Milk may do a body good for some, but for half of the world's adults, it causes cramps and diarrhea. Now, a new study indicates that the ability to digest milk arose more than once in humans descended from cattle herders--a finding that sheds light on how culture can have a rapid and dramatic effect on our genome.
The textbook tale of lactose tolerance runs this way: All humans digest mothers' milk as infants, but until cattle were domesticated 9000 years ago, weaned children no longer needed to digest milk. As a result, many shut down the enzyme lactase, which breaks down the lactose into sugars. After cattle were domesticated, however, it became advantageous for their keepers to digest milk, and lactose tolerance evolved.
Not everyone can stomach milk equally, however. Northern Europeans, who tend to be descended from cattle farmers, are much more lactose tolerant than most Asians, who were not as dependent on cattle. In 2002, researchers identified a genetic mutation that regulates the expression of lactase and allows Finns and other northern Europeans to drink milk as adults. But researchers were surprised that the mutation appeared at lower frequency in milk-drinkers living in southern Europe and the Middle East--and it was missing in African pastoralists.
To see if lactose tolerance evolved differently in these populations, geneticist Sarah Tishkoff of the University of Maryland, College Park, organized a team to collect blood samples from 470 Tanzanians, Kenyans, and Sudanese from 43 ethnic groups. The researchers sequenced the DNA of 110 individuals and tested them for milk tolerance. They found three new mutations in the same stretch of DNA as the European variant. The mutations turned up in varying frequencies in the Masai and other Nilo-Saharan populations in Tanzania and Kenya, in Afro-Asiatic speaking Kenyans, and in the Beja from Sudan. People with any of the variants had higher blood sugar after drinking milk, a sign that lactose was being digested.
The researchers also found that the most common variant arose as recently as 3000 to 7000 years ago--and spread rapidly. The timing correlates with the domestication of cattle in the region about 8000 years ago, says Tishkoff, whose team reported its findings online 10 December in Nature Genetics.
"This is extremely significant because it shows the speed with which a genetic mutation can be selected for," says zooarchaeologist Diane Gifford-Gonzalez at the University of California, Santa Cruz. Anthropologist Ken Weiss at Pennsylvania State University in State College adds that the study is an elegant example of how evolution can find several different solutions to the same problem, especially in the face of strong selection. "There is not just one way to tolerate milk, but several ways," he says. "It's very nice work because it shows that evolution isn't just about picking one gene and driving it."
For a more in-depth news item on this topic, stay tuned for the 15 December issue of Science.