The Effects of the Equator

Variations in human skin colour are due largely to geography and the sun's ultraviolet radiation (UV rays). Depending on where you live in the world, there is usually a common pattern in the colours of skin.  Those that live closest to the equator will have a higher percentage of people with darker skin. Those that live at the poles will have drastically lighter skin. Basically, there is a high occurrence of  dark skin in the tropical areas, and light skin in temperate. The reason behind this is that darker skin is required to avoid vitamin D toxicity. While in equatorial areas, the sun is much stronger and dangerous. As people started moving around the world, natural selection created different shades of skin. Those that continued to be in open areas retained their darker skin for protection. However, people that were in shaded protected areas, or places with less frequent exposure to sunlight developed lighter skin. Light skin colour is necessary outside the tropics to allow for vitamin D synthesis. Since the sun isn't as strong it is much less of a danger, and is ironically necessary. Too much vitamin D can lead to hypercalcemia. The darkest skin is found in the lowest latitudes of the open grasslands of Africa.