Human skin color

Human skin color can range from almost black to pinkish white in different people. In general, people with ancestors from sunny regions have darker skin than people with ancestors from regions with less sunlight. On average, women have slightly lighter skin than men.

Skin color is determined by the amount and type of the pigment melanin in the skin. Melanin comes in two types: phaeomelanin (red to yellow) and eumelanin (dark brown to black). Both amount and type are determined by 4-6 genes which operate under incomplete dominance. One copy of each of those genes is inherited from the father and one from the mother. Each gene comes in several alleles, resulting in a great variety of different skin colors.

Dark skin protects against skin cancer, mutations in skin cells induced by ultraviolet light. Light-skinned persons have about a tenfold greater risk of dying from skin cancer under equal sun conditions. Furthermore, dark skin prevents UV-A radiation from destroying the essential B vitamin folate. Folate is needed for the synthesis of DNA in dividing cells and too low levels of folate in pregnant women are associated with birth defects.

The advantage of light skin is that it lets more sunlight through, which leads to increased production of vitamin D₃, necessary for calcium absorption and bone growth. The lighter skin of women results either from sexual preference or from the higher calcium needs of women during pregnancy and lactation.

The evolution of the different skin colors is thought to have occurred as follows: the haired ancestor of humans, like modern great apes, had light skin under their hair. Once the hair was lost, they evolved dark skin, needed to prevent low folate levels since they lived in sun-rich Africa. (The skin cancer connection is probably of secondary importance, since skin cancer usually kills only after the reproductive age and therefore doesn't exert much evolutionary pressure.) When humans migrated to sun-poorer regions in the north, low vitamin D₃ levels became a problem and light skin color evolved.

Dark-skinned people who live in sun-poor regions often lack vitamin D₃, one reason for the fortification of milk with vitamin D in some countries.

The Inuit are a special case: 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 animal-based diet provides plenty of vitamin D.

Albinism is a condition characterized by the absence of melanin, resulting in white skin and hair; it is caused by a genetic mutation.

Skin color has sometimes been used in an attempt to define human races; see also racism.

See also Human physical appearance.

References:

• Nina G. Jablonski, George Chaplin, "Skin Deep", Scientific American, Vol 287 No 4, October 2002