of anatomically modern humans, analogous to the case of Neanderthals and Denisovans.

Note: This is not that different of a model from Dienekes Pontikos’ ideas in the 2000s, More support for the Afrasian/Palaeoafrican hypothesis, at the high level. Basically the more evidence has come in, the less crazy his model has gotten.

8 thoughts on “Reevaluating “multiple origins” for modern humans

  1. Some still haven’t given up on one explanation for the bottleneck, except for 72-1k BCE rather than 50: Donald Prothero, “When Humans Nearly Vanished: The Catastrophic Explosion of the Toba Volcano”. Published October 2018.

  2. 50,000 years is too recent for Eurasian founding population.

    “In other words, the high genetic diversity of Africans today, and particular groups like the Khoisan, is due to the mixture between an ancient migration from the same population that was the source of “out of Africa” in Eurasia and Oceania, and disparate deeply structured lineages within Africa, that date back 200-400 thousand years ago.”

    This makes sense. Do Khoisan uniparentl markers (mtDNA, Y DNA) show recent common nodes with other Africans? If so, they aren’t a 100,000 year isolate. Being a 100,000 year isolate is impossible anyways. I can’t imagine they share nothing in common with other Africans for that long.

  3. @ Samuel Andrews, how far back do you think the Eurasian founding population goes?

    Razib, question about the Eurasian/OoA bottleneck. Is it an absolute certainty that Eurasians all descend from a single, highly drifted founding population, or could there rather have been several small, discreet sub-populations, that happened to mix-up with one another circa 50,000-60,000 years ago at a certain point, and then that hybridized population expanded to colonize the rest of the world?

  4. “Additionally, I think some earlier “modern” lineages were assimilated in eastern Asia with the latest migration out of Africa. And, some of the ancestry within Africa probably predates the origin of anatomically modern humans, analogous to the case of Neanderthals and Denisovans.”

    Why do you suggest this? I have read about the very early modern human remains in East Asia dating back ~100,000 years ago (such as the teeth at Fuyan Cave), but have not heard of any studies on uniparentals/autosomal DNA that signal a surviving pre-OOA population which contributed to modern populations.

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  5. @Anonymous

    **“Additionally, I think some earlier “modern” lineages were assimilated in eastern Asia with the latest migration out of Africa. And, some of the ancestry within Africa probably predates the origin of anatomically modern humans, analogous to the case of Neanderthals and Denisovans.”**

    I’m also very interested why this is suggested, what lines of inquiry might confirm or disprove this hypothesis, and how this line of inquiry might be pursued.

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