Prehistoric Indo-European Invasion: From North of Sochi

You can not only ride it, you can milk it.
Greg Cochran closely reads the new paper "Ancient human genomes suggest three ancestral populations for present-day Europeans" and predicts that the giant team that put it together has an even bigger news paper coming on the geographic origins of Indo-European speakers, which has been one of the great controversies.
So they must have a paper in the works – one with strong conclusions, if they’re that worried about someone scooping them.  They were too obvious – they need a course in maskirovka. They have got to be looking at ancient DNA from relevant populations: from Kurgan burials, from Russia north of the Caucasus, Tocharian mummies, etc.  They’re going to need strong evidence, and a baseball bat, to get linguists to pay attention.  Looks to me like we”re going to get at least half of the story (the European end) of  the Indo-European expansion out of this, and probably we’ll learn something about Indo-Aryan end as well. 
I will predict this: they’ll pussyfoot about the likely historical process, which undoubtedly was awesomely bloody. The Balkans looks to be where this started, and there a fairly sophisticated agricultural population (with very advanced metallurgy for the time) seems to have been utterly squished.
I don't know about the prehistory, but the history of Bulgaria has largely been one of the locals getting squished, so maybe it was also like that before anybody wrote things down. In the comments, Henry Harpending notes:
Re the Indo-European advantage: consider that Mare’s milk has 190 Calories of fat and protein per gram and 250 Calories of lactose. Five kg. per day from one mare feeds two lactose tolerant children with 2200 Calories and fewer than one non-LT child with only 950 Calories. This is a huge nutritional advantage in an occasionally Malthusian ecology. My bet (hypothesis) is that the early IE people were horse people and that instant doubling of the food supply from a new gene was essentially the cause of the IE expansion. You can’t argue with calories.   
Cochran adds:
The Indo-European invasion is a bit unusual in that it is not agriculturalists displacing foragers. More like agro-pastoralists (with an emphasis on pastoralism, probably) displacing straight farmers. 
Raising cattle probably preadapts for warfare through constant rustling. Valuable portable assets.
A commenter amends:
Or Aggro-Pastoralists, given the carnage they may have inflicted.
By the way, the Sioux Indians expanded radically to the west after they got guns and horses and white man diseases weakened their more densely populated Indian rivals:

Between approximately 1685 and 1876 the western Sioux conquered and controlled an area from the Minnesota River in Minnesota, west to the head of the Yellowstone, and south from the Yellowstone to the drainage of the upper Republican River. This advance westward took place in three identifiable stages: initially a movement during the late seventeenth and early eighteenth centuries onto the prairies east of the Missouri, then a conquest of the middle Missouri River region during the late eighteenth and nineteenth centuries, and, finally, a sweep west and south from the Missouri during the early and mid-nineteenth

century. Each of these stages possessed its own impetus and rationale. Taken together they comprised a sustained movement by the Sioux that resulted in the dispossession or subjugation of numerous tribes and made the Sioux a major Indian power on the Great Plains during the nineteenth century

Not a perfect analogy to the proposed Indo-European expansion, but interesting.