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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, 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.
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."
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