Skin tone amongst humans varies because different groups of people have different amounts of a dark pigment called melanin. Once produced, melanin moves to the top level of skin cells where it protects their DNA from the UV radiation in sunlight. However, since sunlight is also a part of vitamin D production, too much melanin can result in a vitamin D deficiency.
Thus a balance must be struck: too much melanin and you can’t make enough vitamin D, too little and you’re skin is damaged by UV radiation. As such having an appropriate skin tone is an important step in recent human evolution. Since different parts of the world receive different amounts of sunlight, this balance changes across the globe. From this emerges a selection pressure: the less sunlight one is exposed too, the more advantageous it will be to produce less melanin; favouring mutations in skin color genes which do so.
Previous research had identified mutations in SLC24A5 and SLC45A2 (genes involved in the transport of melanin from where it is produced to the epidermis) which drastically reduces the amount of melanin that reaches the top level of skin cells, resulting in paler colouring. The evidence strongly indicates that these genes are responsible for the European skin tone.
Of course, European white isn’t the only skin colour variant in existence. So recent research has attempted to find the mutations in skin color genes which led to another tone – Indigenous American.
There are about 70 or so potential skin genes responsible for this tone so the scientists first sought to eliminate those which couldn’t be responsible. They figured that since having the right skin tone for an environment would be beneficial the genes in question should show evidence of being naturally selected for.
Of the 70 genes tested only 14 showed evidence they were being positively selected. Of these, only 4 contained mutations which correlated with skin tone. These newly identified skin color genes are EGFR – which is responsible for the number of keratinocytes, which influences the number of melanocytes (cells which produce melanin – and OPRM1 – which also influences keratinocytes – as well as variants SLC24A5 and SLC45A2. If you throw your mind back to a few paragraphs ago, you might recall that other mutations in these genes are responsible for the European skin tone.
So, it would seem that only a handful of skin color genes have actually mutated to produce all the different shades seen in humans. In many cases the same set of genes – with a different set of mutations – are involved in several different tones.
Differences in skin colour have often been the source of tension; to understate a fact. I find it slightly ironic that such a source of division is actually a surprising case of similarity between people.
|Quillen EE, Bauchet M, Bigham AW, Delgado-Burbano ME, Faust FX, Klimentidis YC, Mao X, Stoneking M, & Shriver MD (2011). OPRM1 and EGFR contribute to skin pigmentation differences between Indigenous Americans and Europeans. Human genetics PMID: 22198722|