The genetics of hair texture: a mystery

By Razib Khan | October 2, 2011 9:34 pm

Lauryn Hill, Image credit: Lisa Lang


I received an email today from a friend about speculation on the genetics of hair texture. More specifically, curly vs. straight hair. I know that there are a few SNPs which are correlated with straight vs. curly hair (23andMe has actually been involved in this), but the architecture hasn’t been elucidated to nearly the same extent as pigmentation. The hair form markers explain ~5% of the population level variation. In contrast variants at large effect pigmentation genes can explain ~30% of population level variation.

So I did what I often do: check Wikipedia. Wikipedia might not be right much of the time, but it’s a great place to start looking for sources and get a sense of the range of disagreement. What I found was not pretty. Basically there’s not much there, and I was tempted to delete the whole section. The evolutionary explanations for why some populations have curly (or woolly) or straight hair are particularly embarrassing. But honestly I can’t think of anything that’s not laughable. Can you?

CATEGORIZED UNDER: Genetics, Genomics
  • RafeK

    I am going to guess woolly or kinky hair and Asian hair were driven to fixation by a pleitropic effects I can’t see any other reason. It is notable that melanasians who are distantly related to africans and closely related to straight and wavy haired populations but who live in a similar environment with similar ecological adapations(tuber based hoe farming, primary female labor, polygeny etc) also have wooly hair. The variety seen in european hair seems more likely to be a case of frequency dependent sexual selection for novel traits. I think peter frost offers intriguing ideas along these lines though focused on color not form http://evoandproud.blogspot.com/2010/04/puzzle-of-european-hair-and-eye-color.html

  • Grey

    I’d say afro hair might have some kind of heatsink effect e.g

    http://www.google.co.uk/search?hl=en&cp=9&gs_id=o&xhr=t&q=heatsinks&biw=1092&bih=637&gs_sm=&gs_upl=&bav=on.2,or.r_gc.r_pw.,cf.osb&wrapid=tljp1317638092534012&um=1&ie=UTF-8&tbm=isch&source=og&sa=N&tab=wi

    straight hair otherwise

    some curly hair from afro admixture e.g

    http://img.karaoke-lyrics.net/img/artists/12344/tom-jones-226054.jpg

    otherwise curly hair as another european “dependent sexual selection for novel traits”

  • S.J. Esposito

    Not that this isn’t laughable, but I’ve always thought that hair texture has more to do with cooling than with UV protection. The Wiki article seems to focus more on the tightly-packed nature of “woolly” hair acting as a sort of natural sun-blocker. I just don’t think that it makes much sense with regard to location on the body. Most areas of the body that have a higher concentration of hair are high-sweat areas, but not all of them are high-exposure-to-sunlight areas.

    Again, I have nothing to back this up, and I’m very interested to see what others have to say.

  • bob sykes

    Tightly curled hair with open pores (like the “afro”) is likely a mechanism to enhance air flow, wicking and evaporation from the skull in hot climates. Flat hair, even the kind of wavy hair often seen in Europeans likely is an insulator. The openness of the wave, allowing air flow is the important point.

    By the way, no African or Australian wears a full afro. That was an affectation of black Americans 50 years ago.

  • jb

    What I find interesting is how quickly the change occurred, which really does suggest selection. Since we don’t yet know the genes involved, is this a viable candidate for introgression from archaics?

    Also, while “wooly” hair occurs in the tropics outside of Africa, it never seems to be the really tightly curled variety you find within Africa, while within Africa it never seems to be loosely curled (except in areas where it could possibly be explained by back migration from Eurasia) . Maybe the primitive state for modern humans was really something like the hair of Australian Aborigines, while the tightly curled hair of modern Africans also introgressed from archaics?

  • colbyg

    This is the kind of case where I would start by trying to find at least some of the loci that influence hair texture and look for evidence of selection on those genes BEFORE trying to come up with stories about the adaptive significance of different hair textures. And then those stories should be tested – is there a cooling effect from kinky or curly hair (as a one-time owner of an enormous “jew-fro,” I can tell you that it doesn’t feel that way)? Does curly hair block the sun more effectively than flatter, straighter hair?

    It’s not that adaptive explanations are necessarily wrong or even implausible, but what I’m seeing looks like speculation, not science. Neutral explanations for hair texture are easy to come up with – have they been tested?

