As a generic answer, I think the first think we think about when talking about skin color and distance to the equator is that skin pigmentation becomes darker closer to the equator to decrease the risk of skin cancers.
But I read today a hypothesis that skin pigmentation actually functions to prevent vitamin D intoxication by filtering out excessive solar radiation. Vitamin D is a sterol derivative, and water insoluble. So when it accumulates in fatty tissues it can increase the absorption of Ca2+ to such a degree that soft tissue could calcify, or kidney stones may form.
Pretty interesting!