NPR just did an interesting story on research on melanin and skin color shifts over relatively short evolutionary time, just a few thousand years. Those of us with darker skins may well have had ancestors just 2500 or so years ago who were much lighter in skin color, and those of us with lighter skin may have had much darker recent ancestors. The new research suggests that human evolution does not need thousands of years to change something as superficial as skin color. Given that reality, it is amazing and sad that we humans make so much of our thinking and social organization hinge on something as superficial as melanin variation and dark/light skin color.
Here is an interesting map (NPR, George Chaplin) of where darker-skinned people now live on the planet. Notice how skin color generally follows the levels of ultraviolet radiation (sun intensity) on the planet.
The NPR story quotes Anthropology Professor Nina Jablonski at Penn State, who argues that your current skin color
is very probably not the color your ancient ancestors had — even if you think your family has been the same color for a long, long time. … Skin has changed color in human lineages much faster than scientists had previously supposed, even without intermarriage, Jablonski says. …By creating genetic “clocks,” scientists can make fairly careful guesses about when particular groups became the color they are today. And with the help of paleontologists and anthropologists, scientists can go further: They can wind the clock back and see what colors these populations were going back tens of thousands of years, says Jablonski. She says that for many families on the planet, if we look back only 100 or 200 generations (that’s as few as 2,500 years), “almost all of us were in a different place and we had a different color.” … “People living now in southern parts of India [and Sri Lanka] are extremely darkly pigmented,” Jablonski says. But their great, great ancestors lived much farther north, and when they migrated south, their pigmentation redarkened.
Of course, we are all Africans if we go back about 100,000 years, and thus we all come from people who were once likely quite dark-skinned, given that we originated 
photo credit: Hitchster
in equatorial Africa where the levels of ultraviolet light were, and are quite high. Melanin is a type of skin molecule that makes
skin lighter or darker. Kind of like a Venetian blind, it can let UV light in or keep it out. . .. .Humans have had it for a long, long time and what Jablonski and others have learned is that when early humans migrated from the equator, their melanin levels changed.
And skin color can change much faster than earlier estimates suggested:
“Our original estimates were that [skin color changes] occurred perhaps at a more stately pace,” Jablonski says. But now they’re finding that a population can be one color (light or dark) and 100 generations later — with no intermarriage — be a very different color. Figuring 25 years per generation (which is generous, since early humans walked naked through the world — clothes slow down the rate), that’s an astonishingly short interval.
One thing that the NPR story does not deal with is that there is some significant variation in the map, with some far-north peoples having darker skin color than those somewhat to their south. One reason for this, a science blogger suggests, is that of nutrition and agricultural development:
The deleterious consequences of switching many non-agricultural populations to the starch rich diet are well known (obesity, diabetes, etc.). Selection happens, and it seems likely that a genetic revolution was ushered in by the radically altered nutritional universe of the farmer. … Frank W. Sweet published an essay in 2002 which offered that the feasibility of a farming lifestyle at very high latitudes in Europe due to peculiar climatological conditions served to drive Europeans to develop light skins over the last 10,000 years. In short, Sweet argues that the diets of pre-farming peoples were richer in meats and fish which provided sufficient Vitamin D so that skin color was likely light brown as opposed to pink. But with the spread of agriculture Vitamin D disappeared from the diets of northern European peoples and so only by reducing their melanin levels could they produce sufficient amounts of this nutrient to keep at bay the deleterious consequences of deficiencies. This explains why the Sami, who [live far north on the planet] never adopted agriculture, remained darker.
So, sociological factors loom large as well, in this case shifts in agriculture and food eaten. There are other environmental and genetic diversity factors as well, such as timing in evolution and genetic diversity in the initial population. And one must be careful about arguing from biological research on melanin to broader sociological issues.
Still, it never ceases to amaze me that melanin variation is such a powerful factor in the social construction of “race” among human beings, so much so that young people like Mr. Grant are now deceased because of melanin variation’s perception in some white person’s mind. How irrational is that?
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