Saturday, September 7, 2013

Why are we the naked ape?

Infant stump-tailed macaque (source). Other photos showing adults and infants (courtesy of Monte M. Taylor and Christopher H. Taylor).

Why do humans have so little body hair? This question is addressed by Sandel (2013) in his comparative review of hair density in 23 primates and 29 nonprimate mammals. There seems to have been a long-term trend towards hairlessness in our primate ancestors: 

[…] all primates, and chimpanzees in particular, are relatively hairless compared to other mammals. This suggests that there may have been selective pressures acting on the ancestor of humans and chimpanzees that led to an initial reduction in hair density. (Sandel, 2013)

Across species, hair density negatively correlates with body mass. This correlation may exist because bigger primates are of more recent origin. Or hairlessness may be a way to disperse the excess heat generated by a larger body, since the increase in surface area (and hence the ability to dissipate heat) does not keep pace with the increase in mass. Sandel rejects this ‘heat load’ hypothesis:

Wheeler (1984, 1985) hypothesized that the low hair density in humans was associated with increased sweating capabilities. If the low hair density among primates represents a thermoregulatory adaptation, there should be a negative correlation between eccrine sweat gland density and hair density. There are no comparative data on eccrine sweat gland density in primates, but the distribution of eccrine sweat glands (presence vs. absence in certain body regions) is not consistent with the thermoregulatory predictions (Montagna, 1972; Grant and Hoff, 1975). In sum, the negative relationship between hair density and body mass cannot currently be explained. (Sandel, 2013) 

He concludes that the evolutionary trend towards hairlessness cannot be due to anything specifically human, such as bipedality. Denudation of the skin must have begun even before the common ancestors of humans and chimpanzees went their separate ways:

If chimpanzees are indeed relatively hairless compared to other mammals, there may have been a selective pressure acting on the ancestor of humans and chimpanzees that led to an initial reduction in hair density. Current hypotheses for human hair evolution focus on uniquely human traits, such as bipedality or longdistance running. If a reduction in terminal hair density is shared with chimpanzees, we may need to develop hypotheses for human “hairlessness” based on traits that are shared among chimpanzees, bonobos, and humans. (Sandel, 2013). 

Social signaling?

One cause may have been a growing tendency among primates to replace fur coloration with skin coloration as a means to provide conspecifics with key information about oneself: age, sex, social rank, availability for mating, etc. (Higham, 2009). Increasingly complex social relations would have created more information to signal, thereby driving selection for denudation of the body surface. Since social status can change over a short span of time, skin might have edged out fur as a better way to convey this information to others.

In ancestral humans, the key signaler seems to have been the adult female, as Charles Darwin noted: 

As woman has a less hairy body than man, and as this character is common to all races, we may conclude that our female semi-human progenitors were probably first partially divested of hair; and that this occurred at an extremely remote period before the several races had diverged from a common stock. As our female progenitors gradually acquired this new character of nudity, they must have transmitted it in an almost equal degree to their young offspring of both sexes; so that its transmission, as in the case of many ornaments with mammals and birds, has, not been limited either by age or sex. […] 

The females of certain anthropoid apes, as stated in a former chapter, are somewhat less hairy on the under surface than are the males; and here we have what might have afforded a commencement for the process of denudation. (Darwin, 1871, pp. 377-378)

If we consider women’s skin, particularly its visual and tactile properties, it tends to be softer, smoother, paler, and more pliable. These are also the properties of infant skin. In this and other ways (e.g., face shape, pitch of voice), the adult female body tends to mimic the infant schema, perhaps as a way to trigger the same mental and behavioral responses. There may thus have been a three-stage evolutionary process where human skin lost its body hair through a selection pressure that first targeted infants and then women, with men becoming denuded as a side effect.

Infant skin color and social signaling

Primate infants use both skin and fur coloration to indicate their age class:

The coat color of the newborn infant of all species of Old World monkeys for which information is available is different from that of an adult of the same species. Often this difference is extremely striking, as in the dark-brown fur of the newborn langur. Skin color of the infant langur, baboon, and macaque is pink, in contrast to the almost black skin of the older infant or adult. The infant’s pink face, hands, and feet and its large pink ears are in sharp contrast to its dark brown fur. The natal coat color is present during the first two or three months of life, when the infant most needs protection and nourishment from its mother and older monkeys. It is almost certainly more than coincidence that the duration of coat color difference coincides with a period of dependency, when it is essential that the young be sheltered and protected by older animals (Jay, 1962)

