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.

Out of Africa-Out of Asia-Out of Africa

Roberta Estes recenty posted about the fact that Spencer & I will be revisiting The Journey of Man, fifteen years after he wrote the book and made the documentary.

As part of the preparation, I’m revisiting some uniparental results, in particular, recent ones. This preprint in bioRxiv Carriers of mitochondrial DNA macrohaplogroup L3 basic lineages migrated back to Africa from Asia around 70,000 years ago., caught my attention:

Background: After three decades of mtDNA studies on human evolution the only incontrovertible main result is the African origin of all extant modern humans. In addition, a southern coastal route has been relentlessly imposed to explain the Eurasian colonization of these African pioneers. Based on the age of macrohaplogroup L3, from which all maternal Eurasian and the majority of African lineages originated, that out-of-Africa event has been dated around 60-70 kya. On the opposite side, we have proposed a northern route through Central Asia across the Levant for that expansion. Consistent with the fossil record, we have dated it around 125 kya. To help bridge differences between the molecular and fossil record ages, in this article we assess the possibility that mtDNA macrohaplogroup L3 matured in Eurasia and returned to Africa as basic L3 lineages around 70 kya. Results: The coalescence ages of all Eurasian (M,N) and African L3 lineages, both around 71 kya, are not significantly different. The oldest M and N Eurasian clades are found in southeastern Asia instead near of Africa as expected by the southern route hypothesis. The split of the Y-chromosome composite DE haplogroup is very similar to the age of mtDNA L3. A Eurasian origin and back migration to Africa has been proposed for the African Y-chromosome haplogroup E. Inside Africa, frequency distributions of maternal L3 and paternal E lineages are positively correlated. This correlation is not fully explained by geographic or ethnic affinities. It seems better to be the result of a joint and global replacement of the old autochthonous male and female African lineages by the new Eurasian incomers. Conclusions: These results are congruent with a model proposing an out-of-Africa of early anatomically modern humans around 125 kya. A return to Africa of Eurasian fully modern humans around 70 kya, and a second Eurasian global expansion by 60 kya. Climatic conditions and the presence of Neanderthals played key roles in these human movements.

It was really hard for me to parse all the coalescene and phylogenies here. I know some readers are pretty well versed in this area so I’m curious about critiques and reactions to the above.