http://www.newscientist.com/article/mg2 ... -from.html
Where does white skin come from?
THE idea that early humans became fair-skinned as they migrated north out of Africa so they could make enough vitamin D to stay healthy has been questioned again, reopening a debate that many think is settled.
In equatorial Africa and in the tropics, melanin - the pigment that makes skin dark - provides protection against the intense sunlight. But melanin can also block the ultraviolet radiation (UVB) that triggers vitamin D production in the skin. This is an advantage in the tropics, where UVB radiation is barely filtered by the atmosphere above.
But UVB intensity falls dramatically at higher latitudes, where melanin can pose a problem for dark-skinned people. Vitamin D deficiency can lead to rickets and women with the disease often develop a deformed pelvis, making it difficult for them to reproduce.
According to the vitamin D hypothesis, when humans left Africa tens of thousands of years ago and reached Europe, natural selection weeded out the melanin. While people with lighter skin could produce adequate levels of vitamin D, those whose skin remained dark were more likely to suffer from rickets. The hypothesis arose when studies from the early 20th century showed that blacks in the US were two to three times as likely to suffer from the disease as whites.
"It is a very attractive hypothesis and very few people have taken issue with it," says Ashley Robins of the University of Cape Town Medical School, Observatory, South Africa.
Robins is one of the few, arguing that adequate vitamin D wouldn't necessarily have been a problem. Melanin is not an absolute screen against UVB, he says. Dark-skinned people in higher latitudes need to be exposed to about 6 to 10 times as much sunlight as white-skinned people for the vitamin D in their blood to reach acceptable levels. This equates to about 2 to 3 hours of sunlight about 3 times a week for Africans living in, say, the UK. "Early humans would have had that amount of exposure every day," says Robins. "And that would certainly have overwritten any melanin barrier. I'm pretty certain that you would not have got vitamin D deficiency and rickets."
Robins also points to studies showing that while black volunteers have significantly lower blood levels of vitamin D than white volunteers after a whole-body dose of UVB, the difference narrowed and even disappeared when levels of metabolites derived from vitamin D were compared (American Journal of Physical Anthropology, DOI: 10.1002/ajpa.21077). This suggests that in darker-skinned people, enzymes from the liver and kidneys were working harder to keep the levels of the active metabolites the same, regardless of the skin pigmentation. "There seems to be a compensatory mechanism," says Robins. "That's another reason why the vitamin D hypothesis fails."
But Michael Holick, an expert on vitamin D at the Boston University School of Medicine, says Robins is wrong. Rickets is a debilitating disease with serious consequences, says Holick. "De-pigmentation would have had to occur within a few generations. Otherwise, you would not have been able to procreate in northern European environments."
Asta Juzeniene, of the Oslo University Hospital in Montebello, Norway, points out that the consequences of vitamin D deficiency go beyond rickets. She says a lack of the vitamin has also been linked to diabetes, cancer, rheumatoid arthritis, multiple sclerosis and heart disease.
Juzeniene and her colleagues recently reviewed alternate hypotheses for why humans might have evolved lighter skin (Journal of Photochemistry and Photobiology B: Biology, vol 96, p 93). One highly controversial idea involves sexual selection: once sensitive light skin was no longer hazardous, as in Africa, it was selected for sexual attractiveness. The other idea is that dark skin was more prone to frostbite in higher latitudes, and hence would have come under negative selection pressure, a claim that comes from studies of soldiers during the Korean war, when black soldiers suffered far more frostbite than white soldiers.
One idea is that dark skin was more prone to frostbite in higher latitudes and was selected against
Juzeniene is not convinced by these alternatives. "The vitamin D hypothesis is the most likely hypothesis although there is still no consensus about it," she says.
Robins, on the other hand, is keen on the frostbite theory for the evolution of lighter skin. "If darker skin people are going to have frostbite, and babies and mothers' nipples are going to be frostbitten, then like sunburn, this is going to be a potent selective force," he says.
George Bernard Shaw: “The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.”