Red hair. Freckles. Pale skin. Soulless. What good are they?
Seriously — I have a red-haired son and brother and cousins, and you’ve got to wonder why evolution has spawned all these strange color variants — there’s no known advantage to ginger-ness, and plenty of disadvantages.
The biochemical cause of these differences are known. Pigmentation is produced by the deposition of a complex light absorptive polymer called melanin in cells. We vertebrates produce primarily two forms of melanin, eumelanin and pheomelanin.
Eumelanin is a dark pigment; cells loaded up with it look black or dark gray. If you’re dark-haired, it’s because the keratinocytes are stuffed full of eumelanin, which is also an excellent barrier to UV damage.
Pheomelanin is a reddish-brown pigment. We all make it to varying degrees — nipples and lips get their reddish color from it — but some people make it in much larger quantities than they do eumelanin. Red hair is stuffed full of pheomelanin rather than eumelanin.
But here’s the thing: pheomelanin is a lousy barrier to UV. In fact, pheomelanin is prone to photodamage (exposure to light causes it to break down) that produces carcinogenic byproducts. It’s also synthesized biochemically by a process that consumes glutathione (GSH), an important cellular anti-oxidant. So making red hair actually depletes the body of a protective substance, and has the side-effect of producing carcinogens.
Just to add further insult, studies in birds have found evidence of a physiological trade-off: increased pheomelanogenesis (that is, birds with more red plumage) is correlated with reduced brain size. More pheomelanic birds are also more sensitive to environmental stress — they’re more fragile.
You see the problem: WHY DO GINGERS EVEN EXIST? It just seems to be a bad thing all around.
I can give you a couple of hypothetical reasons, though.
Don’t assume that because a trait is deleterious it must be culled by natural selection. Red hair is not a serious detriment to survival; it could simply be that it persists as a part of a biochemical pathway that isn’t easily blocked, or that it’s disadvantages aren’t great enough to have led to its removal over time. Pheomelanin is all over the place among the vertebrates, though, so I suspect this isn’t the case; it seems to have some utility.
Sexual selection. In birds in particular, pheomelanin is used as a marker for sex; you can’t get rid of it without losing bright, bold signals that males use to advertise their availability. Similarly, there’s nothing sexually unattractive about red hair in humans, and many people find it extremely attractive. Gillian Anderson and Ewan McGregor are good-looking people; their coloring helps distinguish them from others.
And here’s another new idea: the pheomelanin pathway exists as a sink for excess cysteine.
That last hypothesis is interesting because it fits with a couple of observations, and also makes some predictions.
Why would you need a pathway to get rid of cysteine? Well, just on general principles you’ll often see pathways that balance the synthesis and breakdown of compounds in the body, but also there is some evidence that excess cysteine contributes to some diseases: In birds, there is clear physiological evidence that excess cysteine levels contributes to metabolic acidosis, thinning of egg shells, and growth inhibition. In humans, elevated cysteine levels are associated with rheumatoid arthritis, Parkinson’s and Alzheimer’s disease, cardiovascular disease, and systemic lupus erythematosus. There has not, however, been any evidence yet that being a ginger reduces the incidence of those diseases, and also note that these are diseases of the aging, and are less likely to have been a source of significant selection pressure in human evolution.
It’s also an open question that the authors of this proposal ask: “if pheomelanogenesis has adaptive value because it removes excess cysteine, do pheomelanic humans (i.e. phenotypes typically having red hair, fair skin, freckles, and green irides) avoid risk of diseases associated with excess cysteine more easily than eumelanic individuals?” This question hasn’t been answered yet, so at this point that third explanation is very much a tentative hypothesis looking for verification.
So, all you lovely red-haired people, there might actually be an advantage to your coloring…but what you really need to know is that you should minimize exposure to the sun!
But you already know that from experience, I’m sure.
Galván I, Ghanem G, Moller AP (2012) Has removal of excess cysteine led to the evolution of pheomelanin?: Pheomelanogenesis as an excretory mechanism for cysteine. Bioessays 34: 565–568.
I must include a counterpoint from Tim Minchin.