The intensity of UV light was a strong environmental factor in the evolution of skin color. Dark indigenous people living near the equator had more offspring than their lighter counterparts; darker skin protects cell nuclei from damage. At higher latitudes, where UV light was limited, those populations with lighter skin and having the ability to synthesize vitamin D were favored.
Different Selection at Different Latitudes
There is a clear relationship between skin color and geography. Generally speaking, the darkest native people live in sub-Saharan Africa and in Melanesia. The lightest native people live in high northern latitudes. This pattern follows that of UV radiation, with the highest intensity radiation at or near the equator and the lowest at high latitudes. The ancestors of Homo sapiens probably had light skin. A major challenge to these populations in the African savannah was the sun. Those populations acquiring traits that helped them survive in their environment left more offspring than those that did not. Over time, Homo sapiens individuals in populations in Africa with darker skin (having lots of the pigment eumelanin) left behind more offspring than those with lighter skin. The three primary hypotheses for positive selection of dark skin are not mutually exclusive and relate to protecting DNA from UV damage in skin cell nuclei. These are 1) protecting skin from developing cancer; 2) preventing the photolysis of folate (vitamin B9); and 3) preventing skin, sweat glands and blood vessels from severe damage.
Figure (above):The intensity of UV light also varies across Earth, with the highest near the equator (red) and the lowest (blue) at high latitudes.
As humans migrated northward, the selection pressures for dark skin relaxed. Additionally, these populations met a new challenge: insufficient synthesis of Vitamin D, which requires the interaction of skin cells and UV light. Individuals with lighter skin could synthesize more vitamin D under low UV light conditions than those with darker skin. Their children were stronger than those that could not synthesize sufficient vitamin D. Modern societies fortify their food with both vitamin D and folate. This practice reflects the diversity of skin color in present day societies.