Cochran/Harpending on Proto-Indo-European expansion

Their book (The 10,000 Year Explosion: How Civilization Accelerated Human Evolution) hasn't yet been released, but much of it is now readable on Google Books. Here is Cochran and Harpending's take on the spread of Indo-European:
Milk and the Kurgans

Improved variants of the Kurgan hypotheses fit many facts, but what they don't do is explain why the Proto-Indo-Europeans expanded at the expense of neighboring peoples [. . .]

We suggest that the advantage driving those Indo-European expansions was biological--a high frequency of the European lactose-tolerance mutation (the 13910-T allele). The usual story about lactose tolerance is that it's the result of a cultural innovation, the domestication of cattle. That innovation led to selection for a new mutation that extended lactase production into adulthood. But there's more to the story.

Initially, selection favored individual carriers of the lactose-tolerance mutation, but the mutation was rare and had little social effect. Cattle were used for plowing and pulling wagons, for their beef, and as a source of secondary products like leather and tallow. But when the lactase persistence allele became common, so that a majority of the adult population could drink milk, a new kind of pastoralism became possible, one in which people kept cattle primarily for their milk rather than for their flesh. This change is very significant, because dairying is much more efficient than raising cattle for slaughter: It produces about five times as many calories per acre. Dairying pastoralists produce more high-quality food on the same amount of land than nondairy pastoralists, so higher frequencies of lactose tolerance among Indo-Europeans would have caused the carrying capacity of land to increase--for them.

Standard ecological theory indicates that when two similar populations use the same resources, the one with the greater carrying capacity always wins. In more familiar terms, the Proto-Indo-Europeans in our scenario could raise and feed more warriors on the same amount of land--and that is a recipe for expansion. The same basic idea is behind theories of the expansion of farming through local population growth (called demic expansion): Farming produces more food per acre, therefore farmers will outnumber foragers, and so farmers will expand at the expense of foragers.

Proto-Indo-Europeans probably were most competitive in areas where grain agriculture was marginal. In the steppe, the problem was limited rainfall. Since raising cattle there had been competitive with grain farming even before dairying arose, milk-drinking Indo-Europeans would have had an absolute advantage and should have spread rapidly over the steppe. In much of northern Europe, shorter growing seasons must have interfered with production of cereal crops such as wheat, particularly when agriculture was new there, as those crops had had little time to adapt to the local climate. Eventually, other cereal crops, such as oats and rye that could do well in those climates, were developed--probably by accident, starting as weeds in wheat or barley fields. But that happened in the Bronze Age, long after the introduction of farming. Dairying may have been more productive than grain farming in northern Europe during the late Neolithic. Even if it was not, it may have been close enough to let other advantages of the pastoral way of life tip the scales. It seems clear that the Proto-Indo-European form of patoralism did have other advantages in intergroup competition.

As the Proto-Indo-Europeans became dairymen, they should have come to rely more and more on their cattle and less on grain farming. As that happened, they would have become mobile, which is a military advantage, especially against farmers. Farmers have homes and villages that they must defend, whereas pastoralists can fight at a time and place of their choosing.

[. . .]

Back in the early days of their expansion, the Indo-Europeans appear to have encountered farmers in the Balkans who had been farming since about 6000 BC, but who weren't under a powerful central government. Around 4200 BC, things went sour. Ancient village sites were abandoned, advanced work in metals and ceramics became rare, and the inhabitants shifted to easily defended sites such as caves, hilltops, and islands. We find an increasing number of Kurgan burials similar to those found earlier on the steppe. (Interestingly, the bodies in those Kurgan burials averaged almost four inches taller than the earlier peoples of the region--milk does a body good.)

We suspect that pre-state farmers had a lot of trouble with invading Indo-European pastoralists. It wasn't just that dairying was productive and conferred increased mobility. It made cattle very valuable, and cattle are far easier to steal than heaps of grain: They can walk. It looks as if the early Indo-Europeans spent a lot of time rustling each other's cattle, fighting over cattle, planning revenge for previous raids, and in general raising hell. They became a warrior society.
So far, so plausible. Now it starts to get a bit pat.
European languages and culture spread past those regions in which dairying was favored--for example, into southern Europe and Iran--but strong states probably limited their expansion into the Middle East.

As much as anything, those peripheral expansions were probably driven by what might be called historical momentum: Peoples with a long record of success in war and raiding kept expanding even in areas where they had no special ecological advantages. Something similar happened when the Indo-Aryans moved into India: Internal weakness, possibly even collapse, of the Indus civilization may have allowed that expansion to occur.
In Understanding Human History (pdf), Michael Hart attributes conquests of more southerly peoples by Indo-Europeans to higher intelligence among the latter, which, while it may not be a complete explanation, I find more compelling than "historical momentum". Continuing:
Today the LCT 13910-T variant has reached almost 100 percent frequency in some parts of northern Europe; it is common in northern India and can even be found at low levels among some pastoral peoples of sub-Saharan Africa, such as the Fulani and Hausa.

Moreover, there is reason to think that this historical phenomenon has happened at least three times. Cattle herders of East Africa in the region of the Upper Nile and further south are lactase-tolerant[sic] milk drinkers dure to a younger mutation of their own. They, too, have expanded: They have become warlike, and there are fascinating parallels between their religions and social structure and those of the ancestral Indo-Europeans. Another separate pair of mutations causing lactose tolerance happened in the Arabian peninsula, driven in this case by the domestication of camels. This may have been an important cause of the explosive growth of Islam and the Arab conquests of the sevention century AD and later.