Skin Melanin and genes

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.
An albino child sitting with his Tanzanian family.

The evolution of the different skin tones is thought to have occurred as follows:[citation needed] 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[4].

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[5].

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 ~140,000 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 Afro-textured 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

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