Skin: Its biology in black and white

Human skin comes in a range colours, grading from very dark brown near the equator to near ivory near the poles. This colour gradient resulted from the action of natural selection. The original skin tone for members of the Homo lineage, including Homo sapiens, was dark brown. Diversification of skin colouration occurred as some Homo sapiens populations dispersed to regions of lower ultraviolet radiation (UVR), mostly outside of Africa. Humans who began to inhabit higher latitudes faced reduced opportunities to make vitamin D in the skin because of lower and more seasonal levels of UVB from ambient sunlight. Depigmented skin evolved under these conditions, and genetic evidence indicates that mutations leading to loss of melanin pigmentation occurred multiple times in hominin history when groups invaded regions of lower UVB. Because similar skin colours have evolved multiple times under the same environmental conditions, classification of humans according to skin colour is fraught with problems. The colour-based races defined in the 18th century are congeries of physical and behavioural traits that do not exist. The persistence and propagation of these concepts has caused serious problems in many places, and can only be countered by effective education.