Remember that Wang et al. preprint at bioRxiv on the genetic prehistory of the Greater Caucasus? Well, it’s just been published at Nature Communications under a new title: Ancient human genome-wide data from a 3000-year interval in the Caucasus corresponds with eco-geographic regions.
The authors also re-worked a few other parts of the manuscript, including the abstract and figures, but most of it looks pretty much the same as the bioRxiv version from May 2018. It’s hard for me to believe that this process took more than half a year, so I’m guessing this is just how long it takes sometimes to get a paper into this journal.
In any case, the supplementary information includes a Peer Review File (see here) with a couple of interesting comments in regards to the Proto-Indo-European (PIE) homeland debate. Emphasis is mine:
Reviewer no 2: This hypothesis about the Caucasus source of Proto-Indo-European has been advanced also for slightly other reasons by David Reich and Kristian Kristiansen, so I think it should be elaborated here by the authors and they should marshall their new results to add whatever support they can. However, this hypothesis should rest on showing a sustained admixture between Maikop and Yamnaya to serve as a bridge to Yamnaya from the Caucasus (because the authors accept Yamnaya as connected to later PIE.) It is difficult to see in the results presented here a sustained gene flow from Maikop into Yamnaya, that would sustain this hypothesis. On lines 410 and 432 the authors preferred to see the Anatolian Farmer genes that appeared in Yamnaya as flowing from southeastern Europe, with a 20% WHG component, not from Maikop, without the WHG component. If most of the c. 15% Anatolian Farmer found in Yamnaya came from the west, it leaves very little room for gene flow into Yamnaya from Maikop. If the 3% WHG that makes the difference between a western and Caucasian source of Anatolian Farmer is strongly supported by their data, that makes a Caucasian origin of PIE less likely because it reduces gene flow from Maikop into the steppes. In fact it suggests that very little south-to-north gene flow occurred during the Maikop period (except into 2 individuals from a distinct, small, local genetic group different from Maikop and Yamnaya). This is puzzling and unexpected, but also it fails to support the bridge that seems to be needed.
Reply: We’re afraid that this might be a misunderstanding. There is indeed very limited gene flow between the Caucasus and the steppe groups (apart from the examples highlighted). However, we have based our PIE-related speculations on the observation that the CHG/Iranian (green) ancestry component is increasing already during the Eneolithic north of the Caucasus. This led us to propose that this might be the actual ‘tracer dye’ of an early PIE spread, which could then also accommodate the spread of PIE south of the mountain range where this ancestry component also rises in frequency resulting in a relatively homogenised dual ancestry (Anatolian + Iranian farming-related ancestry) in Chalcolithic times (see also brown arrow in Figure 2).
A misunderstanding? Perhaps, but the impression I get from reading both the preprint and paper is that the authors really wanted Maykop to have been the source of Indo-European languages on the Pontic-Caspian steppe, even if they didn’t spell this out directly. So I’m not surprised by the peer reviewer’s line of inquiry.
I think what actually happened was that the authors got it in their heads long ago that the PIE homeland was south of the Caucasus, simply because that’s what they saw when they looked at the spread, across space and time, of some exceedingly broad and very ancient genome-wide genetic components, including one such component with roots in the Caucasus and surrounds that is found both in Yamnaya and Hittite samples. And they penciled in Maykop, probably based on archeological data, as the most likely vector for the spread of this potential PIE «tracer dye» onto the steppe.
But that didn’t work out once they had a good look at their ancient DNA from the Caucasus, and it seems they couldn’t come up with a coherent alternative theory. Little wonder, considering what their ancient DNA showed: a profound genetic differentiation between the Eneolithic/Bronze Age populations of the Caucasus and the Pontic-Caspian steppe, especially in terms of paternal ancestry, which is crucial in linguistics debates.
Whatever. I’ve already said way too much on this topic, so I’m now moving on. But I’m certainly looking forward to the genotype data from this paper. Analyzing it is going to be a hoot.
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The PIE homeland controversy: January 2019 status report
R-V1636: Eneolithic steppe > Kura-Araxes?