For many years, skin color has been cited as an example of adaptation through natural selection in humans. In general, pigmentation in indigenous populations prior to European contact (beginning around 1500) followed a particular geographical distribution, especially in the Old World. This pattern pretty much holds true today. As Figure 15-1 shows, populations with the most pigmentation are found in the tropics, while lighter skin color is associated with more northern latitudes, especially among the long-term inhabitants of north western Europe. Three substances influence skin color haemoglobin, the protein carotene, and, most important, the pigment melanin. Melanin is a granular substance produced by cells called melanocytes, located in the outer layer of the skin (Fig. 15-2). Melanin is extremely important because it acts as a built-in sunscreen by absorbing potentially dangerous ultraviolet (UV) rays that are present but not visible in sunlight. So melanin protects us from overexposure to UV radiation, which frequently causes genetic mutations in skin cells. These mutations can lead to skin cancer, which, if left untreated, can eventually spread to other organs and even cause death (see “A Closer Look: Skin Cancer and UV Radiation”). As mentioned earlier, exposure to sunlight triggers a protective mechanism in the form of tanning, the result of a temporary increase in melanin production (acclimatization). This response occurs in all humans except albinos, who carry a genetic mutation that prevents their melanocytes from producing melanin. But even people who do produce melanin differ in their ability to tan. For instance, in all populations, women tend not to tan as deeply as men. More importantly, however, people of northern European descent tend to have very fair skin, blue eyes, and light hair. Their cells produce only small amounts of melanin and, when exposed to sunlight, they have almost no ability to increase production. But in areas closest to the equator (the tropics), where the sun’s rays are most direct and where exposure to UV light is most intense and constant, natural selection has favored deeply pigmented skin. In considering the cancer-causing effects of UV radiation from an evolutionary perspective, keep in mind these three points:
- Early hominins lived in the tropics, where solar radiation is more intense than in temperate areas to the north and south.
- Unlike most modern city dwellers, early hominins spent their days outdoors.
- Early hominins didn’t wear protective clothing.
Under these conditions, UV radiation was a powerful agent selecting for maximum levels of melanin production as a means of protection from UV radiation. Physical anthropologists have long considered this protection to be very important, because UV radiation is the most common cause of skin cancer. There’s an important objection to this hypothesis, however. As we mentioned in Chapter 4, natural selection can act only on traits that affect reproduction. Because cancers tend to occurlater in life, after people have had their children, it should theoretically be difficult for selection to act effectively against a factor that might facilitate the development of cancer. This is probably true in general, but in one African study it was shown that all albinos in dark-skinned populations of Nigeria and Tanzania had either precancerous lesions or skin cancer by the age of 20 (Robins, 1991). This evidence suggests that in early hominins of reproductive age, less pigmented skin could potentially have reduced individual reproductive fitness in regions of intense sunlight.
However, Jablonski (1992) and Jablonski and Chaplin (2000, 2010) disagree that skin cancer was the most important factor; they have provided convincing evidence for another, probably more important explanation for heavily pigmented skin in the tropics. This explanation concerns the degradation of folate by UV radiation. Folate is a B vitamin that is not stored in the body and must be replenished through dietary sources such as leafy green vegetables and certain fruits. Adequate levels of folate are required for cell division, and this is especially important during embryonic and fetal development, when cell division is rapid and ongoing. In pregnant women, insufficient levels of folate are associated with numerous fetal developmental disorders, including neural tube defects such as spina bifida (Fig. 15-3). The consequences of severe neural tube defects can include pain, infection, paralysis, and even failure of the brain to develop. Given the importance of folate to many processes related to reproduction, it’s clear that maintaining adequate levels of this vitamin contributes to individual reproductive fitness.
