The idea that early humans became fair-skinned as they migrated north out of Africa so they could make enough vitamin D to stay healthy has been questioned again, reopening a debate that many think is settled.
In equatorial Africa and in the tropics, melanin–the pigment that makes skin dark–provides protection against the intense sunlight. But melanin can also block the ultraviolet radiation (UVB) that triggers vitamin D production in the skin. This is an advantage in the tropics, where UVB radiation is barely filtered by the atmosphere above.
But UVB intensity falls dramatically at higher latitudes, where melanin can pose a problem for dark-skinned people. Vitamin D deficiency can lead to rickets and women with the disease often develop a deformed pelvis, making it difficult for them to reproduce.