According to a review of the primatological literature, the infant stage is most often identified by a specific fur color. Nonetheless, the infant does have differently colored skin in many species: “deep blue face colouration” (proboscis monkey), “white skin” (silvered leaf monkey), “pink/grey skin” (hanuman langur), “pink face” (spectacled leaf monkey), “pink skin” (capped langur), “pink face” (baboons), “pink flesh” (stump-tailed macaques), and “pale pink skin” (lion-tailed macaque) (Alley, 1980)

The skin seems to have reached its current denudation relatively late in hominid evolution, perhaps even after the fork that led on the one hand to Neanderthals and on the other to modern humans.  Neanderthals survived subzero climates without tailored clothing, and their sites yield only hide scrapers that could have served only to make blankets or ponchos. Microwear analysis shows that these scrapers were used for the initial phases of hide preparation, but not for the more advanced phases of clothing production (Hoffecker, 2002,p. 107). In contrast, modern human sites abound in eyed bone needles and bone awls (Hoffecker, 2002, pp. 107, 109, 135, 252). Further evidence for the relative lateness of tailored clothing is the recent origin of the human body louse, which lives in clothing and first appeared perhaps 83,000 to 170,000 years ago (Toups et al, 2011). Finally, Neanderthal infants seem to have clung to their mothers’ fur: “Chimpanzees have ridges on their finger bones that stem from the way that they clutch their mother’s fur as infants. Modern humans don’t have these ridges, but Neanderthals do” (Cochran and Harpending, 2009).

Denudation would have made the pale pink skin of infants visually more important. This pallor is striking in darker-skinned humans and seems to be appreciated by parents. A life story of a !Kung woman records why she would not kill her newborn child: “Uhn, Uhn … I don't want to kill her. This little girl is too beautiful. See how lovely and fair her skin is?” (Shostak, 2000, p. 70). In Kenya, newborn infants are often called mzungu ('European' in Swahili), and a new mother may tell her neighbors to come and see her mzungu (Walentowitz, 2008). Among the Tuareg, children are said to be born "white" because of the freshness and moisture of the womb (Walentowitz, 2008). The cause is often thought to be a previous spiritual life:

There is a rather widespread concept in Black Africa, according to which human beings, before "coming" into this world, dwell in heaven, where they are white. For, heaven itself is white and all the beings dwelling there are also white. Therefore the whiter a child is at birth, the more splendid it is. In other words, at that particular moment in a person's life, special importance is attached to the whiteness of his colour, which is endowed with exceptional qualities. (Zahan, 1974, p. 385)

Another Africanist makes the same point: "black is thus the color of maturity [...] White on the other hand is a sign of the before-life and the after-life: the African newborn is light-skinned and the color of mourning is white kaolin" (Maertens, 1978, p. 41).


Loss of body hair was a long-term evolutionary trend in ancestral hominids and even ancestral primates, being perhaps a response to a greater need for social signaling. In ancestral humans, the selection pressure seems to have gone through three stages, initially targeting infants and only later women and then men.

Among nonhuman primates, the relatively depigmented skin of infants has long exercised the signaling function of calming aggressive impulses in parents and stimulating protective, nurturing behavior. Women seem to have mimicked infant skin for the same purpose, perhaps because of the longer period of infant dependency and their correspondingly greater vulnerability during this period.


Alley, T.R. (1980). Infantile colouration as an elicitor of caretaking behaviour in Old World primates, Primates, 21, 416-429.

Cochran, G. & H. Harpending (2009). Neanderthals, Steve Sailer’s iSteve Blog, January 10, 2009 

Darwin, C.R. (1871). The Descent of Man and Selection in relation to Sex, London: John Murray, vol. II, 1st edition. 

Higham, J.P. (2009). Primate Coloration: An Introduction to the Special Issue, International Journal of Primatology, 30, 749–751. 

Hoffecker, J.F. (2002). Desolate Landscapes. Ice-Age Settlement in Eastern Europe, New Brunswick: Rutgers University Press. 

Jay, P.C. (1962). Aspects of maternal behavior among langurs, Annals of the New York Academy of Sciences, 102, 468-476. 

Maertens, J-T. (1978). Le dessein sur la peau. Essai d'anthropologie des inscriptions tégumentaires, Ritologiques I, Paris: Aubier Montaigne.

Sandel, A.A. (2013). Brief communication: Hair density and body mass in mammals and the evolution of human hairlessness, American Journal of Physical Anthropology, 152, 145–150. 

Shostak, M. (2000). Nisa: The Life and Words of a !Kung Woman, Harvard University Press. 

