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.
Researchers have identified a mutation in a gene that confers
several distinct traits to East Asians, including thicker hair.
The traits — thicker hair shafts, more sweat glands,
characteristically identified teeth and smaller breasts — are the
result of a gene mutation that occurred about 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 one another.
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.
Seeking to understand if the gene was the cause of thicker hair in
East Asians with the variant gene, a team of researchers led by
Yana G. Kamberov and Pardis C. Sabeti at the Broad Institute in Cambridge, Mass.,
decided to test the gene in mice, where its effects could be more
easily explored.
Mice already have EDAR, an ancient mammalian gene that plays a
leading role in the embryo in shaping hair, skin and teeth. The
Broad team engineered a strain of mice whose EDAR gene had the same
DNA change as the East Asian version of EDAR.
When the mice grew up, the researchers found they did indeed have
thicker hair shafts, confirming that the changed gene was the cause
of East Asians’ thicker hair. But the gene had several other
effects, they
report in Thursday’s issue of the journal Cell.
One was that the mice, to the researchers’ surprise, had extra
sweat glands. A Chinese member of the team, Sijia Wang, then tested
people in China and discovered that they, too, had more numerous
sweat glands, evidently another effect of the gene.
Another surprise was that the engineered mice had less breast
tissue, meaning that EDAR could be the reason that East Asian women
have generally smaller breasts.
East Asians have distinctively shaped teeth for which their version
of EDAR is probably responsible. But the mice were less helpful on
this point; their teeth are so different from humans’ that the
researchers could not see any specific change.
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 role of sweat glands 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 about 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 blond 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 60 to 90 percent of
American Indians, a population descended from East Asians.
The Broad team is studying EDAR as part of a larger plan to
identify all the genetic variants responsible for recent human
evolution. Many researchers, including Dr. Sabeti, have devised
ways of scanning the human genome to detect the fingerprints of
natural selection. But these scans have typically identified large
chunks of the genome that contain many genes. There is often no way
to tell which gene was the target of natural selection.
A team led by Dr. Sabeti and Sharon R. Grossman of the Broad
Institute has now refined the usual scanning methods and identified
412 sites on the genome that have been under selection. Each site
is small enough that it contains at most a single gene.
Each race has a different set of selected regions, reflecting the
fact that the human population had dispersed from its African
homeland and faced different challenges that led to genetic
adaptation on each continent. About 140 of the sites affected by
natural selection are in Europeans, 140 in East Asians and 132 in
Africans, the authors report in another article published Thursday in
Cell.
Inserting some of the other selected genes into mice might help
explain why they were favored, and point to critical turning points
in recent human evolution, Dr. Sabeti said.
In the case of EDAR, putting the gene into mice has only magnified
the mystery of why it was selected. But the researchers are not
discouraged. “A reflection of good science is that a step forward
opens up a lot more questions,” Dr. Akey said.