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Out of Africa to Out of Eden (well, perhaps not yet)

The recent African origins hypothesis for modern humans had several things going for it. First, most of the old fossils that look like modern humans were in Africa. Chris Stringer and others were pushing the African origins of our modern lineage before genetics came to the fore. But of course, you also have DNA. The mtDNA, Y, and autosomal DNA, which tends to show a pattern where Africans are more diverse, and non-Africans are nested within phylogenies of Africans.

In the 2000s the “Out of Africa” model got a little out of control. The stylized narrative was that a small tribe of East Africans developed some genetic mutation that allowed them to exterminate all other human lineages (e.g., language). This is best encapsulated in Richard Klein’s The Dawn of Human Culture. The British science fiction author Stephen Baxter used this idea as a frame in his novel Evolution (the innovation in this novel was religion though). In this view modern humanity was an African saltation, a great leap forward.

We’re at a different point now. The idea of admixture and/or introgression from non-African lineages into African modern humans is widely accepted. Additionally, both genomic inference and paleontology are pushing the roots of modern humanity much further than ~50,000-60,000 years before the present.

So it’s not as surprising to see a paper like this, The earliest modern humans outside Africa:

To date, the earliest modern human fossils found outside of Africa are dated to around 90,000 to 120,000 years ago at the Levantine sites of Skhul and Qafzeh. A maxilla and associated dentition recently discovered at Misliya Cave, Israel, was dated to 177,000 to 194,000 years ago, suggesting that members of the Homo sapiens clade left Africa earlier than previously thought. This finding changes our view on modern human dispersal and is consistent with recent genetic studies, which have posited the possibility of an earlier dispersal of Homo sapiens around 220,000 years ago. The Misliya maxilla is associated with full-fledged Levallois technology in the Levant, suggesting that the emergence of this technology is linked to the appearance of Homo sapiens in the region, as has been documented in Africa.

Now, the reality is that Israel is arguably part of “Greater Africa” biogeographically. So it isn’t that surprising. Or it shouldn’t be.

But, this reinforces the reality that anatomically modern humans were geographically already widespread ~200,000 years ago. I would say that this informs and updates our estimation of the plausibility of the Jebel Irhoud modern humans in Morocco, who flourished ~300,000 years ago. It also makes more sense of the reality that most of the ancestors of the Khoisan likely diverged from other modern lineages ~200,000 years ago (or more, depending on who you talk to). Finally, it makes recent archaeological finds of modern humans or their artifacts in East Asia tens of thousands of years before the great expansion of neo-African humanity50,000-60,000 years before the present much more plausible.

There has been some genetic evidence for modern(ish) human expansion before the 50,000 year date. So this isn’t resting only on paleontological evidence.

Where does this leave us? In The Guardian David Reich observes that ‘It’s important to distinguish between the migration out of Africa that’s being discussed here and the “out-of-Africa” migration that is most commonly discussed when referring to genetic data. This [Misliya] lineage contributed little if anything to present-day people.’

Obviously, this is an important point. But we know that the first modern humans to settle Europe did not leave any descendants either.  The modern human settlement of Europe was still nevertheless important. Second, these early wave humans may have given modern populations adaptive variants that are present at high frequencies in modern lineages.

Finally, there’s the issue that this may reorient our understanding about the demographic origins of human populations. Ever so slightly our priors as to an African genesis for our modern lineage are getting weaker. You have two very old modern fossils on the northwest and northeast fringe of the continent. Ten years back the arguments was between those who argued for an East African origin (most), or a minority who favored a Southern Africa one. Now the whole continent, and perhaps even Arabia, are game.

Ultimately, as always, ancient DNA is going to be the final arbiter.

10 thoughts on “Out of Africa to Out of Eden (well, perhaps not yet)

  1. Very interesting. It seems odd that humanity would leave Africa multiple times over hundreds of thousands of years, but for some reason that 50,000-60,000 years ago wave was the one that swamped everything else. What were they bringing to the metaphorical battlefield that their close relations already outside of Africa didn’t have?

