There is a new paper out in Nature, mostly about the spread of the Indo-Europeans. They confirm that the Yamnaya ( from the Ukraine) were the main ancestors of the Corded Ware culture (northern Europe, from Germany far on east). The Sintashta culture, (base of the Urals down to the Caspian, first with chariots, and ancestors of the Indo-Iranians), looks like a migration of Corded Ware to the east.
The Yamnaya were also the source of the Afansievo culture in the Altai, possible ancestors of the Tocharians.
The European lactose tolerance mutation is not common in the early Bronze Age, but it does exist, particularly in the Yamnaya, where the gene frequency is about 30% (in this study – zero in the Haak study) This suggests that the European mutation originated on the steppe (not Bavaria !) – which would explain why the same mutation is fairly common in northern India and Pakistan. It’s not clear if it was common enough to have social and demographic significance back then – since it’s dominant, a gene frequency of 30% would mean that 51% of the Yamnaya were lactose tolerant. In other words, I said that I must be wrong about lactose tolerance mattering in the Indo-European expansion, but I am no longer sure about that. But these samples are small – time to excavate more kurgans. Conceivably the frequency varied significantly by social class. Anyhow, it had to have been spread by a massive invasion – it couldn’t have spread that fast as a Fisher wave.