The first appearance of white skin in humans
Scientists Find A DNA Change That Accounts For White Skin
Washington Post By Rick Weiss (Dec. 16, 2005)
Scientists 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, helping to give rise to the lightest of the world's races.
The findings show 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 of the human genome--the complete instructions for making a human being.
The work raises a raft of new questions--not least of which is why white skin caught on so thoroughly in northern climates 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.
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 discovery, described in the Journal Science, was an unexpected outgrowth of studies Cheng and his colleagues were conducting on the 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. It showed that whites
with northern and western European ancestry have a mutated version of
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, an 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.
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.
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