Another very interesting article from John Noble Wilford of the NY Times. Researchers in Turkey have found evidence of tuberculosis in the cranium of a 500,000 year-old Homo erectus fossil. This sets a record by several hundred thousand years for the oldest known evidence for TB - the previous examples were only a few thousand years old from Egyptian and Peruvian mummies. But researchers are pushing their interpretations of the find's significance to another level:"But the discovery’s importance, scientists say, is the support it gives to the theory that dark-skinned people who migrate out of tropical climates tend to have lower levels of vitamin D, a condition that can adversely affect the immune system as well as the skeleton.
While the presumably dark skin of human ancestors protected them from the intense ultraviolet radiation from the African sun, the adaptation became a liability when they moved into the temperate latitudes of Eurasia, as the pigment melanin blocked much of the attenuated sunlight. The reduction of absorbed vitamin D from sunlight compromised their immune systems."
(snip)
"In Africa, Dr. Jablonski noted, sunlight is so strong that even with dark skins as protection against its deleterious effects, enough light is absorbed to provide high levels of vitamin D. The risk of vitamin D deprivation increased as dark-skinned people moved to temperate zones, then as now. In the ancestral migrations, she said, it probably took hundreds of generations for immigrants to evolve lighter skins for absorbing more sunlight."
This is the first and earliest physical data point that may support the theory that the lower levels of melanin in the skin of high-latitude humans is an adaptation to increase vitamin D absorption.
2 comments:
In The Third Chimpanzee, Jared Diamond makes a fairly convincing case that skin pigmentation may not be adaptive to sun levels at all. Why would the Tasmanians, who are about as far away from the equator as the French be so dark? They had plenty of time to evolve a more optimal coloration. Why would Scandanavians be so pale when they only migrated to that region in the comparatively recent past?
This conclusion seems to be reaching a rather long ways.
-R.A.W.
One possible explanation is that the southern hemisphere gets a lot more UV radiation than the northern due to differences in the ozone layer over each pole. Dark skin may well be protective so far south for that reason.
As for the blonde Swedes, it doesn't take very long for a population to change color. Maybe someone reading can cite the number of generations necessary, given enough selective pressure and enough people to work with, but I bet you it is fewer than you think. Evolution can work fast when it has to.
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