http://www.economist.com/news/science-and-technology/21597881-homo-sapiens-became-black-beat-cancer-skinny-skin-colour

This article explains one possible reason why humans have darker skin than chimpanzees (after humans and chimps parted ways). Because of evolutionary pressures, human skin was almost always black. This may be due to the role of melanin in black skin. Melanin helps absorb ultraviolet light, prevents DNA damage and protects mutations, protects against skin cancer. Interesting read!