Walentowitz, S. (2008). Des êtres à peaufiner. Variations de la coloration et de la pigmentation du nouveau-né, in J-P. Albert, B. Andrieu, P. Blanchard, G. Boëtsch, and D. Chevé (eds.) Coloris Corpus, (pp. 113-120), Paris: CNRS Éditions.

Zahan, D. (1974). White, Red and Black: Colour Symbolism in Black Africa, in A. Portmann and R. Ritsema (eds.) The Realms of Colour, Eranos 41 (1972), 365-395, Leiden: Eranos.


Sean said...

Those 'Other photos showing adults and infants' are well worth a look. Mark Pagel and Sir Walter Bodmer theorised hairless was related to freedom from parasites, I suppose Lord Kalvan of Otherwhen will be subscribing to that idea.

It ought to be obvious that the loss of body hair is related to sexual selection, as what body hair humans have appears at puberty and is not visually obvious. Once prospective mates get close, those same areas waft sexually alluring smells. Moreover, women's axillary and pubic hair thins after menopause, as does their crowning glory.

Anonymous I said...

Out of curiosity, Peter, why do you think that Europeans are overall hairier than most other populations? If sexual selection for feminine traits acted more strongly on Europeans, then would we not expect them to have less body hair, as well as more trouble growing beards?

Sean said...

Are Europeans really hairier than South Asians or Australian aborigines? It is known that North Europeans have less body hair than south Europeans, although I haven't seen anything about whether north European women's axillary and pubic hair is denser--like their scalp hair is. Africans are not hairy, but they have a very tough outer layer of skin; European skin is very infant-like compared to that of Africans. The heavy beard growth of European men is supposed to be related to making them look tough. I wonder if they needed beards to look that way after sexual selection of women had feminised their features

JayMan said...

Judith Rich Harris proposed a similar hypothesis, particularly noting that neanderthals were likely quite hairy, but she postulated that parental selection was involved in the evolution of hairlessness:

Judith Rich Harris: Parental Selection: A Third Selection Process in the Evolution of Human Hairlessness and Skin Color

Ken Fabian said...

Peter, this is a subject I've been interested in for a long time. What you've written is interesting but to my mind the scholarship around 'hairlessness' has been disappointing for some serious errors of omission and your post suffers by perpetuating them. Even the term 'hairlessness' appears to embody a falsehood!

I suggest that we cannot work out how our patterns of hair evolved if we don't correctly identify the functions hair has and, from Darwin to the present day I suggest academia has failed to adequately do so. This is not a good look for experts on human evolution.

Hairs are fully functional sensory organs. Move or vibrate them and we feel it. They extend our sense of touch beyond the surface of the skin - with goosebumps, even further - and are so sensitive the air off the wings of a passing insect are easily felt. The fine hairs on my face near eyes and on my nose are so small they can barely be seen with the naked eye but are so sensitive that if they are disturbed it's a struggle to NOT rub or scratch. Surely this is a useful and important function that should never have been overlooked. It relates directly to an alternative evolutionary hypothesis for the evolution of furlessness - ectoparasites - and, given how dramatically and rapidly infectious disease can alter a population, any physical characteristics that alter relative susceptibility by variants within a population need to be given full consideration.

A recent study by Isabelle Dean and Mike Siva-Jothy showed that fine body hair gives sensory awareness of the presence of bedbugs (compared to hairless skin), whilst noting that finer hairs would give greater (finer?) sensitivity, so this function is not entirely unnoticed. But... were you aware of it?

Compared to coarse, dense fur, finer, sparser hairs give greater sensitivity to - and therefore awareness of - the presence of ectoparasites, prompting some social nitpicking or perhaps replacement of bedding. If they avoid the infectious diseases that ravage their furrier relatives, then an otherwise less well adapted variant becomes the dominant survivors.

I would also make the point that furlessness is universal in the juvenile form, suggestive of some kind of neotony. Could that be a consequence of distinct mutation rather than accumulation via sexual selection? I think the sexual selection hypothesis doesn't seem to explain that universal juvenile form. For the variations of hair patterns within the adult form, maybe, but I'd expect that pre-puberty juvenile form to be more variable if it were the result of sexual selection.

Tyrion lannister said...

Why should a hairless sexual partner have been more of a concern for males than for females? A male hairy body seems to be just as unattractive to a female as a hairy female body would be to a male:

1) Women from New Zealand and U.S rate as the most attractive those male bodies lacking any trunk hair, with a steady decline in attractiveness as hirsutism became more pronounced.