    The story also gets confused by the much larger, more clear history of evolution involving hair in humans – the loss of heavy hair over most of the body. It becomes quite reasonable, in this case, to assume that hair was retained on the top of the head (and armpits, pubic area, etc) for reasons with some adaptive significance, but there is a gap between “head hair has adaptive significance” to “variation in hair texture between ancestral human populations has adaptive significance” that is very difficult for me to cross given the current state of knowledge about the genetic basis of hair texture. Again: find genes. Get sequences. Test for evidence of natural selection. THEN start making up cute stories about wicking and cooling differences.

  • Insightful

    It should be noted that there is variation in African hair texture without Eurasian intrusion. Khoisan hair is more tightly coiled than bantu hair and in fact is the most tightly-coiled hair on earth, so much so that it is called ‘peppercorn’ hair..

  • colbyg

    “What I find interesting is how quickly the change occurred, which really does suggest selection.”

    No, it does not necessarily suggest selection. Hair texture alleles might be adjacent to highly selected loci on the chromosomes, which would lead to them coming along in crossing-over events in meiosis. There are other neutral or linkage-based possibilities. This is Genetics 101 stuff; come on.

    “Also, while “wooly” hair occurs in the tropics outside of Africa, it never seems to be the really tightly curled variety you find within Africa, while within Africa it never seems to be loosely curled (except in areas where it could possibly be explained by back migration from Eurasia) .”

    Yes; which kind of hurts the “rapid change = selection” argument. People in Southeast Asia have certainly been there long enough to see a return to very curly hair if it really does benefit individual fitness that much; one could counter that point by suggesting that the requisite genetic variation for re-evolving the trait had been lost, but it comes back to the original problem: we don’t know (almost) anything about the genetic basis for hair texture!

  • http://emilkirkegaard.com Emil

    I can’t think of much benefit from hair colors. As for hair textur, since africans hav som curly-ish hair (afro), and they live in a warm climate, perhaps it has somthing to do with heet dissipation? Here’s an idea: non-curly hair tend to cling to the skull wen wet (becus of swet), and curly hair tends to do this to a lesser degree. It is better for heet dissipation if the hair does not cling to the skull.

    This does not sound particularly plausible, but does anyone no about the hair textur of non-african tropical populations? If they also hav curly hair, then perhaps ther is somthing about it nontheless.

    Other than that, it cud be a by-product of somthing els.

  • http://washparkprophet.blogspot.com ohwilleke

    It is entirely possible that hair texture and pigmentation are primarily ancestry indicative and selectively neutral.

  • pconroy

    @10,

    I don’t agree with pigmentation being selectively neutral – as even across taxa (genera ?), there is selection for blue eyes in Northern latitudes. Husky and other Spitz-type dogs have blue eyes, as do many humans in these same areas.

    I’d guess that it’s something got to do with low light levels, and allowing more light to hit the retina – so selection for enhanced vision.

    Likewise, as heat exits through the head, and wooly/nappy hair allows for a layer of insulating air between the head and the sun, this is an probably an adaption to regulate heat in hot/tropical climates. So selection for heat regulation.

    Just as Arabs don’t go around in shorts and t-shirts, but wear loose sheet like clothing instead – to trap a layer of air between themselves and the surrounding hot desert, to both insulate from heat and slow water evaporation.

    You could extend the metaphor to Aboriginals too, as they have a protruding browridge and recessed eyes, which act as natural sun vizors, necessary when focusing at distance in a very sunny climate. Perth is the sunniest place on earth.

  • Ghoghogol

    Elaine Ostrander’s work on dogs has shown that three genes are responsible for the vast array of coats in known dog breeds. FGF5 long or short coat; KRT71 straight, wavy or curly; and RSPO2 wiry hair and furnishings

    It is probably not possible that such a parsimonious process applies to human hair variations.

  • pconroy

    @12 – well this is what 23andMe has to say on Pigmentation:

    Genes related to skin, eye, and hair color
    This group includes genes that are thought to be associated with human skin, eye, and hair color. A few of these genes (TYR, OCA2, TYRP1, SLC45A2) underlie a certain type of albinism, oculocutaneous albinism, in which skin, eyes, and hair are all lightened in color. Many redheads carry changes in another gene, MC1R. One study estimated that 74% of the variation in eye color in Europeans can be attributed to the OCA2 gene. In addition to having a large effect on our appearance, pigmentation genes are thought to affect our chances for cancer, since fair skin is a risk factor for skin cancer.
    The heritability of one aspect of pigmentation—skin color—is estimated to be 83%, suggesting that genes play larger role than the environment for this particular trait. Exposure to ultraviolet radiation is a major environmental factor that affects skin color, while age can affect eye or hair color.
    Genes in this set: MITF, EDN3, BNC2, ADAMTS20, DCT, TYRP1, SLC24A5, SLC45A2, LYST, OCA2, EDA, KITLG, TYR, MC1R, POMC, ASIP

