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Why Whitey Don't Freeze (2/22/06)
A long time ago, all humans were black. Among other things, generous doses of the skin pigment melanin blocked much of the sun's harmful UV rays, helping them to survive in the blistering tropics of central Africa. As our ancestors began to spread out across the globe, however, most of the populations that found themselves in new environments didn't stay black. As they got further north, people tended to get paler and paler, arguably culminating in the blindingly white asses of the Scandinavians.
The usual explanation for this phenomenon is that as people moved into colder climates with less sunlight (and more clothes were needed to protect from the cold) they needed to allow more UV light into their skin in order to produce enough vitamin D. (UV light breaks down a precursor molecule to vitamin D, providing a building block needed by your metabolism in order to make vit-D.)
Setting aside for the moment the possibility that black folks just stood out against the glaciers of Norway too much and got eaten by polar bears, a more important factor in eliminating dark skin from populations in cold climates may have been physics rather than nutrition. As kids, most of us have had the experience of stepping barefoot on an asphalt street in the middle of summer and almost instantly having the soles of our feet char-broiled. The reason asphalt (and other dark objects) get so hot on sunny summer days is because dark materials more easily convert sunlight into heat. So, a dark object sucks up a lot of visible and infrared light (the sun puts out huge amounts of infrared), converting it to heat (which warms the object.) If you've looked inside a thermos, you've probably noticed that most are lined with a reflective silver material: That's to prevent heat from escaping as infrared light. Likewise, mylar 'emergency blankets' are highly reflective to prevent the escape of infrared light.
What's less obvious is that the process works just as well in reverse: Dark objects more readily give up their heat in the form of infrared light. That's why ultra-fast aircraft like the SR-71 Blackbird are painted black: The dark color rapidly radiates heat away from the surface of the plane; otherwise friction with the air could make the wings hot enough to cause structural failure at very high speeds. (The skin of the tiny but freakishly fast X-43 experimental craft can reach several thousand degrees from the heat produced by driving it through the atmosphere at up to 7,500 MPH.)
So, what does this have to do with race and climate? Simply, a person with dark skin will lose heat more quickly than a person with pale skin. The genes for light skin color that came to dominate the Northern European populations literally made people better insulated against the cold.