Got Milk?

People are always interested in human evolution. Are we still evolving is a common question. Here is a very recent example of human evolution that was the subject of a paper by a professor and an undergraduate student at Cornell just last year.

Although all mammalian infants drink their mothers' milk, humans are the only mammals that can drink milk as adults (despite their well known fondness for the product most cats are also lactose intolerant). But not all of us can digest milk. In fact most adults in the world, about 60%, are lactose-intolerant: the majority of humans stop producing significant amounts of lactase sometime between the ages of two and five. Lactase is an enzyme required to digest milk. A relatively recent genetic change caused some populations, including many northern Europeans, to continue producing lactase into adulthood; these lactose-tolerant populations are the exception and not the rule. Lactose intolerance is an autosomal recessive trait, while lactase-persistence is the dominant allele.

It has been known for some time that people from different parts of the world have very different rates of lactose intolerance - ranging from 2% intolerant in Sweden to 98% intolerant in Thailand and 100% in Native American populations.

The Cornell study published last year showed that it is primarily people whose ancestors came from places where dairy herds could be raised safely and economically, such as in Europe, who have developed the ability to digest milk. The mutation that originally produced the lactose tolerant gene (or rather the lactase-persistence gene) is thought to have occurred as recently as 4000 years ago. You can read more about the story at the Wikipedia lactose intolerance entry.