From the Director | May 2010

Archaeology is a subfield of anthropology. We study linguistics, social organization, and physical anthropology on our way to our degrees and careers breathing dust and diesel fumes under the hot sun. This diverse background gives me license to stray into physical anthropology, with a brief review of the evolutionary genetics of skin color.

My interest in this topic was rekindled by the 2010 census. Question 9 gave me a list of color/ethnicity/nationality/language terms as options to describe my “race.” I answered the way I knew they wanted me to answer, but it seemed a little nonsensical. The question reflects a general confusion in our society surrounding our concept of race. In one sense, confusion is a good thing, because there is nothing anthropologically absolute about races. Racial categories are simply “us vs. them” in a cultural sense, and in the United States, “we” (the majority of the population descended from Europeans) currently apply a complex mix of skin color, language, and national origin to distinguish “races” as is convenient for us. However, that convenience can be elevated to bigotry, which is not a good thing and has no biological basis. Witness the candidacy of the first American president with African genetic heritage, which elicited statements such as, “I’ll never vote for a black man.”

The first and most persistent racial divisions in our society are based on skin color. Skin color is genetically controlled, is consistent within reproductively circumscribed populations, and is visually obvious—very convenient for distinguishing “us” from “them.” Geographic patterns of skin color (darkness) are so consistent across world populations that anthropologists have always expected to find a powerful evolutionary explanation for it, but that explanation had eluded us until recently. Dr. Nina Jablonski, in an article in the 2004 Annual Review of Anthropology, has synthesized decades of research that explains why different populations have different skin colors. The underlying mechanism in turn explains why the color of someone’s skin is completely independent of other human qualities (language, culture, and ability).

European world travelers encountered people of darker skin colors as soon as they strayed from their northern climes. With only a few exceptions, and before the mass migrations of the past 500 years, skin colors were uniformly darker toward the equator. However, the biochemical mechanisms that produce darker skin vary between geographically separated populations. The strength of the darker-color-with-lower-latitude tendency, and the independent underlying pigmentation mechanisms within the skin of different populations, suggests a very strong evolutionary imperative that has resulted in dark skin at different times in human history.

Physiologists and anthropologists have sought to explain variations in skin color for more than a century. A linkage with the intensity of the sun at lower latitudes was suspected, especially since all peoples “tan” as a temporary response to sun exposure. Sun intensity includes several wavelengths of potentially damaging ultraviolet light (UVA and UVB), and melanin (the most common of several dark skin pigments) has the ability to intercept or block damaging UV wavelengths. Conversely, UV from the sun is essential to vitamin D production and calcium metabolism, and people with dark skin are at a distinct disadvantage toward the poles, where sun intensity is less.

Evolution requires a strong selective principle, something that either affects mortality or fertility in a way that changes the frequency of a particular genetically determined trait within a population. The vitamin D connection is a strong selective pressure. Without vitamin D supplements, darker skinned women living toward the poles suffer calcium deficiencies and reproduce poorly. In a population with a range of skin colors at high latitudes, lighter skinned women would have more healthy children than darker skinned women, quickly shifting the genetics of the population toward lighter skin colors.

But how does UV translate into a strong selective pressure toward dark skin in populations living near the equator? Skin cancer comes to mind, but skin cancers generally strike individuals after their reproductive lives are well past, so that cancer has little impact on overall population genetics. The “ah ha!” moment was when physiologists discovered that UV radiation destroys a necessary precursor chemical for sperm production. Simply put, light skinned men at lower latitudes will be less fertile than dark skinned men, shifting the population genetics rapidly toward darker skin.

Women and men therefore have conflicting needs concerning UV exposure. Women’s reproductive needs push populations lighter, while men’s needs push populations darker. The result is the best compromise for a given latitude, and there is a slight tendency within any given population for women to be lighter skinned than men. An interesting exception actually proves the rule. One of the anomalies in the geography of skin color is the dark skinned Inuits, who, like Icelanders and Finns, live at high latitudes. But their diet is so high in vitamin D that there is no need for women or men to synthesize the vitamin from sun exposure. Therefore there is no selective pressure pushing the population toward lighter skin, and the population remains dark.

Since populations have migrated between latitude zones during the tens of thousands of years of Homo sapiens development, there have been multiple opportunities to shift population genetics from dark skin to light and then back. Once a population has reduced or even eliminated a specific set of genetic instructions that produce dark skin, new mutations that accomplish the same result can be selected during the next migration toward the equator. This explains why the appearance or shade of dark skin is different in Africa and Asia—the underlying melanin systems are the result of different mutations becoming dominant in the populations.

In today’s world of long-distance population movements, technology has come up with ways to lessen selective evolutionary pressures. Vitamin D supplements can support the calcium metabolism of dark skinned women in the high latitudes of northern Europe, while clothing, sun screen, and buildings can sustain the reproductive potency of light skinned men at low latitudes. Remove these technological supports and the forces of natural selection would quickly (within a few centuries) begin to restore the “natural” geography of skin color. These principles would apply equally to a displaced group of Icelanders as it would to a displaced group from Zambia.

Simply put, skin color has everything to do with where on the globe your ancestors lived (as mediated by what they ate), and nothing to do with any other human characteristic or potential. Why we in the United States characterize “race” in terms of skin color is purely cultural and historical, initially reflecting the needs of northern European peoples to differentiate “us vs. them” as they encountered people who were from different areas of the world. The skin color difference made it easy to both rationalize and implement the enslavement of dark skinned Africans, and the inherited nature of skin color made it possible to maintain boundaries by proscribing social (and genetic) mixing. The nonsensical quality of the categories on the census form is evidence of the internal illogic of our concept of race. We can hope that as bigotry and prejudice based on appearance subside, there will be less and less need to maintain and gather statistics on these false categories.

Editor's note: Nina G. Jablonski wrote a popular study, Skin: A Natural History (University of California Press, 2006). Thanks to David Noble for bringing it to our attention.