To help things run more smoothly in the comments, I urge everyone taking part in the debates here about the colonization of the Eurasian steppe and the Indo-European homeland question to read carefully the following three papers. They're all open access:
1) Paleoecology, Subsistence, and 14C Chronology of the Eurasian Caspian Steppe Bronze AgeIn particular, please note the latest calibrated radiocarbon-based dates of the main archaeological cultures being discussed:
2) The Steppe and the Caucasus During the Bronze Age Mutual Relationships and Mutual Enrichments
3) New Radiocarbon Dates and a Review of the Chronology of Prehistoric Populations from the Minusinsk Basin, Southern Siberia, Russia
- Khvalynsk, Eneolithic, 4300–3800 cal BC
- Steppe Maikop, Early Bronze Age, 3800–3000 cal BC
- Yamnaya, Early Bronze Age, 3000–2450 cal BC
- Afanasievo, Early Bronze Age, 2900-2500 cal BC
- Early Catacomb, Early Bronze Age, 2600–2350 cal BC
Of course, Yamnaya are in large part of Eastern European hunter-gatherer (EHG) origin but with roughly 50% of Near Eastern-related ancestry from an unknown population (Haak et al. 2015). Paper #2 linked to above provides tentative isotopic evidence that the latter might be the Steppe Maikop people or their descendants (see paragraph 4 on page 58).
However, the Khvalynsk population from the Samara region harbors around 25% of the same or very similar Near Eastern-related ancestry (unpublished data courtesy of David Anthony). And, as per the dates above, Khvalynsk existed before Steppe Maikop.
Thus, although the increase of the Near Eastern-related ancestry on the steppe from the Khvalynsk to the Yamnaya periods can be tentatively attributed to Maikop influence, this cannot be the initial source of this type of ancestry on the steppe.
Moreover, dates older than 3,000 cal BC for Afanasievo appear to be spurious (see paper #3 above). If so, what this means is that Afanasievo is around the same age as Yamnaya, or perhaps a little younger, and thus the generally accepted hypothesis that Afanasievo derives from Yamnaya or pre-Yamnaya looks safe.
Now, it's especially important that everyone concerned is aware of the key climatic shifts on the steppe, because climatic changes are often invoked as likely causes of major population movements within and out of the steppe. So I'm re-posting here Table 1 from paper #1 (click to enlarge).
I'll update this post as new information comes in, which will hopefully be very soon. There are signals that something big is on the way from the Reich Lab pertaining to the Indo-European homeland debate (for instance, see here).
Near Eastern admixture in Yamnaya: a couple of graphs + some ideas