Toxicity of phaeomelanin

Question

I have been reading an article discussing a study by Tiffany Slater and Maria McNamara in which they identified molecular evidence of phaeomelanin in the fossil record. Slater is quoted as saying "scientists still don’t know how – or why – phaeomelanin evolved because it is toxic to animals".

I am curious what toxicity is being referred to here, since looking into it has only turned up the fact that phaeomelanin is responsible for red haired pigments in humans as well as reddish coloration in other animals, and the only mentions I've seen of toxicity are either phototoxicity that might occur when exposed to certain kinds of light, or increased susceptibility to skin damage from sunlight.

I did find a few studies in the search results whose titles were completely impenetrable to a layman such as myself, so maybe there is more information there that I wasn't able to dig out.

Is there a little more context around this statement that phaeomelanin is toxic that I am missing? I'm especially interested in information in layman's terms as I am not deeply familiar with the field of biology, but I am always curious!

Answer 1

The paper in which Slater et al. reported preservation of phaeomelanin does not include the word or root ‘toxic’, so it is difficult to know whether the quotation is correct and, if so, what was actually meant.

The Wikipedia entry for melanin states that as well as being responsible for the ‘red hair’ in certain people (who seem to survive this), phaeomelanins are concentrated in the lips, nipples, glans penis and vagina (depending on sex) of the rest of us. This suggests that phaeomelanins cannot be toxic, per se. The article does go on to say:

“Exposure of the skin to ultraviolet light increases pheomelanin content, as it does for eumelanin; but rather than absorbing light, phaeomelanin within the hair and skin reflect yellow to red light, which may increase damage from UV radiation exposure.”

However this is not the same as saying that phaeomelanin is toxic. Furthermore, the work reported was not concerned with human skin, but the colouration of feathers in the fossils of birds and feathered dinosaurs — presumably an advantageous trait, that outweighed any deleterious effects of UV radiation. This all suggests to me that a complex system for producing different variants of melanins arose in animals long before the appearance of mammals, and that there was differentiation in relation to which cells migrated to or were present in which tissues.