From today's New York Times:

A Genetic Glimpse Into Recent Human Evolution 

By NICHOLAS WADE 

Gaining a deep insight into human evolution, researchers have identified a mutation in a critical human gene as the source of several distinctive traits that make East Asians different from other races. 

The traits – thicker hair shafts, a greater number of sweat glands, characteristically identified teeth and smaller breasts – are the result of a gene mutation that occurred some 35,000 years ago, the researchers have concluded. 

The discovery explains a crucial juncture in the evolution of East Asians. But the method can also be applied to some 400 other sites on the human genome. The DNA changes at these sites, researchers believe, mark the turning points in recent human evolution as the populations on each continent diverged from each other. 

The first of those sites to be studied contains the gene known as EDAR. Africans and Europeans carry the standard version of the gene, but in most East Asians one of the DNA units has mutated. ...

The finding that the gene has so many effects raises the question of which one was the dominant trigger for natural selection. 

Dr. Sabeti said the extra sweat glands could have been the feature favored by natural selection, with all the other effects being dragged along in its train. 

“We’re the only mammals to have changed their entire hair pattern. So the changes in teeth, hair and breasts — it’s very possible they are the passengers and thermoregulation is the key,” she said, referring to the sweat glands’ role in cooling the body. 

East Asians are sometimes assumed to have evolved in a cold environment because of their narrow nostrils, which conserve heat, and the extra eyelid fat that insulates the eye. But the Broad team calculates that the EDAR variant arose some 35,000 years ago in central China and that the region was then quite warm and humid. Extra sweat glands would have been advantageous to the hunter gatherers who lived at that time. 

But Joshua Akey, a geneticist at the University of Washington in Seattle, said he thought the more likely cause of the gene’s spread among East Asians was sexual selection. Thick hair and small breasts are visible sexual signals which, if preferred by men, could quickly become more common as the carriers had more children. The genes underlying conspicuous traits, like blue eyes and blonde hair in Europeans, have very strong signals of selection, Dr. Akey said, and the sexually visible effects of EDAR are likely to have been stronger drivers of natural selection than sweat glands. 

Yet a third view is held by Dr. Kamberov who believes that each of the effects of the EDAR variant may have been favored by natural selection at a different time. A series of selections on different traits thus made the variant version so common among East Asians. About 93 percent of Han Chinese carry the variant, as do about 70 percent of people in Japan and Thailand, and between 60 and90 percent of Native Americans, a population descended from East Asians.