Despite the fact that we've elected a black president, race is still a problem in the US. We know that humans originated in Africa, so why did some people's skin turn white, anyway?
Scientists used to tell us that people's skins turned lighter when they migrated north and needed more sun exposure in order to get enough vitamin D. Melanin, the pigment that turns skin dark, blocks out this vitamin. Vitamin D deficiency can lead to rickets and even to a deformed pelvis in women, which would interfere with childbirth. But the truth is that much of this is still a mystery.
But Melanin is not an absolute screen against UVB. 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 is the same as 2 to 3 hours of sunlight about 3 times a week for Africans living in Europe.
New Scientist, Anil Ananthaswamy quotes geneticist Ashley Robins as saying, "Early humans would have had that amount of exposure every day, and that would certainly have overwritten any melanin barrier. I'm pretty certain that you would not have got vitamin D deficiency and rickets."
Vitamin D expert Michael Holick thinks Robins is wrong because rickets is a debilitating disease with serious consequences. Holick is quoted as saying, "De-pigmentation would have had to occur within a few generations. Otherwise, you would not have been able to procreate in northern European environments."
A third researcher, Asta Juzeniene of Norway, thinks that the problem might be frostbite. A lack of the vitamin has also been linked to diabetes, cancer, rheumatoid arthritis, multiple sclerosis and heart disease.
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