Skin pigment serves a purpose: it protects us from harmful ultraviolet radiation.
But there is a cost: it blocks production of vitamin D. This phenomena explains why people indigenous to latitudes distant from the equator have very little pigment while those indigenous to latitudes close to the equator have dark pigment. A graphical representation of this can be seen in the map above.
People with little or no natural skin pigment, as in albino patients, who live close to the equator have devastating, disfiguring skin cancers. In contrast, people with dark skin who live in northern latitudes often have vitamin D deficiency.
Graphic from: Chaplin G.© , Geographic Distribution of Environmental Factors Influencing Human Skin Coloration, American Journal of Physical Anthropology 125:292–302, 2004; map updated in 2007.