On the other hand, most adults whose ancestors lived in very hot or very cold climates that couldn't support dairy herding or in places where deadly diseases of cattle were present before 1900, such as in Africa and many parts of Asia, do not have the ability to digest milk after infancy.
"The implication is that harsh climates and dangerous diseases negatively impact dairy herding and geographically restrict the availability of milk, and that humans have physiologically adapted to that," said evolutionary biologist Paul Sherman, a professor of neurobiology and behavior at Cornell. "This is a spectacular case of how cultural evolution -- in this case, the domestication of cattle -- has guided our biological evolution."
Although all mammalian infants drink their mothers' milk, humans are the only mammals that drink milk as adults. But most people -- about 60 percent and primarily those of Asian and African descent -- stop producing lactase, the enzyme required to digest milk, as they mature. People of northern European descent, however, tend to retain the ability to produce the enzyme and drink milk throughout life.
Sherman and former Cornell undergraduate student Gabrielle Bloom '03, now a graduate student at the University of Chicago, compiled data on lactose intolerance (the inability to digest dairy products) from 270 indigenous African and Eurasian populations in 39 countries, from southern Africa to northern Greenland. Their findings will be published in a forthcoming issue of Evolution and Human Behavior.
On average, Sherman and Bloom found that 61 percent of people studied were lactose intolerant, with a range of 2 percent in Denmar
Source:Cornell University News Service