TORONTO--Less than half of all adults can easily digest milk, a trait believed to have first appeared in people who kept dairy animals. Now scientists have traced the genetic roots of milk tolerance to the Ural mountains, well north of where pastoralism is thought to have begun. The surprising result supports a theory that nomads from the Urals were one of two major farmer groups that spread into Europe, bringing the Indo-European languages that eventually diverged into the world's largest family of modern languages.
Practically all babies produce lactase, the enzyme that digests the milk sugar lactose. But in many people, the lactase gene gradually gets turned off after infancy, leaving them unable to tolerate milk. Two years ago, a team led by Leena Peltonen of the University of Helsinki, Finland, and the University of California, Los Angeles, tracked down gene mutations associated with lactose tolerance, which likely play a role in regulating the lactase gene. Now, Peltonen's team has tried to trace the origins of lactose tolerance by looking at these variants in 1611 DNA samples from 37 populations on four continents.
The results suggest that lactose tolerance first appeared in populations living between the Ural mountains and the Volga River, such as Udmurts and Mokshas. The trait most likely developed 4800 to 6600 years ago from an earlier variant differing at just two base pairs that these groups got from intermixing with tribes migrating from the Asian Steppes, the team reported here last week at the annual meeting of the American Society of Human Genetics. The lactose tolerance mutation "probably emerged by chance" and then remained because it was beneficial in the dairy-consuming Ural peoples, who later spread the gene to Europe and the Middle East, says Peltonen.
The finding supports the somewhat controversial theory that nomadic herders known as Kurgans in the southern Urals expanded into Europe 4500 to 3500 years ago, bringing Indo-European languages with them, Peltonen's group concludes. "I find it very interesting," says emeritus population geneticist Luigi Luca Cavalli-Sforza of Stanford University. A competing idea for explaining the mystery of the origin of the Proto-Indo-Europeans is that they were crop-growing farmers from the Anatolia region in modern Turkey (Science, 27 February, p. 1323). But Cavalli-Sforza says if the milk gene study holds up, it reinforces his own view that both theories are correct: Indo-Europeans migrated to Europe in two waves, first from Turkey and later from the Urals.