Recently, in the "Southern Arc" paper, Lazaridis et al 2022, there was a particularly redundant explanation to the "unmixed R1b" in Iran/Armenia. The authors say:
"Despite the Y-chromosome movement southward attested by our data, any association between R-haplogroup bearers and Eastern hunter-gatherer ancestry was lost south of the steppe because these had similar proportions of Eastern hunter-gatherer ancestry as I-Y16419 bearers (the second most prevalent lineage in Armenia). Two Bronze-to-Iron Age sites with substantial sample sizes [unrelated males from Bagheri Tchala (n = 7) and Noratus (n = 12)] have contrasting haplogroup distributions dominated by R-M12149 and I-Y16419, respectively (Fisher’s exact test P < 0.001), suggesting founder events, high genetic drift, or a patrilocal mating system ~1000 BCE in Armenia. During the same period at Hasanlu in Northwest Iran, many individuals have no trace of Eastern hunter-gatherer ancestry at all despite the presence of R-M12149 there (6), suggesting that the initial association of this lineage with Eastern hunter-gatherer ancestry on the steppe had vanished as R-M12149 bearers reproduced with Southern Arc individuals without Eastern hunter-gatherer ancestry (Fig. 6C)"
While this scenario makes sense, it is not the simplest solution, given the genetic evidence which is plainly available, as you will see.
One of the points which I have stressed in an earlier post, is that the CHG grouping is nearly identical to the Iranian Neolithic (Ganj Dareh). That notion isn't simply based on my analysis, as an amateur blogger, for which one may conveniently doubt, but such has been highlighted by, none other, than Lazaridis himself. In the Southern Arc paper, he says:
"Caucasus hunter-gatherers (CHG) (7), Eastern hunter-gatherers (EHG) from Europe (8, 9), Levantine Pre-Pottery Neolithic (10), Balkan hunter-gatherers from the Iron Gates in Serbia (3), and Northwestern Anatolian Neolithic from Barcın (9). These correspond to the four-source ADMIXTURE model, with further distinction between the Anatolian and Levantine ends of the “Mediterranean”interaction zone (11). These five sources should not be unduly emphasized beyond their utility as a descriptive convenience because (i) they could be swapped for related ones [e.g., Neolithic Iran captures much of the same deep ancestry as Caucasus hunter gatherers do (10,11)]"
So the authors of the Southern Arc paper, are acknowledging the genetic overlap between Iran_Neolithic group and CHG. This seems rather insignificant at first, but here's the thing. Why go out of your way to say this in the first place? And moreso, why keep referencing the Iran/CHG pseudo-equivalence and yet advertise CHG as ancestral to Yamnaya. In his other paper Ancient DNA from Mesopotamia suggests distinct Pre-Pottery and Pottery Neolithic migrations into Anatolia,
Lazaridis says,
"These results tentatively suggest that Caucasus hunter-gatherer and Ganj Dareh Neolithic are interchangeable for the purposes of quantifying the amount of inland admixture, although some populations may have a clearer connection with one or the other (e.g., the Neolithic of Armenia with the hunter-gatherers of the South Caucasus rather than Iran, and the geographically intermediate Azerbaijan and Mesopotamia with both)"
The fact that Lazaridis makes subtle these references to through his papers, suggests that he is only doing so to veil his personal bias against a southern origin for PIE. That is to say, it supports his advocacy for a northern more point for PIE (or perhaps Anatolian). This is not to say that the origin of R1b - M269 carries significant relevance to the origin of PIE - It's clear R1b (or any other haplogroup) cannot be used as a marker for PIE, because PIE groups shows diversity within and between other PIE populations since ancient times, but the narrative Lazaridis wants to push, is that Indo-Iranian came from the Steppe in the infamous round-the-Caspian route, from Steppe Theory 101.
That being said, a new paper by Allentoft et all 2022, also highlights the continuous gene flow between Iran/CHG and the Steppe since 7200ybp (again preferring "CHG to IRN"):
Our results thus document genetic contact between populations from the Caucasus [and Iran] and the Steppe region as early as 7,300 years ago, providing documentation of continuous admixture prior to the advent of later nomadic Steppe cultures, in contrast to recent hypotheses, and also further to the west than previously reported12,27
At least their figure uses IRAN not CHG 😄 :
SUMMARY:
1. There was continuous gene flow between CHG and IRAN through early and late history
2. CHG and IRAN are nearly isomorphic, and for the purpose of assessing local admixture can be swapped out, per Lazaridis
CONCLUSION: The possibility of R1b moving from Iran into the Steppe is open