From a news feature about a forthcoming palaeogenomics paper:
"The question of where the Yamnaya come from has been something of a mystery up to now," said one of the lead senior authors Dr Andrea Manica, from Cambridge's Department of Zoology.
"We can now answer that as we've found that their genetic make-up is a mix of Eastern European hunter-gatherers and a population from this pocket of Caucasus hunter-gatherers who weathered much of the last Ice Age in apparent isolation. This Caucasus pocket is the fourth major strand of ancient European ancestry, one that we were unaware of until now," he said
Read more at: 'Fourth strand' of European ancestry originated with hunter-gatherers isolated by Ice Age
Update: the paper is now out and open access at Nature Communications.
The two ancient Georgian genomes belong to Y-chromosome haplogroup J. So it looks like I was right when I said that this type of ancestry mostly entered the European steppe from the Caucasus via female mediated gene flow during the Bronze Age (see here and here).
However, one of the Eastern European Hunter-Gatherers (EHG) from Mathieson et al. 2015 also belonged to haplogroup J. This suggests that there was intermittent gene flow, including some paternal gene flow, between the Caucasus and the steppe well before the Bronze Age.
Nevertheless, it's now even more difficult to accept that Y-haplogroup R1 and the Proto-Indo-Europeans might have originated south of the steppe. Clearly, R1 appears to be a steppe marker from way back, and I seriously doubt that Indo-European languages were introduced into highly patriarchal steppe societies by female migrants from the Caucasus.
Image credit: Nature Communications, dx.doi.org/10.1038/ncomms9912