January 26, 2018

Out of Africa: a theory in crisis

The sensational discovery of modern humans in the Levant 177-194 thousand years ago should cause a rethink of the currently held Out-of-Africa orthodoxy.

By Out-of-Africa, I mean here the origin of anatomically modern humans, as opposed to the earlier origin of the genus Homo or the later origin of behaviorally fully modern humans.

Two main pieces of evidence supported the conventional OOA theory:

1. The observation that modern Eurasians possess a subset of the genetic variation of modern Africans.
2. The greater antiquity of AMH humans in the African rather than the Eurasian palaeoanthropological record.

Both these observations are in crisis.

1a. The oldest African fossil AMH is in North Africa (Morocco, Jebel Irhoud); modern genetic variation does not single out this region as a potential source of modern humans. In short, modern genetic variation has nothing to say about where AMH originated.
1b. Eurasians can no longer be seen as a subset of Africans, given that they possess genetic variation from Denisovans, a layer of ancestry earlier than all extant AMH. While it is still true that most Eurasian genetic material is a subset of that of modern Africans, it is also true that the deepest known lineage of humans is the Denisovan-Sima de los huesos, with no evidence for any deeper African lineage. Within humans as a whole, Africans possess a subset of Eurasian genetic variation.
2a. African priority received a boost by 0.1My by the redating of Jebel Irhoud last year. And, non-African AMH received a boost of 0.05My by the Hershkovitz et al. paper yesterday. A very short time ago, Ethiopia boasted the oldest AMH by 0.07My and now it's tied with the Levant and beaten by Morocco. It's a bit silly to argue for temporal priority based on the spotty and ever-shifting palaeoanthropological record.
2b. It is virtually untenable to consider the ~120,000 year old Shkul/Qafzeh hominins as a failed Out-of-Africa, since it now seems that they may have been descendants from the Mislya Cave population of >50,000 or even >100,000 years earlier.

I had previously supported a "two deserts" theory of human origins in which AMH originated in North Africa (Sahara) and then left Africa >100kya as evidenced by the Shkul/Qafzeh hominins and/or the Nubian technocomplex in Arabia. While I am still convinced that AMH originated somewhere in North Africa or the Near East, I am less certain as to where.

Science 26 Jan 2018: Vol. 359, Issue 6374, pp. 456-459 DOI: 10.1126/science.aap8369

The earliest modern humans outside Africa

Israel Hershkovitz et al.

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.


January 05, 2018

Eurasian origin of mtDNA L3 and Y-chromosome DE

I've argued for a similar scenario for years, so it's nice to see a preprint on the topic.

bioRxiv doi: https://doi.org/10.1101/233502

Carriers of mitochondrial DNA macrohaplogroup L3 basic lineages migrated back to Africa from Asia around 70,000 years ago.

Vicente M Cabrera et al.

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.