Naked Apes?

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My recent post on Sexual Selection mentioned the theory that the ‘hairless’ skin of humans is due to sexual selection.

After writing this I thought I would check out what is known about the evolution of human body hair. One interesting result is this Royal Society paper by Pagel and Bodmer. Their theory is that hairless skin makes it easier to remove ectoparasites like fleas and ticks, and that humans’ loss of body hair was first favoured by natural selection for this reason, then reinforced by sexual selection. [Added: this was a free pdf when I got it a week or two ago, but may now require subscription. For those who want to track it down in a library, the reference is ‘A naked ape would have fewer parasites’, by Mark Pagel and Walter Bodmer, Proceedings of the Royal Society of London (B) (Suppl.), 270, 2003, pp.S117-119. Or you may still be able to Google up a freebie. Try searching Google Scholar for key words ‘Pagel Bodmer naked ape’. When I tried this just now it still gave free access to the full text.]

As P & B recognise, it isn’t clear whether the advantage of removing crawling parasites like ticks would be offset by an increase in attacks by mosquitoes and other flying pests. Another obvious question is why, if hairless skin makes it easier to remove parasites, man is the only primate to have lost his body hair. Other primates certainly spend a lot of time grooming themselves and each other to remove fleas and ticks. P & B suggest that the loss of body hair was related to the invention of fire and clothing, which would have enabled humans (unlike other primates) to regulate the surrounding temperature. The selective advantage of removing parasites more easily would then not be constrained by the need to retain body hair for warmth at night.

Like many hypotheses in human evolution, this one seems vulnerable to the criticism of being a ‘Just So Story’. It is attractive, but difficult to prove one way or the other. And the link between the loss of body hair and the use of clothing becomes less plausible when one remembers that most modern hunter-gatherers in tropical countries (Australian aborigines, Bushmen, various pygmies), who are presumably the best model for our out-of-Africa ancestors, traditionally wore little or no clothing. The aborigines are noted for their ability to sleep naked on cold nights.

I think that any successful theory for the loss of body hair will need to look more closely at the physiology and genetics of human hair development. Humans are not really hairless at all, but have dense hair everywhere except the palms of the hands and soles of the feet. The appearance of hairlessness depends on the length and fineness of the hair, which is under complex hormonal control in different parts of the body and at different stages of the life cycle. And then there is the puzzle of lanugo, the long hair which grows all over the human fetus but disappears before or shortly after birth. Explain that by sexual selection! [Added: Wikipedia has this to say about lanugo: “Lanugo are hairs that grow on the body to attempt to insulate it because of lack of fat. It is a type of pelage. It occurs on fetuses and it is normal for the unborn baby to consume the hair, which then contributes to the newborn baby’s first faeces. Lanugo hair is usually shed and replaced by vellus hair at 36-40 weeks gestation. The presence of lanugo in newborns is a sign of premature birth. It is also a common symptom of serious anorexia nervosa, as the body attempts to insulate itself as body fat is lost.” The ‘insulation’ theory sounds to me like pure guesswork: a fetus in the womb doesn’t need insulation! I suppose the anorexia point might appear to support it, until one reflects that there is unlikely to be an evolved response to anorexia: in pre-modern conditions, anyone who got that thin would be doomed.]


  1. Much of the residual body hair (on our arms and legs for example) could be viewed (or classified?) as fur – in that it only grows to a certain length. In contrast, hair on the scalp requires the occasional services of a barber. What other critter has hair/fur like that on the human scalp?

  2. There have got to be some costs of growing hair in both materials and energy. (There was a family last century with a mutation for an ape-like coat of body hair. They suffered from horrible dentition.) The cost may be more important depending on when in the development cycle it comes. 
    There is also the ‘man is a retarded ape theory.’ That one’s just fun to say.

  3. What’s always puzzled me is why do we have so much hair in the armpits and groins? Most animals (esp. primates) seem to have less hair, er, fur there than in most other places. So why did we lose hair/fur over most of our bodies, but gain it in the pits/groins?  
    Perhaps we were selected to service the razor-and-waxing businesses?

  4. What’s always puzzled me is why do we have so much hair in the armpits and groins? 
    believe it or not, i’ve read this is to trap odors. but hey, i might have read it in desmond morris, and that guy is a rank pervert :)

  5. Re: parasite theory, props to them for at least basing it on one of the main drivers of natural selection in humans. But true, anything that made us more vulnerable to mosquitoes & flies would never have stood a chance w/in the malaria belt.  
    However… malaria in humans is maybe 3000 years old, and in so short a time populations plagued by it have evolved a slapdash genetic response, which leads to greater susceptibility to sickle cell. If that’s possible, what would be implausibe about a scenario where ectoparasites were a greater plague than even malaria? From our narrow p-o-v, malaria is the worst of all, but that wouldn’t have been true 5000 years ago — maybe then, schmalaria was ten-fold worse.  
    We lost our hair to prevent this greater disease even though it would set us up in the future to malaria. In order to prevent both the old & new plagues, populations in the malaria belt evolved a separate defense for the new plague, lest the old one strike again if they simply re-gained their hair.