  2. Glorious thing it was the jaw, if any other.
    Now, does that magic new sequencing machine from 2015 can work with things this old? The aDNA, mtDNA and yDNA of this fella must be sequenced ASAP to find if he has left continuity or if he was a dead end.

    Also, more and more it’s becoming possible to me that Neanderthals and OoA-Sapiens actually interbred many many times – since 300.000 years ago maybe? – closing the gap of these two. We know that Early Neanderthals and Late Neanderthals were different morphologically, with the Later being more “gracile”, this might have been influenced by Sapiens admixture.
    In short, we’re about 99,7% like the Neanderthals today, but it might be the case that we were much less in the past and this might be the difference between those who stayed and those who left Africa.
    Or it all could be standard divergent evolution (comparing the stay/left populations) and convergent evolution (comparing the left/neanderthal populations).

  3. Also, more and more it’s becoming possible to me that Neanderthals and OoA-Sapiens actually interbred many many times – since 300.000 years ago maybe? – closing the gap of these two.

    hm. so thinking about this this should push the genomic estimates of divergence of the lineages further back, as gene flow would reduce genome-wide distance that would have otherwise accrued.

    but yes, some geneticists are wondering about multiple instances of reciprocal gene-flow now (neander<->modern human).

  4. Razib, The Guardian’s take on this news has a bit of David Reich saying that this new guy didn’t contribute to modern humans. My questions:
    1. He’s saying this based on the models or he sequenced it already?
    2. If he has time to do that, if he’s doing that, what about the South Asia paper?
    3. Is there a paper in the making about the genetic structure of this new guy?

  5. 1. He’s saying this based on the models or he sequenced it already?

    inference. the genomics does seem to indicate expansion 50K BP. also archaeology correlates perfectly.

    3. Is there a paper in the making about the genetic structure of this new guy?

    they always try. i don’t know myself.

  6. Is the following a plausible model?

    (1) A population of AMH located in Arabia begins to rapidly expand 50-60K years ago to the north/east. The expanding group absorbs small populations of other AMH, Neanderthals and then later Denisovians as it pushes to every corner of Eurasia and beyond. But these other human groups, perhaps thinly populated, are largely overwhelmed, leaving only a small minority of DNA in the final non-African population.

    (2) Around the same time, this Arabian AMH population expands south into Africa. It also absorbs other AMH and unknown ancient humans along the way. But in Africa the expansion does not replace the other human groups to nearly the same extent as in Eurasia. Most Sub-Saharan Africans instead retain substantial DNA from non-“Out-of-Arabia” humans, and groups like the Pygmies and San are barely impacted by this expansion. As a result, modern Africans retain greater genetic diversity.

    (3) The part of the original population that stays in Arabia and surrounding areas becomes the “Basal Eurasians”.

  7. So, in a Warfare perspective, the Sapiens tried to “invade” Eurasia since about 180.000 to 60.000 years ago, but was repelled mainly by the Neanderthal defense lines in the Middle East. Subsequently to that, Sapiens were captured and reproduced with Neanderthals and vice-versa.
    When the Neanderthal Empire was at its final legs due to unrelenting Cold Ages and their low birth rates, the Tropical+Fast Reproducing Sapiens invaded for good without resistance and took Eurasia for themselves.

    It’s nice to see things in Warfare perspective because peace is not the norm, and we know how many Genocides happened at least since the Neolithic, with the EEF killing the males and taking the female WHG for themselves, then the reverse slowly happening when the WHG retook territory and then when the Yamnaya imposed themselves everywhere. Discrepancies in how the yDNA of a population gets thrashed in favour of the newcomer and how the mtDNA always pile up and add, instead of disappearing shows us these scenarios.

    Their efficiency must also be taken into consideration – Neanderthals thrived due to the great Megafauna, and it was slowly vanishing by the time of their demise, which means that the Hunting Grounds/Territory became even more important and fights would occur because of that. We would see again this form of Territorialism when Agriculture tied people to the land.

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