The genetic advantages in the two examples we have just discussed—for the Europeans in America and for the Africans in Africa—were huge: Those lacking the required resistance to the infectious diseases in play were almost wiped out. There is no reason to think that differences in disease resistance were the only biological differences between Europeans and Amerindians, or the only advantage, but they must have had the largest impact. European colonization could not have prevailed without a huge edge. Africans, too, may have needed a large biological advantage to resist Europeans, considering their technological and social disadvantages. Although we can't be sure, it looks as if anatomically modern humans also needed a fairly large advantage during the Upper Paleolithic as they displaced the Neanderthals, since they had to outcompete populations of archaic humans that must have been better adapted to Eurasian climates than they were.
And yet, there must have been occasions in which smaller biological advantages were enough to drive a population expansion, particularly when those expanding didn't have to cross oceans. Again, we're not saying that all expansions had such causes, but some could have, and the enduring nature of biological advantage makes it a good candidate for the cause of particularly widespread and long-winded expansions.
One of the largest of all known expansions—the spread of the Indo-Europeans—was likely driven by the mutation that conferred lactose tolerance, one of the most strongly selected al-leles that Europeans possess.
"Indo-European" refers to a family of related languages that have spread over western Eurasia, the Americas, and Australasia. In terms of numbers, it is the largest of all language families, with about 3 billion native speakers, half of the human race. The largest Indo-European languages are Spanish, English, Hindi, Portuguese, Bengali, Russian, German, Marathi, and French.
In these languages, basic words in a number of categories are recognizably similar. In each of them, many of the words for numerals from one to ten, body parts (head, heart, and foot), plants and animals (oak, wolf, bear), natural phenomena (air, snow, moon), and close relations (father, mother, daughter) ultimately derive from a common ancestral language. For example, the word for "three" is treis in Greek, tres in Latin, drei in German, tri in Russian, tri in Bengali, and tre in Tocharian A, an extinct language of central Asia.
These languages were first acknowledged as a family when various Europeans in India noted similarities between Indian languages and European languages, particularly in regard to their connections with Latin and Greek. It was then suggested that a wide swath of languages in Europe and India had a common origin, just as it had long been recognized that the Romance languages (Spanish, Portuguese, French, Italian, and Romanian) derived from Latin. Most of those early observations were not followed up, but after Sir William Jones, an eminent scholar and chief justice of India, mentioned the pattern in a lecture on Indian culture in 1786, people began to take the idea seriously. Many people studied Indo-European language over the next two centuries, and today it is the most successful theory in historical linguistics.
People of many races and ethnic groups speak an Indo-European tongue: There is nothing genetic about that. Chinese pilots talk to Japanese air-traffic controllers in English, for that matter. But there is every reason to believe that the ancestor of all of these languages was once spoken by a particular people, living in some particular region. They were relatively few in number, and the region they occupied was small compared to the lands inhabited by Indo-European speakers today. There were many other small ethnolinguistic groups in Eurasia in those days, but this group spread, while others did not. Perhaps there was something unusual about them.
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