Evolutionarily, each of us is African; the first people had darkly pigmented skin and were thus well-adapted to handle the high levels of UVR (ultraviolet radiation) in equatorial regions. UVR has both benefits and costs for the human body. On the plus side, when it strikes the skin it catalyzes Vitamin D, which is essential for bone and immune health; on the minus side, it can cause DNA damage and skin cancers. 

Diagram 1: Skin Color Distribution Diagram 2: UV Intensity

Jablonski, N. G., Chaplin, G., N.G.J., & G.C. (2003). Skin Deep. 

Construct an explanation based on evidence for how natural selection leads to adaptation of populations. (Cause and Effect)

Evolution Of Skin Color