    McEvoy et al. (2006) . “The genetic architecture of normal variation in human pigmentation: an evolutionary perspective and model.” Hum Mol Genet 15 Spec No 2:R176-R181.
    Sturm et al. (2001) . “Human pigmentation genes: identification, structure and consequences of polymorphic variation.” Gene 277(1-2):49-62.
    Duffy et al. (2007) . “A three-single-nucleotide polymorphism haplotype in intron 1 of OCA2 explains most human eye-color variation.” Am J Hum Genet 80(2):241-252.
    Bonilla et al. (2005) . “The 8818G allele of the agouti signaling protein (ASIP) gene is ancestral and is associated with darker skin color in African Americans.” Hum Genet 116(5):402-406.
    Rees JL (2004) . “The genetics of sun sensitivity in humans.” Am J Hum Genet 75(5):739-51.

  • toto

    I understand that hair texture (and probably genetics) are different among Asians and Europeans (I am not sure whether there are also difference between East and South Asians, or whether they both share similar hair texture). So, just like for pale skin, flat hair seem to have evolved independently among the two main branches of non-Africans.

    That does not sound like “selectively neutral” to me. Why would non-Africans somehow stumble on two different ways to have flat hair, which somehow eluded Africans all this time?

    I have been looking for a map that would show the distribution of hair types, but I haven’t found any.

  • Sandgroper

    @11 – “Perth is the sunniest place on earth.” A lot of people really don’t get this. The first feature film was made in Australia, and in the early part of the 20th Century Australia had a bigger movie industry than California due to the quality of the natural light. The light in Perth is literally blinding.

    Aboriginal people have amazingly good eyesight when unaffected, but they suffer excessively from cataracts induced by exposure to UV. The deep set eyes and heavy brows were often interpreted as an archaic feature, but it seems feasible they are adaptive.

    Culled straight from Wikipedia: “Forensic anthropologist Caroline Wilkenson wrote in 2004 that Australoids have the largest brow ridges “with moderate to large supraorbital arches”. Caucasoids have the second largest brow ridges with “moderate supraorbital ridges”. Negroids have the third largest brow ridges with an “undulating supraorbital ridge”. Mongoloids are “absent browridges”, so they have the smallest brow ridges.”

  • Eurologist

    Hair color can be selected for if you live in a both coldish and dark climate. If your diet doesn’t contain seafood, every bit of vitamin D production helps if most of your body is covered by clothing. However, if it is so cold that you would wear a head cover, this obviously doesn’t apply.

    BTW, the opposite selective pressure is likely not skin cancer (because we tend to get it at prehistorically irrelevant high age), but destruction of folic acid by sunlight.

    I think straight hair helps with parasite removal and prevention (perhaps also in association with a head cover), and of course aids against accumulation of snow.

    Finally, I believe that many East Asians have much thicker straight hair than Europeans, so there is another physical parameter.

  • Ghoghogol

    East Asian hair thickness results from variation in the EDAR gene.

    http://www.omim.org/entry/612630

  • quantum

    The variation in hair texture btwn the head and groin might also be insightful. Personal observation among africans is that hair on the groin is usually less curled than that on the head, so much so that africans with wavy hair are sometimes nicknamed “pubes” by their peers.

  • Jacob Roberson

    That “fiber optics” stuff in the WP article is interesting. Although the only ref is a book so who knows how serious it is.

  • Grey

    If curly hair in Europe is an afro legacy then might it be associated with DNA from the earliest waves of settlement?

    @Emil
    “I can’t think of much benefit from hair colors.”

    One of the interesting things about sexual selection is the amount of effort people put into trying to stop it.

    Mating or marriage can be decided in two very distinct ways:
    1) Based on individual characteristics and personal attraction between two individuals
    2) Arranged by families and not based on sexual selection at all

    The farming cultures all seem to have favoured arranged marriages with little or no consideration for the personal attraction of the two individuals concerned. This makes sense if an individual’s instinctive idea of mating-attractiveness evolved around what made a successful hunter-gatherer while a family’s idea of marriage-attractiveness was based on what made a successful farmer.

  • silylene

    I thought straight hair gene in Eurasians came from Neanderthal introgression.

