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Melanin is a natural pigment that is categorized into two main forms, eumelanin and pheomelanin. It's well documented in the science literature that increased eumelanin levels reduces the risk of developing skin cancer, whereas it was recently reported in Nature that pheomelanin may itself be carcinogenic. So my question is:

Does pheomelanin have a useful biological function?

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Can you point to some references that show the UK population has higher pheomelanin? –  leonardo Nov 2 '12 at 21:45
    
@leonardo not at the moment, so I've removed this statement from the question. But going by my observations, there's a greater variety of redheads and blondes here compared to regions near the equator such as Africa and Asia, so I'm assuming it's down to the higher levels of pheomelanin, lower levels of eumelanin. There is also the standard skin colour map: grida.no/graphicslib/detail/… –  Larry Harson Nov 2 '12 at 23:03
    
You seem to have forgotten to cite the paper you mention. –  terdon Nov 2 '12 at 23:40
    
How do you increase the amount of EUMELANIN but not PHEOMELANIN? Do we know if this is currently been looked at by scientists? –  Liam May 7 at 9:04

4 Answers 4

My understanding has been that it aids in sun absorption in the skin, specifically in low sunlight/overcast areas. I don't have source on hand, but that may steer you in the direction you are looking for.

Also, consider that that pheomelanin, red hair specifically, may encourage breeding success, depending on where you are.

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That's my guess as well, but I can't find any references. Are you a redhead? –  Larry Harson Jan 13 '13 at 19:49
    
I'm not a red head, but if you need a source. I found an article with citations. 38 Red Hair and Redhead Facts The article may lead you to some appropriate sources, and I'll post anything I come across. –  nobrandheroes Jan 15 '13 at 4:51

I don't think that any use has been discovered yet for pheomelanin. I can't find sources about the claim that it increases UV absorption so I guess we can't say that yet.

I would simply guess that it's not harmful enough to warrant elimination via natural selection (in places where people are less exposed to UV, that is), and may, in certain societies, have increased sexual fitness. However even this is hard to tell, because historically red hair has been considered both an identitarian, desirable feature (as for Gauls during Roman times who would even bleach their hair with limewater to obtain reddish-blond hair) and abhorrent and cause for discrimination.

Vestiges and by-products of useful mutations may accumulate endlessly if their impact on overall fitness is marginal. In this case the useful effect would be the inhibition of eumelanin the skin, which is desirable in low lit/cold climates. This may in turn cause greater pheomelanin production and allows the normally concealed pheomelanic phenotype to show.

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I don't think that the role of pheomelanin is known yet. To produce larger amounts of it, you need mainly two conditions: A weak (or absent) MC1R signalling and the availibility of the amino acid cysteine. If MC1R signalling is strong, the preferred way goes into the direction of eumelanin. Its known for quite a while that pheomelanin produces reactive oxygen species which either directly cause DNA damage or do it indirectly by using up the cells storage of antioxidants. You can find more details in this review as well as links to a number of original papers.

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Pheomelanin has it normal function in transforming radiant heat to energy but can't protect its host from ultra violet rays from the sun as UV will destroy melanin cells.

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ahh, interesting! Do you have a reference for this? –  Larry Harson May 6 '13 at 14:44

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