  6. believe it or not, i’ve read this is to trap odors 
    Pretty implausible. All that sweat & whatever else saturating all that hair just stinks you up. Girls smell less offensive in part b/c they don’t have days-old sweat soaked into a fur-sponge. Trim under your arms sometime — you’ll notice an instant drop in BO, even sans deodorant.

  7. Why do you assume “trapped” odors shouldn’t be smelled? It’s as like to be “trapped” for enhancement of pheromes. Given regular bathing many women, at least, like the “clean” concentrated smells from these areas.

  8. i’ve read this is to trap odors 
    I think this is supported by comparative ethology. From Body Hot Spots: The Anatomy of Human Social Organs and Behavior (1976), Chapter 5
    The chief human stink spot, like the gorilla’s and the chimp’s, is in our armpit or axilla. . . Like those of other mammals, human social glands are located in areas which are easily disturbed by bodily movement. The tufts of hair in the armpit and crotch area are rubbed together every time the limbs move, disseminating the body scent to the surrounding air.  
    Hey, still good enough for Dr. Mealey’s Human Ethology class, still good enough for me.

  9. We may *need* stinky armpits. I remember reading a controlled study about a decade or so ago where a group of menopausal women suffered less in the way of hormonal imbalances when given certain male hormones, in this study those obtained by rubbing sweat from men which was applied by contact to the areolae of the test subjects. If humans are still the only primates who undergo menopause then having tufts of fur to collect and store sweat may have been an evolutionary means of self defense. 
    I point out that my wife doesn’t read this ‘blog.

  10. As someone who often rides the NYC subway system in the summer alongside many other sweaty “straphangers,” I can assure you that we don’t *need* stanky armpits … 
    Wait: you mean, we didn’t evolve in order to be more pleasant on the NYC subways?

  11. Armpit smell: What a wretched digression; I think btw, that armpit hair has more to with the prevention chaffing – not to trap smells.  
    As for the wasted energy used to grow fur – the wasted energy used to grow extremely long scalp hair is totally baffling (to me) – and the same for beards on men. And why are there little or no beards on Native American men?

  12. There is a highly unpopular hypothesis that explains human nakedness compared to other primates. Besides, it explains: 
    1) Why our head is far from naked 
    2) Why we waste a lot of water just for cooling 
    3) Why we have a webbing between our thumb and forefinger while chimps do not 
    4) Why we are the only primate with a dive-reflex 
    “The Aquatic Ape Hypothesis” is my personal favorite to answering another question: 
    What do you believe is true even if you cannot prove it?

  13. You forgot 5). Why do we enjoy reruns of ”Flipper”, co-starring Jessica Marie Alba?

  14. We Use FIRE! 
    Oh, that and the fact that we don’t need our own hair since we can kill something and use it’s hide, benefitting from hair and from hairlessness.

  15. You are correct Michael: We USE fire. 
    Right now, and in the recent past, but we didn’t use fire during our evolution, a lot of primitive tribes don’t use fire even today. 
    Like I said The Aquatic Ape Hypothesis is highly unpopular, mostly because it was promoted by a woman, a feminist nonetheless. 
    However, its inventor was a German, Wasseraffen sounds more respectable than Aquatic Ape. 
    In addition, it is difficult to prove, because the fossils of such hominids would lay into deep water at the current epoch. 
    What about Flipper? 
    Well I think a better point number 5 would be: 
    5) The larynx descended to allow us to take deep breaths just before diving, that allows us to speak. 
    No other primate has a descended larynx but sea lions and walrus do. 
    Chimps can’t breath through their mouth nor can they be taught to say a simple word, that is because of their larynx.  

  16. I thought the reason humans had less hair than apes was well accepted. 
    The apes in their younger days have the same hair covering as us. 
    To enable intelligence humans are frozen in the younger days of apes (when then had good learning aptitude). 
    Hence being hairless is just a byproduct of needing increased intelligence, and has no benefit in itself. 

  17. We used fire half a million years ago.

  18. To enable intelligence humans are frozen in the younger days of apes 
    this is way too simple. ontogeny recapitulates phylogeny is not totally wrong, but it misleads when taken too literaly, and so does “humans are neotenous apes.”

  19. “I thought the reason humans had less hair than apes was well accepted” 
    - The ‘neotenous ape’ theory is not ‘well accepted’: it is just one of several rival theories. The most popular is probably the ‘bipedality leading to hair loss for thermoregulation’ theory.

  20. How do we “know” that humans *ever* had as much hair as the common ape? Isn’t it possible that hominoids were always relatively naked apes?

  21. The Aquatic Ape Theory raises some interesting observations about how human morphology differs from that of land animals, or is more similar in some ways to sea animals. 
    I had never heard of this theory before reading this thread:)