Saturday, June 15, 2024

Remarks on Skourtanioti 2024 paper on the Genetic History of South Caucasus

This is the first paper with a large number of ancient DNA samples from Georgia. The bulk of them are from eastern Georgia, within the Kur-Araxian basin, which permitted the authors to compare the genetic histories of modern Armenia and Georgia. The time transect starts from the Early Bronze Age and the Kura-Araxes culture. Genome-wide, the Kura-Araxes samples from eastern Georgia are similar to those found in Armenia. The Y-DNA of all three samples is J1 (Z1842). We now have 12 Y-DNA samples from Kura-Araxes, and 9 of them (75%) are J1, which further reinforces the idea that J1-Z1842 expanded with the Kura-Araxes culture and was probably the most important Y-DNA in the northeastern parts of the Kura-Araxes horizon.

Next, in the Middle Bronze Age period (2200-1500 BC), there is an increase in steppe ancestry. The Y-DNA R1b-Z2103 and I2a2b appear associated with the Trialeti-Vanadzor culture. Apparently, those steppe pastoralists had a lower impact on northeastern parts of Georgia, likely due to the geography and high mountains. We see a preponderance of J2 there, while the southeastern regions of Kakhetia and regions bordering Armenia were more strongly affected by the steppe. This situation persists until the Iron Age 2 and Early Antique era (800-300 BC), when in the Urartian-Orontid period, we see a drop in steppe ancestry in Armenia. In Georgia, the drop in steppe ancestry was not abrupt but slow. This difference is another argument favoring the idea expressed in Petrosyan 2023 that this drop in Armenia is related to Urartu activities and post-Urartian events.
Starting from the Iron Age, the genetic histories of Armenia and Georgia begin to diverge. Georgia witnesses' migrations from the south in the Late Antique period to Early Medieval periods (1-400 AD). The authors link those migrants with the spread of Christianity but given that some of them appear in the 1st century AD (NAT004, 1950 ybp), we can assume they were related to the political events of that period. In the 1st century AD, Rome was trying to reinforce its control of Greater Armenia and the South Caucasian kingdoms of Iberia and Albania. Meanwhile, the Parthian Arsacids were trying to counteract them. As a result of these wars, the Arsacids became the rulers of Armenia and tried to impose their cadet branch in Iberia as well. Armenia on the other side gets a Levantine influence in Hellenistic period. From historic point of view, it is quite possible but given the small sample size and issues with models where various periods are not compared with the same source populations, there is a need of extra data to confirm this.
Starting from 400 AD, eastern Georgia witnesses new sporadic waves of migrants from the steppe, this time associated with the Huns and Sarmatians. My feeling is that the Maskut tribes' invasion is also associated with this event. These steppe intruders had a practice of artificial deformation of the skull, which the locals borrowed. The sample size from Armenia is small from those periods, which is probably why we don't see them in Armenia. Another possibility is that they didn't reach Armenia in large number. After the Middle Ages, there are no remarkable events.
Overall, this is a good paper that describes the genetic history of the region. This comparison would be more comprehensive if we had dense sampling of Armenia and Azerbaijan from the relevant periods. We should not forget that the sample size in the post-Urartian period Armenia is incomparable to what we have now from Georgia.
Besides the positive side there are some issues mentioned by other commentators:
- The choice of Iran_Chl as a source population instead of Iran_Neo. This choice can mask the Levant N ancestry present in the South Caucasus since the Neolithic period.
- Mentions of obsolete papers like Pagel 2013, where Pagel and Atkinson calculated the ages of divergence of different linguistic groups in Eurasia.
It must be noted that current genetic data do not forbid the deep origins of Kartvelian from CHG. Given that there was a resurgence of CHG in the Bronze Age Georgia corresponding to the age of divergence of Proto-Kartvelian.
The deep origin of PIE from local farmers is also still on the table. However, the most frequent Y-DNA of Kartvelian people (G2a1 from Fertile Crescent) and those in the CLV cline (R1b, from EHG) create other possibilities also. More data will obviously help us better understand the deep origins of Kartvelian and PIE.

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