The colour of human skin is still frequently at the heart of violent political, social and physical controversy. Yet there is no biological basis to this. A given human population cannot be defined according to its pigmentation since any skin hue blends gradually into another. But there are undoubtedly dark skins, and there are light skins. The difference depends on the amount of a pigment - melanin - found in skin cells. And scientists have found a gene which has a direct influence on melanin content, and hence skin colour: SLC24A5.
Melanin is found in organelles known as melanosomes and it is the concentration of these which make skin dark, or not so dark. SLC24A5 seems to have a pivotal role in melanosome morphogenesis and melanin synthesis by way of calcium transport. If calcium uptake is poor, melanosomes present a lower content of melanin, giving skin a paler shade of brown. So a Scotsman must certainly carry a different version of SLC24A5 than a Nigerian say. Yes. There is a 'light-skin' version of SLC24A5 and a 'dark-skin' version. And populations that are a mixture of white and black 'blood' - such as the Afro-American populations for instance - share a mixture of the two.
There must be a biological advantage in owning a dark skin, or a light skin. Indeed, melanin-rich skins are hugely protected against ultraviolet rays - which is why darker skins are found where the sun hits hard. In such skins, the melanosomes cap the nucleus of skin cells, thereby protecting the cell's DNA from UV irradiation. In cooler latitudes, a lighter skin will let in just enough UV irradiation to synthesize vitamin D, which is important for bone growth.
Everything seems to fit in very nicely. However, light-skinned East Asians share the same version of SLC24A5 as dark-skinned Africans - which just goes to show that the protein is part of a far more complex molecular pathway... But one of the most intriguing aspects of SLC24A5 is that it can shed a brighter light upon our past. Especially on the paths our ancestors followed as they strolled from one continent to another.