Research on the evolution of human skin pigmentation began before information on the DNA sequences of human pigmentation genes was available. Anthropologists deduced that hominids living outside the tropics would have been under evolutionary pressure to lose some of the pigment in their skin as they moved north. This deduction was based on the fact that dark skin and reduced UVB exposure would have made it difficult for people to produce enough vitamin D in their skin to stay healthy and reproduce. Therefore, skin would have had to become lighter. This is the basis for what has long been known as the "vitamin D hypothesis" of human skin pigmentation. Some authorities argued that the body’s ability to store vitamin D could get humans through the months of the year when active production in the skin was not possible. But the time limit for vitamin D storage is about two months, not long enough to see our lean and active ancestors at high latitudes through from the last UVB of autumn to the first UVB of spring. Females, with naturally more subcutaneous body fat than males, would have been able to store more vitamin D, but even these stores would not have been sufficient to meet their year-round requirements. For people at high latitudes, loss of pigmentation or depigmentation was not an option; it was essential to survival.
In the past decade, researchers have accumulated extensive information on patterns of variability in human pigmentation genes and what these patterns mean.7 Many different genes contribute to human pigmentation, including the MC1R locus. Most of the variations in the DNA sequence of the MC1R gene are associated with variations in skin and hair pigmentation. The results of genetic comparisons of the MC1R gene across many living human groups demonstrate that there is little variation in the gene within Africa but a lot outside Africa. The near absence of variation in MC1R was established in the early history of the Homo lineage by strong natural selection, often called positive selection. Since then, a low level of variation in MC1R in native African peoples has been maintained by natural selection working to eliminate extreme variations in pigmentation. In contrast, the gene is highly variable outside Africa, especially in northern Europe, where it is associated with red or blond hair and lightly pigmented skin.
The skin of native northern Europeans contains relatively little melanin, and most of it is yellow-red pheomelanin, not dark brown eumelanin. The pheomelanin is packaged into small melanosomes that are grouped together in membrane-bound clusters. The color of the pheomelanin and the way it is packaged make the skin lookpale. When pale human skin is exposed to UVR, it produces more pheomelanin, not the darker and more protective eumelanin. Pheomelanin reacts with UVR to actually produce dangerous free radicals in melanocytes, just the opposite of what eumelanin does to neutralize these molecules. The production of free radicals in melanocytes is a deadly chemical reaction because it is the first link in the chain of causality leading to the most serious of skin cancers, melanoma. The facts suggest that lightly pigmented skin evolved in people who lived with generally low levels of UVR.
For many years, scientists were puzzled about the gene or genes that were associated with light skin pigmentation. They knew that variant forms of MC1R were associated with red hair and pale skin in northern Europeans, but the curious thing was that different forms of MC1R in this group were associated with different hair colors, but not with different skin colors. This led them to suspect that a gene other than MC1R was responsible for the slight differences in skin color that are observed in various northern European groups: Scandinavians versus Irish and Scots, for instance. ….
The finding that lightly pigmented skin evolved independently in the ancestors of modern Europeans and East Asians is extremely important. It suggests that at least two distinct genetic mutations occurred and underwent positive selection in two regions of the world that receive relatively low levels of UVR. These genetic findings supported the our previous hypothesis that loss of dark skin pigment was an essential adaptation to life in northern regions of the world where the sun shines less brightly. The genetic signature of strong positive natural selection in the European variant of SLC24A5 indicates that it was strongly favored in evolution until it achieved 100 percent frequency in the population. The most likely reason for this shift was that it was associated with a loss of skin pigment that favored vitamin D production under conditions of low UVB…..
Our understanding of human skin-color genetics is still incomplete, but with the tools of modern genetics and comparative genomics, we are making headway. Human geneticists have long known that skin color is controlled by several genes and that these genes interact in complex ways. We now understand that skin, hair, and eye color are all affected by multiple genes and that in some populations some variant forms of the genes account for more of the variation in skin color than in hair color or vice versa. Combinations of different forms of the genes bring about the complex and continuous variation in coloration we see in humans today. Another important outcome of the genetic studies of the past decade is the finding that pigmentation genes evolve on their own and that pigmentation undergoes evolution independently of other traits: it is not intrinsically linked to other features of appearance, constitution, or behavior. Often when people talk of races, like the "white race" or the "black race," as real biological categories, they assume that skin color and other traits of appearance, or even of temperament, are biologically connected in a heritable package. They are not. Skin color is a biological reality; race is not.
Reprinted from Living Color: The Biological and Social Meaning of Skin Color, by Nina G. Jablonski. Copyright © 2012 by Nina G. Jablonski. Reproduced by permission of University of California Press.