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Name: Phoebe N.
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: Sunday, October 27, 2002


Question:
If we are to believe that skin color is darker near the equator because of protection from the sun, and, conversely that the further away from the equator, the lighter the skin because of necessity to absorb vitamin D....why is it that there is such a discrepancy between skin color in Scandinavians and, say, native people of northern Canada or Alaska?


Replies:
Their ancestry is quite different.

Peter Faletra Ph.D.
Assistant Director
Science Education
Office of Science
Department of Energy


Since the human life span is so long, relatively, evolutionary change takes a very long time to show phenotypically. These hypotheses of the evolution of skin color try to answer the question of why dark skin seems to be more common at the equator and why light skin evolved in temperate areas. Recent evidence has forced a change in the old idea that dark skin evolved to protect from skin cancer at the equator. Although that is a benefit of more melanin, the new hypotheses say that dark skin protects against loss of folate, a vitamin that we need in our metabolism. In the regions with less direct sunlight this was not a problem, the new problem was getting enough sunlight to make Vitamin D. Those with lighter skin could get enough sunlight to make Vitamin D, making them more fit for their environment. This process however, took place over millennia. In the meantime humans were migrating all over the globe, crossing the land bridge from Asia into the Americas. In today's mobile society humans can "migrate" at will and you would expect that the populations of various regions would become more diverse. Humans also now have the ability to manipulate their environment, i.e., take vitamins, and cover their skin or go indoors to protect themselves from the environment.

Van Hoeck


There is are "problems" with your reasoning. I am not criticizing, because the same question asked in many different forms has been proposed both in the past and present.

First, it is not evident that people living near the equator have darker skin because of protection from the sun, or that people living in northern climates have light skin to increase the production of vitamin D. Does it? Where are the data? I could suggest that dark skinned people would produce more vitamin D because their skin is dark, and light skinned people would produce vitamin D less efficiently because their skin reflects more sunlight, thus inhibiting the production of vitamin D. That reasoning is called the fallacy of "correlation and causation". The two observations may be correlated, but one does not necessarily cause the other. The skin color correlation also ignores other differences, for example, diet.

Second, indigenous peoples in Australia and New Zealand are very dark - skin but do not live near the equator.

Third, we are observing the global distribution of skin color in a "snapshot" of time in an evolutionary geological context. Genetic changes generally require a very long time -- millions of years -- but our demographic observations are only a few thousand years old, and people can migrate from one region to the other in a matter of decades -- even hours in the last century -- which is much too fast for genetics to "catch up". So we do not know that Scandinavians always lived where they now live, and Africans always lived where they now lived.

Fourth, there may be more fundamental differences in the biochemistry and genetics that are operating, and skin color just happens to be a consequence with no relevance to exposure to sunlight. It is a very complex subject, that I do not think is well understood at all.

Vince Calder



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