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.
Posted: Wed 07 Oct 2009 15:19 Post subject: Sexual selection
Your theory is the main rival, (not Chaplin and Jablonski) according to Peter Frost
"Of course, the revised vitamin-D hypothesis is still plausible, i.e., Europeans became pale-skinned after giving up hunting and gathering for agriculture"
if the Baltic paleness were due to sexual selection, then its dimorphism (the difference in skin tone between males and females) would be greater in northern Europe than in central Africa. In fact, this is not the case. Central Africans are much darker than northern Europeans, of course, but the relative difference between males and females is about the same in both regions
This point may not conclusively disprove Peter Frost's theory, his response is here
Quote:
Even if this selection pressure had acted only on ancestral European women, both sexes would have lightened in color. This is because most skin-color genes are not sex-linked. Nonetheless, some are, so women should have lightened a bit more than men did, thus making skin color more dimorphic in Europeans than in other populations. Yet the reverse seems to be true (Frost, in press; Madrigal and Kelly, 2006). The reason may be a ceiling effect, i.e., as ancestral Europeans approached the limit of maximum skin depigmentation, this lightening would have become increasingly hard to achieve for women—already lighter-skinned—than for men. Even though sexual selection had operated primarily on women, men would have lightened more
White Europeans evolved only ‘5,500 years ago’.
"It was only when early humans gave up hunter-gathering and switched to farming about 5,500 years ago that white skin began to be favoured, say the researchers."
Hmmm, could this 'new' theory and your own be related by any chance?
could this 'new' theory and your own be related by any chance?
It suggests the same hypothesis as I do. Technically speaking though, I think you mean "hypothesis" not "theory." There are several hypotheses trying to explain the known findings, and I believe that the one suggested at the linked-to site is the most popular one right now. But no consensus "theory" has yet triumphed.
In sunny countries such as Nigeria, South Africa, and Bangladesh, [...] Studies indicated that the disease occurs among older toddlers and children and probably is attributable to low dietary calcium intakes, which are characteristic of cereal-based diets with limited variety and little access to dairy products.
However if being a hunter gatherer in Africa provided the required calcium and vitamin D one would expect African hunter gatherer populations ( ie Khoisan and Pygmy) to have an average skin tone darker than those African populations who have cultivated cereal grains for thousands of years.
However if being a hunter gatherer in Africa provided the required calcium and vitamin D one would expect African hunter gatherer populations ( ie Khoisan and Pygmy) to have an average skin tone darker than those African populations who have cultivated cereal grains for thousands of years.
That is true. The Khoisan and also the Oromo of Ethiopia are genetically similar, and ancestral to everybody else. And both groups have lighter skin tone than later Africans. Apparently, their ancestors were displaced and split into two enclaves by the eastward-moving wedge of Bantu-speaking agriculturalists.
Because of this, I have suspected that the extreme dark skin of the agriculturalists is also a later adaptation. Unfortunately, that leaves me explaining both the northern Euro paleness and the Bantu-speaking darkness as consequences of agriculture. Obviously, the hypothesis needs more work.
In sunny countries such as Nigeria, South Africa, and Bangladesh, [...] Studies indicated that the disease occurs among older toddlers and children and probably is attributable to low dietary calcium intakes, which are characteristic of cereal-based diets with limited variety and little access to dairy products.
However if being a hunter gatherer in Africa provided the required calcium and vitamin D one would expect African hunter gatherer populations ( ie Khoisan and Pygmy) to have an average skin tone darker than those African populations who have cultivated cereal grains for thousands of years.
Good point. Black skin is quite mystery in that case. One thing to investigate is the nutrion of Khoisan and Oromo populations. Perhaps they have supplementary vitamin D?
'
FAQ
Search
Register
Log in
