Variations in skin color seem to be among the most obvious indications of human diversity. The question of why these variations exist dates itself all the way back to the time of Hippocrates, with Aristotle himself pioneering a “climatic theory” that associated human physical features (including skin pigmentation) and culture with environmental zones. The eighteenth-century naturalists John Mitchell and Samuel Stanhope Smith adopted the currently accepted theory that a “pronounced latitudinal gradient of skin pigmentation” exists “from dark near the equator to light toward the poles-and related it mainly to differences in sunshine heat experienced by people at different latitudes” (Jablonski & Chaplin, 2010).
However, the underlying question of what evolutionary advantage conferred by the latitudinal effect is very much a matter of modern research. Current hypotheses include that natural selection provided pressure selecting against conditions involving sunburn, skin cancer, the overproduction of vitamin D, and the protection of folates, among others.
The current scientific consensus is that the first humans might not have actually been black or had very dark pigmentation. The chimpanzee, the nearest living relative of modern humans, is nearly completely pale, with all melanin concentrated in hair follicles instead of the skin itself (Greaves, 2014). In addition to other closely-related primates being pale, further research suggests that perhaps the first humans in Africa developed their dark pigmentation in a manner akin to the “Ancestral North Indians” and their settlement in India. In the case of India, a lightly and moderately pigmented population moved into an area of strong UV radiation and appeared to have evolutionarily adapted to the different climatological setting with darker pigmentation (Jablonski & Chaplin, 2010).
The first humans in Africa might have undergone a similar change as their habitats changed from the rainforest to the savanna through climate change (Greaves, 2014). However, the shift to the savanna environment alone does not explain why the phenomena occurred. Internal processes within humans must have been affected by the exposure to increased sunlight, and the question remains as to what factors those are.
In addressing this question, the factors of sunburn and overproduction of vitamin D have been dismissed as selective agents by modern researchers, although disagreement exists over the importance of skin cancer. In part, sunburn was dismissed as a driver of change because it is a temporary ailment that normally has little to no influence at all on reproductive success. The overproduction of vitamin D in tropical regions was also dismissed because recent research has found that photochemical regulation prevents the skin from producing vitamin D beyond a certain threshold (Jablonski & Chaplin, 2010).
However, skin cancer has not been dismissed as a factor by some researchers. In particular, Jablonski and Chaplin (2010) argue that skin cancer matters little to reproductive success because it typically sets in after the age range for human reproduction in most individuals.However, Greaves (2014) takes the view that although the former may be generally true, one historical context might exist where skin cancer did substantially affect reproductively-capable individuals, and that influenced the onset of black skin in Africa.
One of the genes that bears large control over pigmentation variations is MC1R. In most parts of the world, considerable variation exists in the alleles expressed from this gene, leading to a variety of different skin tones. However, in Africa, an unusually low amount of variation exists in the general human population; in other words, one general skin tone seems to be unusually predominant. This suggests that very strong selection pressure has taken place, leading to most individuals in the population to express only one form of the gene and wiping out others (Greaves, 2014).
This means that perhaps individuals with paler skin actually died as a result of their pigmentation, reopening the debate on whether skin cancer did actually affect reproductively viable populations. In the case of Africa, this might very well be the historical reality.
The importance of protecting folates in human development is also a likely factor of selection pressure. Jablonski and Chaplin (2010) indicate that folates function as “the primary mechanism for removing UVR-induced DNA photoproducts” and as repairing vessels for nucleotides. Given that folates deteriorate in strong ultraviolet radiation, too little folate is likely to result in potentially harmful genetic mutations and nucleotide malformations. Accordingly, dark pigmentation would thus receive positive selection pressure in that the increased melanin prevents a higher level of radiation from damaging folates in the skin than in people with less pigmentation.
Thus, the question of whether an adaptive value exists in having various levels of pigmentation appears self-evident. In particular, darker forms of pigmentation likely developed from what were originally lightly-pigmented humans to potentially reduce rates of skin cancer as well as to prevent ultraviolent radiation from interfering with nucleotide repairs as much as possible.
