For centuries, skin color has held powerful social meaning — a defining characteristic of race, and a starting point for racism.
“If
you ask somebody on the street, ‘What are the main differences between
races?,’ they’re going to say skin color,” said Sarah A. Tishkoff, a
geneticist at the University of Pennsylvania.
On
Thursday, Dr. Tishkoff and her colleagues showed this to be a profound
error. In the journal Science, the researchers published the first
large-scale study of the genetics of skin color in Africans.
The
researchers pinpointed eight genetic variants in four narrow regions of
the human genome that strongly influence pigmentation — some making
skin darker, and others making it lighter.
These
genes are shared across the globe, it turns out; one of them, for
example, lightens skin in both Europeans and hunter-gatherers in
Botswana. The gene variants were present in humanity’s distant
ancestors, even before our species evolved in Africa 300,000 years ago.
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The
widespread distribution of these genes and their persistence over
millenniums show that the old color lines are essentially meaningless,
the scientists said. The research “dispels a biological concept of
race,” Dr. Tishkoff said.
Humans
develop color much as other mammals do. Special cells in the skin
contain pouches, called melanosomes, packed with pigment molecules. The
more pigment, the darker the skin.
Skin
color also varies with the kind of pigments: Melanosomes may contain
mixtures of a brown-black called eumelanin and a yellow-red called
pheomelanin.
To
find the genes that help produce pigments, scientists began by studying
people of European ancestry and found that mutations to a gene called
SLC24A5 caused cells to make less pigment, leading to paler skin.
Unsurprisingly, almost all Europeans have this variant.
“We
knew quite a lot about why people have pale skin if they had European
ancestry,” said Nicholas G. Crawford, a postdoctoral researcher at the
University of Pennsylvania and a co-author of the new study. “But there
was very little known about why people have dark skin.”
Since
the early 2000s, Dr. Tishkoff has studied genes in Africa, discovering
variants important to everything from resistance to malaria to height.
African
populations vary tremendously in skin color, and Dr. Tishkoff reasoned
that powerful genetic variants must be responsible.
Studying
1,570 people in Ethiopia, Tanzania and Botswana, she and her colleagues
discovered a set of genetic variants that account for 29 percent of the
variation in skin color. (The remaining variation seems tied to genes
yet to be discovered.)
One
variant, MFSD12, was particularly mysterious: No one knew what it did
anywhere in the body. To investigate its function, the researchers
altered the gene in reddish lab mice. Giving them the variant found in
darker-skinned Africans turned the mice gray.
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