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.
Chart showing the physiological activity of dietary cysteine. This amino acid is used for protein synthesis, and can be recovered by protein breakdown. It is also used for the synthesis of reduced glutathione (GSH), which is the main storage of cysteine and thus also acts as a source of cysteine. When the levels of cysteine are higher than required for these functions, especially for protein synthesis, excess cysteine occurs. This excess, which can be toxic, is partly eliminated by cysteine sulfoxidation, a process mainly catalyzed by cysteine dioxygenase (CDO) in which less toxic products than cysteine such as sulfate and taurine are formed. In birds, excess dietary amino acids are also diverted to the synthesis of uric acid, the main product of excretion in birds. Cysteine also reacts with dopaquinone thus participat- ing in the synthesis of pheomelanin that takes place in melanosomes. If cysteine incorporated into the pheomelanogenesis pathway comes from an excess pool, then pheomelanogenesis could represent an excretory mechanism of cysteine and an adaptive process that may explain the evolution of pheomelanin.
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.
- Log in to post comments
More like this
I just did some volunteering at an Irish festival here in CT. The picture above looks like the line that formed at my table to get tickets and wrist bands!
The next day after that festival, I could close my eyes and still see these freckled arms reaching out.
I'm wondering what are the differences in melanoma rates between the two types of that protein?
-r.c.
I've wondered if it might somehow have developed in Northern European regions as a way of opening up the skin's ability to absorb more of the energy needed to create vitamin D. For a large part of the year there is very little sunshine and the cold temperatures mean that skin is covered for warmth. Current research into Vitamin D is leading towards an understanding that it is far more critical to health than our earlier research implied. Rickets and bone health are only part of the picture, and the immune system seems to be dependant upon it for healthy functioning, for one thing. In the unhygenic conditions of Northern Europe from the dawn of time until just recently (in evolutionary times) a stronger immune system would have been critical to survival. Perhaps the gingers had a leg up on survival during the "dark ages"? I have a son and granddaughter that are gingers and I can see that in a brighter environment the super-pale skin of the gingers is a disadvantage, but in the lands my ancestors came from it must have been an advantage.
Another interesting thing that was passed down to me through my mother. My grandmother always "wanted" a red-headed child or grandchild. Red-heads are very cute children and really draw the eye. I've suspected that other kids know this and pick on the red-heads because of it. Maybe the red-headed kids were more protected by the adults because of both factors, cuteness and the jealousy of other children. That might not have protected the red-heads from the other kids when they were playing in the woods, but it might have gotten them a larger helping of food, a warmer coat, or a warmer bed by the fire, which might have translated into a survival advantage.
Prejudice by Tim Minchin
http://www.youtube.com/watch?v=KVN_0qvuhhw
What about the theory that darker skin prevents folate from undergoing UV damage? In turn, lighter skin enables individuals to make more vitamin D. Both of these vitamins have a direct impact on fertility. Anyone who has spent any time in Ireland or Scotland can see the advantage of the palest skin in extracting the most benefit out of the feeble rays of sun available there. Whereas in equatorial climates protection against sun is paramount.
There is one benefit: did you ever see a Dementor go after Ron Weasely's soul, huh?
I have always assumed it was sexual selection, red hair is awesome, I love it. It glows like the sun and makes you stand out from the crowd. There is nothing as stunningly beautiful as a fabulous head of red hair, the skin that goes with it is less marvellous unfortunately but in an overcast climate this would be less obvious. I live in Australia where the deficiencies of red pigmentation are extremely clear but I still love red hair.
There is another general category of explanations, namely that red hair IS actually disadvantageous, and under negative selection, however the gene(s) which lead to pheomelanin have pleiotropic effects, among which is a trait that is under a positive selection that outweighs the disadvantage of red hair. Perhaps the gene for red hair also leads to more creative problem solving, higher risk tolerance, or any other trait that is selected for.
You should also know (in case your doctor doesn't) that you need more anesthesia during operations since you break it down faster
What about vitamin D production?
"there’s nothing sexually unattractive about red hair in humans, and many people find it extremely attractive."
http://acidcow.com/girls/25926-red-hair-104-pics.html
The hair, yes! The freckled faces.....not so much.
As a woman married to a red head I know their plight - they are fragile things, and must be constantly protected. I'm always telling my husband that my genes will squash his.
May as well get it over with; the fantastic Tim Minchin summarising the prejudice of Ginger-sociology in song http://www.youtube.com/watch?v=IZ9_Jba_PcY
The derivation of “Russian” comes from “Rus” referring to the invasion of red-haired Viking warriors who established Europe’s longest ruling royal dynasty, the Romanovs.
Similarly, red-haired and freckled Irish (cf the quintessential Leprechaun) owe their particular phenotype to a similar invasion of red-haired Viking warriors.
Meanwhile, many Gingers can trace their “allelic-enrichment” back to Alexander the Great and his armies of eager and willing contributors the Afghan and Kashmiri gene pools.
Then there are the red-haired Takla Makan (Tarim) mummies of China – a testament to yet another Ginger invasion from afar.
There seems to be a trend here…
I always wondered whether Gingers can trace their ancestry back to the Neanderthals and whether the closely linked red-haired freckled alleles have yet some other subtle and advantageous pleiotropic effect discernable only at a behavioral level.
Could it be that Gingers indeed have a propensity for libidinous fiery tempers confounding erstwhile Harvey-Weinberg equilibria?
Very interesting posting. I am a red head and I can say that having red hair is a gift and a burden. People either love it or hate it. Strangely enough though I don't tend to get sunburn very often though I do feel sick if I am in the sun for not even that long.