Studies have shown that UV radiation rapidly depletes serum folate levels in both laboratory animals and lightskinned people. These findings have implications for pregnant women, children, and the evolution of dark skin in early hominins. Jablonski (1992) has proposed that the earliest homininsmay have had light skin covered with dark hair, as seen in chimpanzees and gorillas (who have darker skin on exposed body parts, such as the face and hands). But as loss of body hair occurred in hominins, dark skin evolved rather rapidly as a protective response to the damaging effects of UV radiation on folate.
The maintenance of sufficient levels of folate and, perhaps to a lesser degree, the occurrence of skin cancer have no doubt been selective agents that have favored dark skin in populations living where UV radiation is most intense. Therefore we have good explanations for darker skin in the tropics. But what about less pigmented skin? Why do indigenous populations in higher latitudes, farther from the equator, have lighter skin? There are several closely related hypotheses, and recent studies have added strength to these arguments.
As hominins migrated out of Africa and into Asia and Europe, they faced new selective pressures. In particular, those populations that eventually occupied northern Europe encountered cold temperatures and cloudy skies, frequently during summer as well as winter. Winter also meant many fewer hours of daylight, and with the sun well to the south, solar radiation was very indirect. What’s more, people in these areas wore animal skins and other types of clothing, which blocked the sun’s rays. For some time, researchers proposed that because of reduced exposure to sunlight, the advantages of deeply pigmented skin in the tropics no longer applied, and selection for melanin production may have been relaxed.
However, relaxed selection for dark skin doesn’t adequately explain the very depigmented skin seen in some northern Europeans. In fact, natural selection appears to have acted very rapidly against darker skin as humans moved to northern latitudes. This is because the need for a physiological UV filter was outweighed by another extremely important biological necessity, the production of vitamin D. The theory concerning the role of vitamin D is called the vitamin D hypothesis.
Since the early twentieth century, scientists have known that vitamin D is essential for the mineralization and normal growth of bones during infancy and childhood because it enables the body to absorb calcium (the major source of bone mineral) from dietary sources. Vitamin D is also required for the continued mineralization of bones in adults. Many foods, including fish oils, egg yolk, butter, cream, and liver, are good sources of vitamin D. But the body’s primary source of vitamin D is its own ability to synthesize it through the interaction of UV radiation and a form of cholesterol found in skin cells. Therefore adequate exposure to sunlight is essential to normal bone growth.
Insufficient amounts of vitamin D during childhood result in rickets, a condition that leads to skeletal deformities, especially in the weight-bearing bones of the legs and pelvis. Thus, people with rickets frequently have bowed legs and pelvic deformities (Fig. 15-4). Pelvic deformities are of particular concern for pregnant women because they can lead to a narrowing of the birth canal. Without surgical intervention, both the mother and her infant can die during childbirth, thus allowing natural selection to act powerfully in favor of any mechanism that provides proper bone mineralization.
In addition to its role in bone mineralization, vitamin D has many other critical functions. In the body, vitamin D is converted to a different molecule called 1,25D, which can attach directly to DNA, and act to regulate more than 1,000 different genes (TaveraMendoza and White, 2007). Some of these genes are involved in cell replication, and because 1,25D influences this activity, it appears to provide some protection against certain cancers, especially prostate and colon cancer (Lin and White, 2004). (Cancer is caused by uncontrolled cell replication.) Moreover, 1,25D reduces inflammation and may eventually be used as a basis for treating certain diseases, including multiple sclerosis (Tavera-Mendoza and White, 2007).
Other genes influenced by 1,25D produce proteins that act as natural antibiotics to kill certain bacteria and viruses, one of which is Mycobacterium tuberculosis (M. tuberculosis), the bacterium that causes tuberculosis (TB). Liu et al. (2006) demonstrated how 1,25D is involved in the destruction of M. tuberculosis in infected cells. The fact that exposure to UV radiation is necessary for vitamin D synthesis probably explains why, in the early twentieth century, TB patients often improved after being sent to sanitariums in sunny locations.
