December 10, 2006
|Ability To Digest Milk A Very Recent Adaptation||Science/Technology|
Geneticists have determined that the human ability to digest milk in adulthood evolved just several millenia ago, soon after humans domesticated cattle. At least four different populations developed lactose tolerance independently, with a different mutation involved in each case. NYT:
A surprisingly recent instance of human evolution has been detected among the peoples of East Africa. It is the ability to digest milk in adulthood, conferred by genetic changes that occurred as recently as 3,000 years ago, a team of geneticists has found.
The finding is a striking example of a cultural practice — the raising of dairy cattle — feeding back into the human genome. It also seems to be one of the first instances of convergent human evolution to be documented at the genetic level. Convergent evolution refers to two or more populations acquiring the same trait independently.
Throughout most of human history, the ability to digest lactose, the principal sugar of milk, has been switched off after weaning because there is no further need for the lactase enzyme that breaks the sugar apart. But when cattle were first domesticated 9,000 years ago and people later started to consume their milk as well as their meat, natural selection would have favored anyone with a mutation that kept the lactase gene switched on.
Such a mutation is known to have arisen among an early cattle-raising people, the Funnel Beaker culture, which flourished some 5,000 to 6,000 years ago in north-central Europe. People with a persistently active lactase gene have no problem digesting milk and are said to be lactose tolerant.
Almost all Dutch people and 99 percent of Swedes are lactose-tolerant, but the mutation becomes progressively less common in Europeans who live at increasing distance from the ancient Funnel Beaker region.
Geneticists wondered if the lactose tolerance mutation in Europeans, first identified in 2002, had arisen among pastoral peoples elsewhere. But it seemed to be largely absent from Africa, even though pastoral peoples there generally have some degree of tolerance.
A research team led by Sarah Tishkoff of the University of Maryland has now resolved much of the puzzle. After testing for lactose tolerance and genetic makeup among 43 ethnic groups of East Africa, she and her colleagues have found three new mutations, all independent of each other and of the European mutation, which keep the lactase gene permanently switched on. [...]
Genetic evidence shows that the mutations conferred an enormous selective advantage on their owners, enabling them to leave almost 10 times as many descendants as people without them. The mutations have created "one of the strongest genetic signatures of natural selection yet reported in humans," the researchers write.
The survival advantage was so powerful perhaps because those with the mutations not only gained extra energy from lactose but also, in drought conditions, would have benefited from the water in milk. People who were lactose-intolerant could have risked losing water from diarrhea, Dr. Tishkoff said.
Diane Gifford-Gonzalez, an archaeologist at the University of California, Santa Cruz, said the new findings were "very exciting" because they "showed the speed with which a genetic mutation can be favored under conditions of strong natural selection, demonstrating the possible rate of evolutionary change in humans." [...]
"There is a lot of genetic variation between groups in Africa, reflecting the different environments in which they live, from deserts to tropics, and their exposure to very different selective forces," she said. [Emphasis added]
Somebody tell Kansas.
Kansas. The home of Dorothy and Toto. So all I can think of is, Kansas doesn't look like Kansas anymore, if I only had a brain, and the big and powerful Oz (who's actually a very little man hiding out).
Humans can tolerate milk, but still, we're the only animal that drinks milk as an adult, and drinks the milk of another animal. This last point is even more peculiar when you consider some men won't touch the breast milk of their wives, but will happily drink 2%. Go figure.
Posted by: Jeff at December 10, 2006 08:33 PM
There is no need to tell Kansas...every time the evangelicals sneak (and I do mean SNEAK) onto the school board and mess with our science standards, we just calmly wait until the next election and boot them off....as we just did (again) this November. They (the Evangelicals, pentacostals, etc.) just haven't figured out yet that a majority of Kansas doesn't want our science standards messed with, and we definately don't want our schools to turn into "Jesus Camp".
Posted by: b_heart11 at December 11, 2006 08:35 AM