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..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 synthesize tyrosine, and has a genetic basis.
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