The Yamnaya outlier

For a while now I’ve been arguing that the more exotic and southern, non-Eastern European Hunter-Gatherer (EHG), part of the Yamnaya genotype mostly made its way into the Pontic-Caspian steppe via female mediated gene flow, probably as a result of the practice of female exogamy with populations in the North Caucasus. For instance:

A plausible model for the formation of the Yamnaya genotype

The genotype data for the ancients that I focused on in that blog entry is now freely available in the Mathieson et al. 2018 dataset via the Reich Lab website (see here). So let’s take a closer look at one of these samples, Yamnaya_Ukraine_outlier I1917, easily the most Caucasus/Near Eastern-shifted Yamnaya individual in the ancient DNA record to date.
Yes, it’s a female, with a very Near Eastern mtDNA haplogroup to boot, so she cannot, in all likelihood, be the result of mixture between a Caucasus/Near Eastern father and Eastern European mother. This is where she plots in my Principal Component Analysis (PCA) of ancient West Eurasia.

Unlike the rest of the Yamnaya samples, Yamnaya_Ukraine_outlier I1917 is sitting much closer to modern-day North Caucasians, such as Chechens and Lezgins, than to the vast majority of Europeans, ancient and modern-day. Looking at this plot, it’s tempting to think that she might represent an as yet unsampled ancient population from the North Caucasus, or nearby southernmost steppes, that once bridged the genetic gap between Eastern Europe and the Caucasus, but has since disappeared from the scene.
Using the Global25/nMonte method, I can try to infer which ancient populations this so called outlier is most closely related to. This might give me some clues as to the origin of at least a part of the southern ancestry in Yamnaya. I’ve chosen to go with Yamnaya_Samara I0429 as the Yamnaya reference, because he’s both one of the oldest and least Caucasus-shifted Yamnaya individuals in my dataset (marked on an earlier version of the above plot here). And at the risk of overfitting, I’ll throw in six other reference options. The relevant datasheets are available here and here.

[1] distance%=2.3661 / distance=0.023661
Yamnaya_Ukraine_outlier:I1917
Yamnaya_Samara:I0429 43.95
Armenia_EBA 28.3
Armenia_ChL 9.95
Iran_ChL 8.1
Ukraine_N 5.6
Trypillia 4.1
CHG:KK1 0

It’s a fascinating result, even if somewhat overfitted. The Global 25/nMonte method is more sensitive to recent genetic drift than formal statistics-based modeling, so I’m not surprised that Caucasus Hunter-Gatherer (CHG) KK1 is not shown to be an important source of ancestry for Yamnaya_Ukraine_outlier I1917.
Instead, the chronologically more proximate samples from Early Bronze Age and Chalcolithic Transcaucasia (Armenia_EBA and Armenia_ChL, respectively) top the list after Yamnaya_Samara I0429, which does make good sense. Moreover, minor gene flow from Neolithic foragers and farmers from what is now Ukraine (Ukraine_N and Trypillia, respectively) is implied, and obviously this is also very plausible. Thus, it seems likely that population movements from the Armenian Plateau did have an impact on at least a part of the Yamnaya poulation, even if mostly on its maternal side.
Another question worth asking is whether, conversely, people like Yamnaya_Ukraine_outlier I1917 were, by and large, an important source of southern admixture in Yamnaya. At least according to this Global 25/nMonte model, perhaps they were.

[1] distance%=4.6173 / distance=0.046173
Yamnaya_Samara
EHG 43.25
Yamnaya_Ukraine_outlier:I1917 41.95
CHG:KK1 14.8
Armenia_ChL 0
Armenia_EBA 0
Iran_ChL 0
Trypillia 0
Ukraine_N 0

See also…
Unleash the power: Global 25 test drive thread
Another look at the genetic structure of Yamnaya
Late PIE ground zero now obvious; location of PIE homeland still uncertain, but…

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