    7 MAY 2010 VOL 328 SCIENCE
    page 715
    Table 2
    “KR241 205 V/M 21 Keratin-associated protein, formation of a rigid and resistant hair shaft”

  • rimon

    As a person with very thick, dark curly hair who has spent time in the Middle East, it seems to me that this hair type (which most Middle Eastern people have) is totally maladaptive for the climate. Dark hair just absorbs the heat and thick long hair is hot and uncomfortable. I always thought that thin blonde hair would be much more comfortable in the summer there. I would be interested in anyone’s ideas about what evolutionary purpose this kind of hair serves in such a hot climate!

  • Sandgroper

    As someone who has shaved his head, I can attest to the fact that in cold weather, you lose a great deal of heat from your head very quickly – so much so that it is physically painful, and you can quickly develop hypothermia if you are not wearing a warm hat, even if you are otherwise warmly dressed. The head loses far more heat than any other part of the body proportionatly in terms of surface area.

    So hair acts as very effective insulation to prevent too much heat loss from the head.

    Likewise in very hot weather, with a shaved head you feel a lot cooler in the shade, but in the sun your head can very quickly become uncomfortably hot, and it could easily result in sunstroke.

    So I have to conclude that one of the things that hair does is that it acts as insulation for the head, to prevent it from taking in or giving out too much heat. Logically, the thicker and coarser the hair, the better insulator it should be. It may well be more uncomfortable in hot weather, but it will prevent your brain from overheating, whereas thin hair would not do this as well. However even thin hair works, because the hair traps a layer of air next to your scalp.

    You would think that blonde hair would act as a better insulator in hot climates and dark hair better in colder climates, so it’s tempting to conclude that hair colour did not derive from this – originally all human hair was dark, and light coloured variants developed for some other reason – I’m inclined to believe it was just loss of pigmentation, or maybe sexual selection, but I always found it difficult to be persuaded by Peter Frost’s theory on that. You get selection today on hair colour among Europeans, I went through a period when I would only date blonde girls with fine hair (which was pretty funny of me considering I wound up marrying a Chinese), but you get so much variation in this that I have difficulty believing this – not all gentlemen prefer blondes. And in hunter gatherer groups potential mates were not so plentiful that you could afford to be too choosy.

    As to hair form, crinkly or straight, and coarse or fine, I can’t think of any obvious really credible explanation without doing some experiments. Maybe it is that crinkly hair allows dissipation of heat away from the scalp while still acting as an effective insulator in hot climates, while straight hair is more effective in preventing heat loss in colder climates, but I’m just guessing. I find it hard to believe that such an effect would be sufficient to drive selection. It may be that different hair forms simply derived from founder effects.

    The other thing that hair does is that it provides protection to the brain and scalp for when you bump your head. So the other function that hair performs is that it protects the brain from impacts, and the thicker it is the more effective it is in having a cushioning effect for the brain and skull. I’m inclined to believe that selection could have been quite strong for that.

    But these are just off the top of the head (sorry!) guesses.

    Given also that there may be some pleitropic effect, it is not surprising that there is no clear obvious answer.

  • Grey

    @rimon
    “As a person with very thick, dark curly hair who has spent time in the Middle East..what evolutionary purpose this kind of hair serves in such a hot climate!”

    Maybe it’s simply transitional i.e the first stage of moving away from tropical hair?

    Maybe it’s all sexual selection once away from the tropics?

    If the mating-attractiveness of males from an individual female’s point of view is partially hardwired to be those traits that would be good for a successful hunter-gatherer while the same male’s marriage-attractiveness from the point of view of the female’s family would revolve around traits that would make for a good farmer then you could see how a conflict could develop after the transition to agriculture.

    The same might be true for female’s in some ways but for those traits that are hard-wired to be attractive because they signal fertility and reproductive advantage the family of the male would be looking for the same things as the male would himself if given a choice. So where the family is choosing mates, if there is any element of sexual selection in the choice, it is likely to be directed more at females i.e a pretty girl with good hips might be as attractive to the male’s family as to the male.

    From the family’s point of view it’s a bit like stock-breeding.

    So if there’s anything related to straighter hair that signals reproductive advantage in females then maybe once away from the tropics it gets selected for?

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This blog is about evolution, genetics, genomics and their interstices. Please beware that comments are aggressively moderated. Uncivil or churlish comments will likely get you banned immediately, so make any contribution count!

About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at http://www.razib.com

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