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.