Development of different human skin colors: a review highlighting photobiological and photobiophysical aspects.

Skin color has changed during human evolution. These changes may result from adaptations to solar ultraviolet radiation (protection of sweat glands, sunburn, skin cancer, vitamin D deficiency, defence against microorganisms, etc.), and/or sexual selection. Migration to areas with high levels of UV is associated with skin darkening, while migration to areas with low levels has led to skin lightening. However, other factors may have played roles. Temperature and food have probably been secondary determinants: heat exchange with the environment is dependent on ambient temperature, and a high intake of food rich in vitamin D allows a dark skin color to persist even at latitudes of low UV levels, as exemplified by Inuit's living at high latitudes. Future studies of human migration will show if skin lightening is a faster process and has a higher evolutionary impact than skin darkening. Maybe due to that some American Indians have kept a relatively light skin although they live under the equator. The following hypotheses for skin darkening are reviewed: shielding of sweat glands and blood vessels in the skin, protection against skin cancer and overproduction of vitamin D, camouflage, adaptation to different ambient temperatures, defense against microorganisms, protection against folate photodestruction. Hypotheses for skin lightening are: sexual selection, adaptation to cold climates, enhancement of vitamin D photoproduction, and changing food habits leading to lower intake of vitamin D. The genetical processes behind some of the changes of skin color will be also briefly reviewed.