Human skin color

Human skin color can range from almost black (due to very high concentrations of the dark brown pigment melanin) to nearly colorless (appearing reddish white due to the blood vessels under the skin) in different people. Skin color is determined by the amount and type of melanin, the pigment in the skin. Variation in skin color is largely due to genetics. As a general pattern people with ancestors from tropical regions and higher altitudes (hence greater UV light exposure) have darker skin than people with ancestors from subtropical regions. This is far from a hard and fast rule however, because many light skinned groups have managed to survive at the equator by way of social adaptation. The same can be said of dark skinned groups living at subtropical latitudes

Melanin comes in two types: pheomelanin (red) and eumelanin (very dark brown). Both amount and type are determined by four to six genes which operate under incomplete dominance. One copy of each of those genes is inherited from each parent. Each gene comes in several alleles, resulting in the great variety of different skin tones.

The evolution of the different skin tones is thought to have occurred as follows: the haired primate ancestors of humans, like modern great apes, had light skin under their hair. When Hominids evolved relative hairlessness (the most likely function of which was to facilitate perspiration), they evolved dark skin, which was needed to prevent low folate levels since they lived in sun-rich Africa. When humans migrated to less sun-intensive regions in the north, low vitamin D₃ levels became a problem and light skin color re-emerged. Sexual selection and diet may have played a part in the evolution of skin tone diversity, as well.

The Inuit and Yupik are special cases: even though they live in an extremely sun-poor environment, they have retained their relatively dark skin. This can be explained by the fact that their traditional fish-based diet provides plenty of vitamin D.

Brown skin is the likely ancestral (or original) skin color among modern humans (Harding et al. 2000). This is due to modern humanity's common origin in equatorial Africa ~200,000 years ago (Tishkoff, 1996). Dark skin was crucial in this UV rich context given that a thick coat of UV protective body hair had long been selected against by this time (Rogers et al. 2004) most likely in order to facilitate the evaporation of perspiration (ie the cooling of the body). This trait (dark skin) continues to be under strong selection in equatorial regions such as Africa, India, and New Guinea (Harding 2000 p 1355). Geneticists estimate that a relatively small group of humans left Africa ~60,000 years ago, and that the descendants of this group went on to populate the entire non-African world. Those migrants that settled in non-African equatorial regions (such as the mentioned India, New Guinea, and/or Australia) retained most of the ancestral sequence at the MC1R locus (Harding 2000 p 1355), a gene strongly associated with determining skin color. Specifically, Harding et al. (2000 p 1355) found that the haplotype sequences for Indians and New Guineans are virtually identical to those of continental sub-Saharan Africans (except for a small number of variants at silent sites).

The retention of the ancestral trait at the equator is due to natural selection for melanin pigment production which serves to protect the body from harmful UV rays (Jablonski 2006). Notably, given that hair is a part of the skin, the retention is also analogous to that which occurred for Natural afro-hair prior to pre-Holocene admixture events among people who settled in India and Australia. However, certain evidence suggests that, unlike skin color, Afro hair ceased to be under strong selection once dark skin arose ~1 million years ago (Harding 2000) (rather, it remained as a vestigial trait among Africans, Andamanese, and Melanesians and changed to straight in the north for adaptive reasons—see hair texture). In fact, dark skin is so selectively advantageous at the equator that initially light skinned native Americans who migrated to Mexico and/or South America experienced renewed selective pressure towards the evolution of dark skin.

According to (Norton et al., 2006), light skin observed in Europeans (with deep red and/or yellowish skin tones), non-Indian Southeast Asians, East Asians and North Africa (Maghreb) is due to independent genetic mutations in at least three loci. They concluded that light pigmentation is at least partially due to sexual selection, however Jablonski postulates that the predominant reason revolved around the facilitation of vitamin D production in northern Eurasia (see hair texture).

Health related effects

Dark skin (melanin) protects against ultraviolet light; this light causes mutations in skin cells, which in turn may cause skin cancers. Light-skinned persons have about a tenfold greater risk of dying from skin cancer under equal sunlight exposure, with redheads having the greatest risk. Furthermore, dark skin prevents radiation of UV-A rays from destroying the essential folic acid, derived from B vitamins. Folic acid (or folate) is needed for the synthesis of DNA in dividing cells and folate deficiency in pregnant women are associated with birth defects.

While dark skin better preserves vitamin B, it can also lead to vitamin D deficiency at higher latitudes which in turn can cause fatal cancers affecting the colon, lung and prostate. Dark-skinned people are also at higher risk for rickets, cardiovascular disease, diabetes and multiple sclerosis. An American study by the USDA found 87% of African Americans to be Vitamin D deficient. To address this issue, some countries have programs to ensure fortification of milk with vitamin D.

The advantage of light skin at high latitudes is that it allows more sun absorption, leading to increased production of vitamin D₃, necessary for calcium absorption and bone growth. The lighter skin of women at higher latitudes most likely results from the higher calcium needs of women during pregnancy and lactation. However, some have postulated that it may also derive from sexual selection.

Albinism is a condition characterized by the absence of melanin, resulting in very light skin, eyes, and hair; it is caused by an inability to convert tyrosine to melanin, and has a genetic basis.