Why Can We Eat Milk?
December 17, 2006 5:10pm CST
I guess you know that Chinese cheese (tofu) is made from soybean... That's because Chinese and many peoples from Southeast Asia are intolerant to milk and dairy products. These products may be rich in protein and calcium, but for half of the world's adults, they provoke cramps and diarrhea. Milk intolerance is due to the fact that the digestive apparatus can not digest lactose, the milk's sugar, and this one is fermented by gut's microbes, provoking the above symptoms. A recent study points to the fact that the ability to digest milk appeared more than once in humans descended from cattle herders, showing how cultural items can have a rapid and dramatic effect on our gene pool. All humans can digest mothers' milk as infants, but until cattle/goat/sheep domestication 9000 years ago, weaned children no longer needed to digest milk. That's why the gene of the lactase enzyme, which breaks down the lactose into glucose (which is readily absorbed into the blood), is shut down after weaning. But after cattle/sheep/goat domestication, this gene gave an advantage for those keeping it functional during adult life, as they became lactose tolerant and could consume milk, an aliment which confers more proteins and minerals than other food. But as many East Asian cultures did not experience a pastoral culture, they did not develop lactose tolerance. In 2002, researchers identified a genetic mutation that tunes the activity of lactase gene, allowing Finns and other northern Europeans to be lactose tolerant. But the same mutation appeared at lower frequency in lactose tolerant people from southern Europe and the Middle East and it was completely missing in African herders. To understand how lactose tolerance evolved differently in these populations, geneticist Sarah Tishkoff of the University of Maryland, College Park, made DNA analysis on samples from 110 Tanzanians, Kenyans, and Sudanese from 43 ethnic groups. Three new mutations in the same stretch of DNA as the European variant were found. The mutations appeared 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. Clinical analysis proved that these people were digesting lactose. The most common variant might have arisen as recently as 3000 to 7000 years ago and spread quickly. “The timing correlates with the domestication of cattle in the region about 8000 years ago,” says Tishkoff. "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. “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”, said anthropologist Ken Weiss at Pennsylvania State University. “There is not just one way to tolerate milk, but several ways”. "It's very nice work because it shows that evolution isn't just about picking one gene and driving it." source: news.softpedia.com