Improved variants of the Kurgan hypothesis fit many facts, but what they don't do is explain why the Proto-Indo-Europeans expanded at the expense of neighboring peoples with similar technology. Effective use of the horse in warfare doesn't seem to have occurred early enough to explain Proto-Indo-European expansion—but even if it had, what would have stopped other peoples from rapidly acquiring horses and using them in the same way? The Plains Indians certainly managed to master light cavalry warfare in short order: Why couldn't non-Indo-European peoples have done the same?
Later empires succeeded in part thanks to a snowball effect: The larger they grew, the stronger they were, until they were stopped by geographic barriers or long lines of communication. Once the Romans unified Italy, they were hard to stop. But as far as we can tell, nothing like this happened in the Indo-European expansion. It was too early for that kind of imperial organization. There was no central command, no capital, no state. If a peripheral Indo-European tribe had a dustup with neighboring non-Indo-Europeans, it had to win on its own, more or less. At most they had local allies. In order to expand as much as they did, early Indo-Europeans must have had some kind of edge, and in order to expand again and again over millennia, they had to have an edge that was hard to copy.
To solve the mystery, let's start with what we know about the Proto-Indo-Europeans from the linguistic evidence. We know that the Indo-Europeans weren't especially skilled at grain agriculture or adapted to it, since they were primarily pastoralists. They were removed from the first centers of farming in the Middle East. We also know that that the Proto-Indo-Europeans were rather backward in the realms of technology and social complexity. Sumerians invented the wheel, writing, and arithmetic and had cities and extensive irrigation systems at a time when the Proto-Indo-Europeans had, at most, domesticated the horse.
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.17 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 the 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 that pastoral way of life tip the scales. It seems clear that the Proto-Indo-European form of pastoral-ism 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. Herodotus tells us how Darius, the head of the Persian Empire, decided to invade the Russian grasslands in 512 BC, then held by the Scythians. Scythians were a people whose way of life was probably similar to that of the Proto-Indo-Europeans, but further developed in that they had fully mastered the horse. They appear to have been milk drinkers early on: In fact, this is mentioned in the Iliad.19,
When Darius invaded, the Scythians kept retreating farther and farther into the sea of grass: They had no cities or fields and thus had nothing to lose by retreating. Darius eventually realized that his expedition had been fruitless and turned back before his army ran out of supplies.19
Darius at least had a powerful state and a powerful army: He could cope with Scythian invasions, even if he couldn't conquer Scythia. 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. That general tendency of pastoral society—a gift for causing trouble—was a key theme in
Eurasian history for millennia. The threat receded as agricultural peoples built strong states, intensified again in the Middle Ages as states weakened and steppe techniques improved (reaching an apogee with Genghis Khan), and ended only with the invention of gunpowder.
Our picture of the Indo-European expansion begins with a very rapid spread across the steppe as soon as the increased frequency of the lactase-persistence mutation became common enough to allow the switch to a dairying economy. This rapid spread would have resulted in a population that spoke similar dialects over a wide region all the way from the Ukraine to the Urals—similar because there hadn't been time for linguistic divergence. The wave of advance continued on into Europe, where dairying was ecologically competitive with early agriculture and produced a far more aggressive culture. Most likely, Indo-European culture also became more warlike as their mobility, superior numbers, and better nutrition allowed them to win battles more often than other peoples. Their victories, in turn, may have led to further advantages in military efficiency: Success feeds success.
Judging from their relatively low contribution to the European gene pool, Indo-Europeans appear to have practiced elite dominance, conquering rather than exterminating and replacing the previous inhabitants. A relatively small elite population can often impose its language on the rest of the population. In addition, the Indo-Europeans would have added the lactose-tolerance allele to the local mix. Although it appears to have been rare or nonexistent in Europe before the Indo-European invasions, it became common in those areas where a dairying economy was favored, particularly in northern Europe.20 Indo-
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 weaknesses, possibly even collapse, of the Indus civilization may have allowed that expansion to occur. Today the LCT 13910-T lactase 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 milk drinkers due to a younger mutation of their own.21 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.22 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 seventh century AD and later.23
If this picture is correct, the occurrence of a single mutation in a particular group of pastoralists some 8,000 years ago eventually determined the spoken language of half of mankind. It may not be possible to reconcile this with Tolstoy's ideas of the unimportance of the individual in history. Of course, champions of individual importance have typically emphasized ideas, intelligence, and character—not digestion.
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