Skin disease in people of colour

Evolution of skin pigmentation

Skin pigment is made up of eumelanin (brown/black) and pheomelanin (yellow/red). The relative synthesis of pheomelanin is largely controlled by the MC1R locus. Dark skin primarily shows an accumulation of pheomelanin and light skin an accumulation of eumelanin. A person’s skin colouration strongly correlates with ambient ultraviolet radiation (UVR) at their latitude of geographic origin. Populations closest to the equator tend to have darker skin, whereas those at higher latitudes have lighter skin. The adaptation to develop different skin colours has been the subject of much interest. About 1.2 million years ago, evolution of human ancestors ( Homo ) led to almost complete loss of all fur, an adaptation thought to favour thermoregulation in hot savannah environments of Africa. Loss of fur was associated with some negative consequences including increased exposure to UVR. Subsequent evolutionary pressures led to the synthesis of permanent dark pigmentation over the entire body surface. The precise evolutionary pressure has been debated extensively and several theories including the need to protect from skin cancer, and eumelanin as a regulator of antimicrobial activity or skin barrier function have largely been disproven. Instead, it seems that the strongest case for an evolutionary driver, which would modify reproductive capacity, is the effect of UVR on folate metabolism. Folate is sensitive to UVR mediated photodegradation, and reduction of this critical vitamin would expose the subject to defective DNA biosynthesis and increased intrauterine defects.

Another effect of cutaneous UVR exposure is the induction of vitamin D ₃ synthesis. Thus, subsequent migration by early humans to latitudes with lower intensities of UVR exposed them to increased risk of vitamin D deficiency. Possibly because females need vitamin D to mobilize calcium during pregnancy, selection pressure then reversed and favoured lighter skins. However, from genetic evidence, it seems that the development of light skins in different populations arose through differing genetic mutations in pigmentation regulators.

Racial differences in hair and eye colour

Hair and eye colour are also varied across geographic populations, and their colour, like skin, is determined by melanin synthesis. It is not clear why hair colour appears to be much less variable in populations of non-European ancestry. Although hair and eye colour appear not to have been under strong evolutionary selection, some authors have suggested that sexual selection may account for the high prevalence of blond and red hair in Europe. Facial hair is associated with increased ability to perform aggressive facial displays, and may offer signals of age or social dominance but is not thought to command a selection advantage. There is a wide spectrum of iris coloration from blue, green, hazel, brown and dark brown, with European ancestry populations generally showing the lightest colours. However, the evidence for evolutionary pressure to drive the prevalence of such differences is limited.