There is racial variation within Homo sapiens. That’s a simple fact but its been the source of unending controversy, prejudice and human misery. The issue becomes all the more prickly when evolution is brought into the picture, with many attempting to argue several horrific racist acts had their roots in evolutionary ideas regarding the inferiority of certain groups.
Of course, attempting to justify racism with evolution is a fundamentally flawed idea. This is partly due to the fact that evolution notes no groups are objectively inferior or superior, just differently adapted. Cheetahs run fast yet we are smarter, who is truly superior?
Mostly, however, racist evolutionary arguments are flawed due to the fact that the biggest source of racial variation within humans appears to be skin colour! Not strength, intelligence or any other trait one might used to justify claims of inferiority but skin tone.
Skin tone varies because different groups of people have a different amount 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. So mutations which decrease the amount of melanin production will be favoured.
But what are these mutations? One would expect them to be positively selected (since they’re conferring an advantage). They should also be correlated with the skin tone we’re examining since they’re responsible for the skin tone we’re examining.
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. Further, they have been positively selected for in Europeans and is correlated with their lighter skin tone, providing further evidence they are the genes we’re looking for.
Of course, European white isn’t the only skin colour variant in existence. So recent research has attempted to find the mutations which led to another tone – Indigenous American.
There are about 70 or so potential genes responsible for this colouring so the scientists first sought to eliminate those which couldn’t be responsible. So they ran a variety of statistical tests to find out which genes weren’t being positively selected for since, as you should remember, the responsible genes should be.
At this point I should confess I am not really trained in genetics, statistics or genetic statistics. So I can only take their word for it that the statistics they used are valid and the conclusions they draw supported. If someone has information to the contrary I’ll listen.
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 genes were 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, mutations of which are responsible for the European skin tone.
It’s very rare a paper doesn’t end without a call for further research since that means the researchers will receive further funding. The result of this is that scientists will typically try and find as many flaws in their conclusion as they can to justify a follow-up study. Whilst this is normally good, the research is sometimes of such a high quality that these objections are bordering on absurd, as is the case here.
But “bordering” is the operative word in that phrase. They aren’t quite yet absurd, so they do raise some valid issues. For example, the other 70 genes might be positively selected for but their statistics couldn’t detect that. Or the genes they did find correlated with skin tone actually coded for another trait which was correlated with skin tone.
However, those are pretty minor objections and so one is safe in concluding that we have found 4 genes, mutations of which are responsible for European and Indigenous American skin tones, important developments in recent human evolution.
Provided I can trust their statistics 😉
|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|