Likely Yamnaya incursion(s) into Northwestern Iran

Despite being stratigraphically dated to 5900-5500 BCE (ie. the Chalcolithic period), ancient sample Hajji_Firuz_ChL I2327 from Narasimhan et al. 2018, belongs to Y-haplogroup R1b-Z2103 and shows minor, but unambiguous, Yamnaya-related ancestry on the autosomes. Why is this a problem? Because both R1b-Z2103 and the Yamnaya culture are dated to the Bronze Age, and Yamnaya samples from Kalmykia and Samara regions of what is now western Russia are exceptionally rich in R1b-Z2103.

Thus, pending a successful radiocarbon (C14) dating analysis, it seems unlikely that Hajji_Firuz_ChL I2327 was alive during the Chalcolithic. Rather, it appears that he's partly of Yamnaya or closely related origin and has been wrongly dated. His remains are likely to be from a secondary burial from the Bronze Age that collapsed into the layer below, right into a Chalcolithic bin ossuary burial full of much older bones.

This scenario is strongly corroborated by data from two other ancient individuals from what is now Northwestern Iran:

- Hajji_Firuz_BA I4243 (also from Narasimhan et al. 2018 and from the same site as Hajji_Firuz_ChL I2327) was initially also stratigraphically dated to the Chalcolithic, but is now labeled as a Bronze Age sample after a radiocarbon (C14) analysis of the remains revealed a date of 2465-2286 calBCE. Moreover, this individual packs around 50% Yamnaya-related ancestry.

- Iran_IA F38 (from Broushaki et al. 2016) from an Iron Age burial at Tepe Hasanlu, which is just a few miles from Hajji Firuz, also belongs to Y-haplogroup R1b-Z2103 and harbors some sort of steppe ancestry on the autosomes (see here).

Below is a Principal Component Analysis (PCA) showing how this trio compare in terms of genome-wide ancestry to C14-dated Chalcolithic samples from Hajji Firuz and the nearby Seh Gabi. The relevant datasheet is available here.

Clearly, they're shifted "north" relative to the C14-dated Chalcolithic samples and thus closer to the ancient Eastern Europeans, suggesting that they carry ancestry from north of the Caucasus that was missing, or at least much less pronounced, in the region before the Bronze Age.

I used D-stats of the form D(Outgroup,Pop1)(Pop2,X), in which Pop1 are Eastern European Hunter-Gatherers (EHG) and Pop2 the C14-dated Chalcolithic samples, to test this more directly. And indeed, the D-stats showed that Hajji_Firuz_ChL I2327 and Hajji_Firuz_BA I4243 shared significantly more alleles with EHG than the Chalcolithic samples did (Z≥3).

Mbuti EHG Hajji_Firuz_ChL Hajji_Firuz_ChL_I2327 D 0.0151 Z 3.737
Mbuti EHG Hajji_Firuz_ChL Hajji_Firuz_BA_I4243 D 0.0496 Z 12.072

Mbuti EHG Seh_Gabi_ChL Hajji_Firuz_ChL_I2327 D 0.0188 Z 4.803
Mbuti EHG Seh_Gabi_ChL Hajji_Firuz_BA_I4243 D 0.0531 Z 12.832

To further elucidate this EHG-related signal, I ran a series of mixture models with the qpAdm software. Below are among the statistically most successful models that I could find for Hajji_Firuz_ChL I2327, Hajji_Firuz_BA I4243 and Hajji_Firuz_ChL. Note that each of these models features Yamnaya from Samara as the proxy for the northern, EHG-related ancestry.

Hajji_Firuz_ChL_I2327
Barcin_N 0.087±0.045
Seh_Gabi_ChL 0.768±0.045
Yamnaya_Samara 0.145±0.033
chisq 8.802
tail prob 0.551007
Full output

Hajji_Firuz_ChL
Barcin_N 0.232±0.027
Seh_Gabi_ChL 0.736±0.029
Yamnaya_Samara 0.033±0.018
chisq 4.269
tail prob 0.93439
Full output

Hajji_Firuz_BA_I4243
Barcin_N 0.201±0.038
Seh_Gabi_ChL 0.244±0.042
Yamnaya_Samara 0.554±0.031
chisq 9.209
tail prob 0.512363
Full output

Yamnaya_Samara-related admixture appears in all of the models. But considering the standard errors, the Yamnaya_Samara-related ancestry proportion for Hajji_Firuz_ChL is very close to zero. Moreover, importantly, Hajji_Firuz_ChL can be modeled successfully without any such ancestry, while Hajji_Firuz_ChL I2327 and Hajji_Firuz_BA I4243 can't (refer to the full output files for the details).

I also tested whether ancient samples from what is now Armenia might make better proxies than Yamnaya for the northern ancestry harbored by Hajji_Firuz_ChL I2327. That's a negative (see here and here).

I don't have a clue who these people were who brought Yamnaya or, at least, Yamnaya-like ancestry to the South Caspian region during the Bronze Age and perhaps also the Chalcolithic. It's rather unlikely that they were the early Iranians, who probably arrived in the region from Central Asia during the Late Bronze Age or even Iron Age (for instance, see here). Perhaps they were the Hittites? Indeed, in his book In Search of the Indo-Europeans, archaeologist James Mallory suggested that the ancestors of the Hittites and other Anatolian-speakers entered the Near East via the Caucasus route:

Most arguments for an Indo-European invasion from the northeast concern the appearance of a new burial rite at the end of the fourth and through the third millennium BC. At that time, both north of the Black Sea and the Caucasus, burials on the Russian-Ukrainian steppe were typically placed in an underground shaft and covered with a mound (kurgan in Russian). Before 3000 BC there begin to appear in the territory of the indigenous Transcaucasian (Kuro-Araxes) culture somewhat similar burials such as the royal tomb of Uch-Tepe on the Milska steppe. As tumulus burials are previously unknown in this region, some would explain their appearance by an intrusion of steppe pastoralists who migrated through the Caucasus and subjugated the local Early Bronze Age culture. More importantly, a status burial inserted into a mound at the site of Korucu Tepe in eastern Anatolia has been compared with somewhat similar burials both in the Caucasus and the Russian steppe. The discovery of horse bones on several sites of east Anatolia such as Norsun Tepe and Tepecik are seen to confirm a steppe intrusion since, as mentioned earlier, the horse, long known in the Ukraine and south Russia, is not attested in Anatolia prior to the Bronze Age.

Another option, however, is that they belonged to some other extinct Indo-European group, such as the Gutians (see here). In any case, keep an eye out for more Bronze Age samples from this part of the world. I have a strong feeling that, unlike their Neolithic and Chalcolithic predecessors, they will be rich in steppe ancestry and R1b-Z2103.

See also...

The Hajji Firuz fiasco

Yamnaya: home-grown

Big deal of 2018: Yamnaya not related to Maykop

Ahead of the pack

Late PIE ground zero now obvious; location of PIE homeland still uncertain, but...