Corded Ware and Yamnaya

The latest from Davidski:

There's been a lot of talk lately about the finding that the peoples associated with the Corded Ware and Yamnaya archeological cultures were close cousins [...]. As I've already pointed out, this is an interesting discovery, but, at this stage, it's difficult to know what it means exactly.

It might mean that the Yamnayans were the direct predecessors of the Corded Ware people. Or it might just mean that, at some point, the Corded Ware and Yamnaya populations swapped women regularly (that is, they practiced female exogamy with each other).

• In any case, I feel that several important facts aren't being taken into account by most of the interested parties. These facts include, in no particular order: despite being closely related, the Corded Ware and Yamnaya peoples were highly adapted to very different ecological zones - temperate forests and arid steppes, respectively - and this is surely not something that happened within a few years and probably not even within a couple of generations
• both the Corded Ware and Yamnaya populations expanded widely and rapidly at around the same time, but never got in each others way, probably because they occupied very different ecological niches
• despite sharing the R1b Y-chromosome haplogroup, their paternal origins were quite different, with Corded Ware males rich in R1a-M417 and R1b-L51 and Yamnaya males rich in R1b-Z2103 and I2a-L699

I suppose it's possible that the Corded Ware people were overwhelmingly and directly derived from the Yamnaya population. But right now my view is that, even if they were, then the Yamnaya population that they came from was quite different from the classic, R1b-Z2103-rich Yamnaya that spread rapidly across the steppes.

Indeed, perhaps what we're dealing with here is a very early (proto?) Yamnaya gene pool located somewhere in the border zone between the forests and the steppes, that then split into two main sub-populations, with one of these groups heading north and the other south?

I think it's most likely, personally, that Yamnaya and Corded Ware are derived ultimately from separate but adjacent earlier sources that practiced a lot of female exogamy over many generations, probably spoke closely related languages or even dialects of the same proto-Indo-European—in part because we can't yet determine that some of the Indo-European daughter languages of the Balkans in particular didn't actually come from a Yamnaya rather than Corded Ware source. I do suspect that they weren't ever really exactly the same population source, however, as the paternal Y-DNA haplogroups seem to be different, as well as to be rooted in more ancient populations. The paternal split between east and west on the steppe seems to have been quite deep rooted, going back quite a bit longer than the Yamnaya and Corded Ware peoples to their predecessors. However, the latest is that Yamnaya essentially spread from the western edge of its range, rather than developing exactly out of Khvalynsk. While the eastern and western steppe cultures that preceded Yamnaya appear to have had a very similar economy and material culture, and probably broadly similar genetics, we're starting to get enough fine scale structure detail to be able to describe some movement and fluctuations which probably resulted in intrusions from adjacent people's haplogroups, and stuff like that. 

It's important to not be too mesmerized by the haplogroups, though. While the different paternal haplogroups between Corded Ware and Yamnaya are both interesting and probably mean something, the fact that at an autosomal level the two populations were still very closely genetically related means something too. They may have been two broadly integrated cultures that had different male lines in ascension in the different regions. David Anthony has proposed some kind of pseudo-Marxist class-based solution to the haplogroup differences. I think the point is that we don't really know. But as Davidski points out, Anthony's solution is probably not the most parsimonious, nor is it necessary.

And besides, there's more structure yet to come. While Davidski says that Corded Ware was rich in R1a-M417 and R1b-L51, the reality is that the R1b-L51 seems to be a slightly later development, during the Bell Beaker phase. If the Corded Ware themselves were rich in it, most of those samples haven't yet been published (although he hints that he's aware of unpublished samples.) In terms of what's published, we have, rather: 1) Yamnaya rich in R1b-Z2103 and to a lesser extent I2a-L699, Corded Ware rich in Rla-M417 and Bell Beaker rich in R1b-L51. The Bell Beaker haplogroup seems to have been quite rare in the early expansions of the Corded Ware (although, again, hints of unpublished results that will partially bridge that gap are common) and had an explosive radiation after the Corded Ware was already established... but mostly, only in the Western half of the Corded Ware horizon. The eastern half remains heavy in R1a-M417 and its subclade Z93.

In fact, one paper on academia.edu that I've read suggests the same thing; a Sredni Stog origin for Indo-European, through Corded Ware, but also suggests that the WSH element is an illusion of sorts. It represents an admixture (roughly 50/50) between the EHG and the CHG populations, but it does not necessarily imply that all "WSH" admixture later reported actually comes from a single source of that admixture of CHG and EHG. This is, again, in keeping with explaining the structure on the steppes, and promoting the idea that Khvalynsk and Sredni Stog were not necessarily as closely related (or closely tied, maybe is a better word) than has been often proposed in the past. Rather, the paper proposes that the WSH should remain broken out as CHG and EHG respectively. It also proposes that rather than there being a CHG/EHG hybrid in the steppes for centuries or even millennia, that the CHG and EHG components should be seen as representing populations that came together in the region, yet probably retained some coherence as separate populations for some time. The early Corded Ware people are, therefore, on average 40% EHG, 30% CHG, 22½% EEF and 7½% WHG. The admixing between them probably started way back in the Dnieper-Donets phase, since all four populations were on contact that long ago in the western steppe and forest steppe, before even the Sredni Stog, much less Corded Ware were formed. 

Sredni Stog, based on so far very limited sampling, seems to show about 1/5th EEF admixture. The earlier Dnieper-Donets culture seems to be almost exclusively EHG with some WHG admixture. The EEF admixture probably came from extended contacts, which are fairly well documented anyway, with the most westerly EEF Cucuteni-Trypillia culture. The Khvalynsk culture has no EEF admixture, but the CHG admixture that it shows is supposed to have happened earlier in the East rather than the West.

Still trying to untangle what all of this means, but I do tend to think that Davidski's models are the best ones for the development of a PIE-speaking population, in part because he's the most inclined to rigorously include the best fine-scale data.