The influence of latitude and skin pigmentation on levels of 1,25D in the body has been demonstrated by epidemiological studies of modern populations. For example, one study revealed that 92 percent of more than 400 girls in several northern European countries were severely deficient in 1,25D during the winter months. Also, the fact that African Americans appear to have about half the amount of 1,25D seen in European Americans illustrates the role of increased pigmentation in reducing vitamin D levels in more northern latitudes (Tavera-Mendoza and White, 2007). This fact is significant because African Americans have a higher incidence of TB than European Americans (Liu et al., 2006).
As you can see, vitamin D is an immensely important factor in the body’s response to a number of conditions, many of which influence reproductive success. This evidence substantially supports the vitamin D hypothesis and argues for strong and rapid positive selection for lighter skin in northern latitudes. Furthermore, the vitamin D evidence is strongly supported by recent genetic studies.
At least 100 genes are thought to be involved in pigmentation in vertebrates. One of the more important of these genes is called MC1R, which affects coloration in all mammals. The human version of this gene has at least 30 alleles, some of which are associated with red hair combined with fair skin and a tendency to freckle (Lin and Fisher, 2007). As we mentioned in Chapter 13, research on Neandertal DNA has revealed that some Neandertals probably had red hair and fair skin. This research revealed the presence of an MC1R allele that reduces the amount of pigment in skin and hair, but it’s not an allele that occurs in modern humans. Therefore less pigmented skin developed in two hominin species but through different mutations in the same gene. This fact strongly reinforces the hypothesis that there is a significant selective advantage to having lighter skin in higher latitudes.
Lastly, evidence for the importance of vitamin D is provided by the recent discovery of yet another gene, called SCL24A5, which we’ll refer to simply as SCL (Lamason et al., 2005). This gene and its effects on pigmentation were first discovered in zebrafish, and just to emphasize (yet again) the concept of biological continuity or connections between species, we’ll point out that approximately 68 percent of the sequences of DNA bases in the human and zebrafish SCL genes are the same (Balter, 2007).
Like MC1R, the SCL gene is involved in melanin production. This gene has two primary alleles that differ by one single base substitution; that is, one allele arose as a point mutation. The original form (allele) of the gene is present in 93 to 100 percent of Africans, Native Americans, and East Asians. However, and most importantly, virtually 100 percent of Europeans and European Americans have the more recent (mutated) allele that inhibits melanin production. These frequencies provide yet more compelling evidence of very strong selection for lighter skin in northern latitudes. In fact, it appears that natural selection favored the mutated allele to the point that it became the only SCL allele in northern European populations. Butthere’s a question that has yet to be answered. (Actually, there are several questions, but we’ll mention only one.) In East Asians, the frequency of the original melanin-producing allele is the same as in sub-Saharan Africans, yet on average, skin color in East Asians is fairly light. Lamason and colleagues (2005) argue that this means that in East Asian populations, there are other, as yet unidentified genes that interact with the SCL locus to reduce skin pigmentation. Certainly, several other genes that contribute to skin pigmentation have been identified, but none has yet been shown to have the same degree of variation between populations.
Jablonski and Chaplin (2000) have looked at the potential for vitamin D synthesis in people of different skin color based on the yearly average UV radiation at various latitudes (Fig. 15-5). Their conclusions support the vitamin D hypothesis to the point of stating that the requirement of vitamin D synthesis in northern latitudes was as important to natural selection as the need for protection from UV radiation in tropical regions.
Except for a person’s sex, more social importance has been attached to skin color than to any other single human biological trait. But there’s absolutely no valid reason for this. Aside from its adaptive significance relative to UV radiation, skin color is no more important physiologically than many other biological characteristics. But from an evolutionary perspective, skin color provides an outstanding example of how the forces of natural selection have produced geographically patterned variation as the result of two conflicting selective forces: the need for protection from overexposure to UV radiation on the one hand and the need for adequate UV exposure for vitamin D synthesis on the other.