Thursday, December 22, 2005

Scientists Find A DNA Change That Accounts For White Skin

Interesting study. Note this comment herein: "Several sociologists and others said they feared that such revelations might wrongly overshadow the prevailing finding of genetics over the past 10 years: that the number of DNA differences between races is tiny compared with the range of genetic diversity found within any single racial group." This is consistent with observations of differences between men and women, namely, that there are more differences within a gender than between them. One wonders why such studies continue? -Angela

By Rick Weiss

Scientists said yesterday that they have discovered a tiny
genetic mutation that largely explains the first appearance
of white skin in humans tens of thousands of years ago, a
finding that helps solve one of biology's most enduring
mysteries and illuminates one of humanity's greatest sources
of strife.

The work suggests that the skin-whitening mutation occurred
by chance in a single individual after the first human
exodus from Africa, when all people were brown-skinned. That
person's offspring apparently thrived as humans moved
northward into what is now Europe, helping to give rise to
the lightest of the world's races.

Leaders of the study, at Penn State University, warned
against interpreting the finding as a discovery of "the race
gene." Race is a vaguely defined biological, social and
political concept, they noted, and skin color is only part
of what race is -- and is not.

In fact, several scientists said, the new work shows just
how small a biological difference is reflected by skin
color. The newly found mutation involves a change of just
one letter of DNA code out of the 3.1 billion letters in the
human genome -- the complete instructions for making a human

"It's a major finding in a very sensitive area," said
Stephen Oppenheimer, an expert in anthropological genetics
at Oxford University, who was not involved in the work.
"Almost all the differences used to differentiate
populations from around the world really are skin deep."

The work raises a raft of new questions -- not least of
which is why white skin caught on so thoroughly in northern
climes once it arose. Some scientists suggest that lighter
skin offered a strong survival advantage for people who
migrated out of Africa by boosting their levels of
bone-strengthening vitamin D; others have posited that its
novelty and showiness simply made it more attractive to
those seeking mates.

The work also reveals for the first time that Asians owe
their relatively light skin to different mutations. That
means that light skin arose independently at least twice in
human evolution, in each case affecting populations with the
facial and other traits that today are commonly regarded as
the hallmarks of Caucasian and Asian races.

Several sociologists and others said they feared that such
revelations might wrongly overshadow the prevailing finding
of genetics over the past 10 years: that the number of DNA
differences between races is tiny compared with the range of
genetic diversity found within any single racial group.

Even study leader Keith Cheng said he was at first
uncomfortable talking about the new work, fearing that the
finding of such a clear genetic difference between people of
African and European ancestries might reawaken discredited
assertions of other purported inborn differences between
races -- the most long-standing and inflammatory of those
being intelligence.

"I think human beings are extremely insecure and look to
visual cues of sameness to feel better, and people will do
bad things to people who look different," Cheng said.

The discovery, described in today's issue of the journal
Science, was an unexpected outgrowth of studies Cheng and
his colleagues were conducting on inch-long zebra fish,
which are popular research tools for geneticists and
developmental biologists. Having identified a gene that,
when mutated, interferes with its ability to make its
characteristic black stripes, the team scanned human DNA
databases to see if a similar gene resides in people.

To their surprise, they found virtually identical
pigment-building genes in humans, chickens, dogs, cows and
many others species, an indication of its biological value.

They got a bigger surprise when they looked in a new
database comparing the genomes of four of the world's major
racial groups. That showed that whites with northern and
western European ancestry have a mutated version of the gene.

Skin color is a reflection of the amount and distribution of
the pigment melanin, which in humans protects against
damaging ultraviolet rays but in other species is also used
for camouflage or other purposes. The mutation that deprives
zebra fish of their stripes blocks the creation of a protein
whose job is to move charged atoms across cell membranes,
obscure process that is crucial to the accumulation of
melanin inside cells.

Humans of European descent, Cheng's team found, bear a
slightly different mutation that hobbles the same protein
with similar effect. The defect does not affect melanin
deposition in other parts of the body, including the hair
and eyes, whose tints are under the control of other genes.

A few genes have previously been associated with human
pigment disorders -- most notably those that, when mutated,
lead to albinism, an extreme form of pigment loss. But the
newly found glitch is the first found to play a role in the
formation of "normal" white skin. The Penn State team
calculates that the gene, known as slc24a5, is responsible
for about one-third of the pigment loss that made black skin
white. A few other as-yet-unidentified mutated genes
apparently account for the rest.

Although precise dating is impossible, several scientists
speculated on the basis of its spread and variation that the
mutation arose between 20,000 and 50,000 years ago. That
would be consistent with research showing that a wave of
ancestral humans migrated northward and eastward out of
Africa about 50,000 years ago.

Unlike most mutations, this one quickly overwhelmed its
ancestral version, at least in Europe, suggesting it had a
real benefit. Many scientists suspect that benefit has to do
with vitamin D, made in the body with the help of sunlight
and critical to proper bone development.

Sun intensity is great enough in equatorial regions that the
vitamin can still be made in dark-skinned people despite the
ultraviolet shielding effects of melanin. In the north,
where sunlight is less intense and cold weather demands that
more clothing be worn, melanin's ultraviolet shielding
became a liability, the thinking goes.

Today that solar requirement is largely irrelevant because
many foods are supplemented with vitamin D.

Some scientists said they suspect that white skin's rapid
rise to genetic dominance may also be the product of "sexual
selection," a phenomenon of evolutionary biology in which
almost any new and showy trait in a healthy individual can
become highly prized by those seeking mates, perhaps because
it provides evidence of genetic innovativeness.

Cheng and co-worker Victor A. Canfield said their discovery
could have practical spinoffs. A gene so crucial to the
buildup of melanin in the skin might be a good target for
new drugs against melanoma, for example, a cancer of melanin
cells in which slc24a5 works overtime.

But they and others agreed that, for better or worse, the
finding's most immediate impact may be an escalating debate
about the meaning of race.

Recent revelations that all people are more than 99.9
percent genetically identical has proved that race has
almost no biological validity. Yet geneticists' claims that
race is a phony construct have not rung true to many
nonscientists -- and understandably so, said Vivian Ota Wang
of the National Human Genome Research Institute in Bethesda.

"You may tell people that race isn't real and doesn't
matter, but they can't catch a cab," Ota Wang said. "So
unless we take that into account it makes us sound crazy."


  1. So all those white folk are mutant descendants of the brown folk.

    Wait till O'Reilly figures that one out.

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