Friday, July 07, 2017

Genetic Data Indicates Another Migration of Archaic Homo sapiens Out of Africa

Ars Technica is running a story based off of a Nature Communications article about a migration of archaic Homo sapiens out of Africa that took place more recently than the one at c. 650 kya that was hypothesized to have given rise to the Neandertals. Cathleen O'Grady writes:
The picture painted by nuclear DNA (nDNA) is that, between 765,000 and 550,000 years ago, our ancestors in Africa diverged into two groups. One group would eventually lead to our own species, although we wouldn't make an appearance until around 200,000 years ago. The other group would lead to Neanderthals and the closely related Denisovans. This proto-Neanderthal/Denisovan group left Africa for Eurasia at some point; sometime around 430,000 years ago, they diverged into distinct Neanderthals and Denisovans.

But the picture painted by mtDNA is different. Neanderthal mtDNA is more similar to modern humans than it is to Denisovan mtDNA. And the divergence date between us and them, when estimated based on mtDNA, is much more recent—between 498,000 and 295,000 years ago.

Some researchers have suggested that you can explain this mixed genetic evidence if Neanderthals interbred with another, more recent African group of humans. This would provide them with different mtDNA after they split from Denisovans. And that, in turn, means that there must have been humans, closely related to our own species, who left Africa for Europe far earlier than previously suspected.
It is not such a stretch to suggest that the recent finds from Jebel Irhoud may be a population related to this mystery population.  The Jebel Irhoud crania show modern characteristics in the face and have been placed in the modern Homo sapiens clade (if provisionally).  Whether or not they have enough traits found in later humans to be the stem group of anatomically modern Homo sapiens is not currently known.  Jebel Irhoud is a hop, skip and a jump to Gibraltar, which we know had already provided an avenue of migration for earlier populations of early Homo which gave rise to Homo heidelbergensis in Europe.

As the story notes, there are remarkable similarities in stone tool technologies between the northern  African Middle Stone Age and the European late Acheulean and early Mousterian tools.  There are Levallois flakes found in both areas, as well as similarly-made knives and points.  This suggests that populations in North Africa had some contact with those of southern Europe.  As we push the age of the modern human clade back in time, these discoveries help to frame the appearance of modern humans and how they interacted with the populations around them.  Exciting times!

The Nature Communications article, Deeply divergent archaic mitochondrial genome provides lower time boundary for African gene flow into Neanderthals can be found here and was free when I read it.

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