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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.

Perfectly evolved for the crappy weather of her home country.

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.

skin color genes

This graph, from the paper, shows something important. I think.

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

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acollectionofatoms · 10th January 2012 at 12:40 am

Great Post. Science is amoral, moral superiority isn’t a scientifically valid concept.
Also I like the Humor on the Graphs.
I’ve noticed that people from Southwestern Europe (like me) tan more easily, than those from Northern Europe, any idea why this may be?

    sahelanthropus · 10th January 2012 at 11:31 am

    From a cursory googling it would seem to be that a tan is the result of exisiting melanin being altered and more melanin being produced, so if you had extra melanin by default or produced more in reaction to sunlight it would result in an easier tan.

Jack East · 14th January 2012 at 3:17 am

My understnading is that darker skin color protects folic acid in a warm sunny climate. Folic acid is destroyed by sunlight. Low folic acid levels cause neurological defects in foetuses. The most famous is spina bifida. Such babies in the past did not survive and reproduce. Low Vit D levels also affect survivability to adulthood and reproduction. Skin cancer is a later in life disease and does not affect reproduction.

    Adam Benton · 14th January 2012 at 5:46 pm

    No doubt the protection of folic acid is an important role of melanin. However, vitamin D deficiency can affect young people too – notably in the form of rickets.

      acollectionofatoms · 15th January 2012 at 7:55 pm

      So the Mediterranean climate would select for the ability to tan easily, to prevent Low Vitamin D and rickets.
      Keep up the good work Mr. Benton.

        Adam Benton · 16th January 2012 at 1:21 am

        Tanning seems to be a good way of balancing the two demands, with the skin lightning/darkening in response to the amount of sunlight. As such, it seems plausible that increased tanning ability would be selected for in an area with highly variable sunlight – such as the Mediterranean – although I do not know if this conclusion is true or not.

        Plausibility =/= validity, after all.

      Jack East · 16th January 2012 at 3:56 am

      I commented that low Vit D reduces survivability. Light skin is to remove blocking of Vit D production in a lower UV light environment.

      Balancing Vit D production against folic acid destruction is a role of meleanin.

      The Mediterranean latitude would select for easy tanning to better balance Vit D production against folic acid destruction.

      The further north one goes the less uv that reaches the ground. Artic peoples get their Vit D from marine mammal blubber.

acollectionofatoms · 16th January 2012 at 3:44 am

It does seem highly plausible.
The production of Melanin would only occur when it is needed, saving the costs of producing it year round.
I just read that some Northern Europeans are unable to tan, so the production of melanin appears to be costly enough to have been selected against when it’s not needed.
What we need to show is that there is greater seasonal variability of UV rays in say Spain than in Ireland.
This topic appears worthy of appearing in a journal, would you care to co-author one?

    ScienceDefined · 1st February 2012 at 2:59 pm

    I would have thought this was well known already…

    Don’t forget that skin colour also reflects our ability to retain and lose heat. nearer the equator people have evolved to have darker skin partially so they lose heat quicker. Nearer the poles heat obviously needs to be retained.

    Tanning might just be an adaptation to help with heat retention as well. Whereas those who can’t tan, who live near the poles, probably can’t because most people would never have travelled by foot from norway to ecuador. So tanning would never have been advantageous..

Cris · 16th January 2012 at 6:02 pm

You assert “there is racial variation within Homo sapiens” and call this a “simple fact.” It is anything but. There is indeed variation within Homo sapiens, all kinds of it. Calling this variation “racial” doesn’t tell us much, and none of the evolutionary anthropologists or biologists I know use the adjective “race.” It is uninformative. Humans vary in all kinds of interesting ways, often in geographic clines, and we can categorize this variation using scientific concepts rather than social-political ones based on skin pigmentation.

    Adam Benton · 16th January 2012 at 7:04 pm

    Ultimately though there are these social-political concepts that define people according to skin colour and so, in a discussion of skin colour, I believe it is not unreasonable to mention them; if only to say how futile discriminating based upon them is.

Anonymous · 8th June 2016 at 10:41 am

Being an Indian it is a fact that 50% of the 1 billion population is dark skined. People have a bias towards lighter tone skin since it is sexier ! This is a fact and there is no not much more to say about it !!!

    Adam Benton · 8th June 2016 at 2:03 pm

    I’m not sure how objective one can be about these “facts” as such preferences have a strong cultural component. For example, in Europe it was often “sexier” to have lighter skin as it indicated one was wealthy enough to not have to work in the fields. Now, the reverse is true as tans indicate one is wealthy enough to holiday often.

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