2) Chinese women rate male bodies with no or little trunk hair as most attractive.

3) African women have not a specific preference for one type of male hairiness, anyhow the most hirsute bodies are not judged as most attractive

Tyrion lannister said...

The genes that are responsible for hair loss have not been clearly identified and we know very little about the specific inheritance pattern for hair (male and female). This may be due to multiple genes that are responsible for hair loss. The same probably applies to body hair growth/loss as well. Otherwise there is no direct correlation with the hair loss gene from the scalp and body hair genetics. There is also no connection between body hair and the absence of head hair.

How does evolutionary theory can explain the prevalence of male baldness in much of the white race (the Irish being the big exception)? That a man 50,000 years ago had an accidental genetic mutation which caused him to lose his hair, and the women in his tribe were more attracted to him with his bald head than to all the other hairy men, and so he had more offspring than the hairy ones, and so the genetic mutation for baldness spread through the population?

My point is that we don’t have the slightest idea why male baldness exists and why it is so common, and there is no remotely plausible genetic “scenario” by which it came into existence via an accidental mutation that was then selected. Or maybe baldness just appeared (by a random genetic mutation), but didn’t have to be selected to survive, it just wasn’t selected against? why these factors should have come into existence in the first place, affecting some members of the population to the point of total male pattern baldness, affecting others less, affecting others not at all. Currently, baldness is a negative sexual signifier and maybe bald men incur a mating disadvantage. Although the causes of hairloss in men don’t seem to affect reproductive fitness significantly nowadays so it is difficult to imagine under what circumstances they might have a great impact on fitness. Receding hairlines and baldness decreased facial attractiveness and perceived aggressiveness.

Reader said...

Tyron Lannister, I'm sorry but you're totally wrong.

Male baldness evolved as a signal of maturity to young women. For this reason, young women find young men their age with a receding hairline *very attractive*, so much so that balding young men actually get married and start families quite early, in some cases in their early-/mid-20's (from what I've observed).

There is very much a link between body hair and head hair. Men with hairy bodies have a lot of male androgen (DHT). This spells trouble for head hair, but it makes those men handsome to women and allows them to get married early.

The fact that women "don't like" hairy men may be a public convention of sorts, but doesn't seem to translate into their *actual* sexual decisions. Anecdotally, the majority of men I know who are married to very hot women are hairy and balding. Scientifically, if women truly didn't like hairiness, it would have been weeded out of the gene pool.

Bones and Behaviours said...

This seems to fit well with the 'sexy ape' theory proposed by Desmond Morris. If human fine hairs are sensitive to bedbugs as Ken Fabian suggests then they're also sensitive to human caresses as well.

I also have doubts about whether neanderthals had to be hairy bodied to keep themselves warm during the European Middle Paleolithic, when the Tasmanians and Fuegians were walking around naked in such cold climates very recently - and if they were still partially hairy I doubt their hair was very good at insulation. You points out that among primates hair density negatively correlates with body mass, but forget that the neanderthals were so stockily built.

I don't know why neanderthals retained the ridge that Harpending and Cochran mention as present on their finger bones, but before modern humans invented ways to carry infants whilst the females were out foraging juvenile hominins needed to cling to their mothers somehow - although in neanderthals it may have no longer been functional and I wonder about the frequency of this trait among not only fossil hominins but different modern humans as well (populations with different kinds of infant care). And if you are right, the near-complete hairlessness of modern humans might well be restricted to the Homo sapiens line, having co-evolved with increased parental care as is indicated by the rapid and striking shift toward a more superficially platyrrhine-like than typically hominin-like, neotenous upper facial region during later Pleistocene human evolution. Neanderthals and other fossil Homo would still be hairy on their backs and the small bodied hominins would maybe resemble chimps.

Tyrion lannister said...

“Male baldness evolved as a signal of maturity to young women. For this reason, young women find young men their age with a receding hairline *very attractive*, so much so that balding young men actually get married and start families quite early, in some cases in their early-/mid-20's (from what I've observed).”

Can you prove that ridiculous argument?
First, little is known about women’s preferences for hair length and hair quality in men. PANCER and MEINDL [1978] found that women prefer men with long hair.

MUSCARELLA and CUNNINGHAM [1996] photographed men wearing wigs ranging from full hair to complete baldness. Both receding hairlines and baldness decreased facial attractiveness and increased perceived age and social maturity. These authors proposed that baldness is an adaptive process and serves as an indicator of old age and reduced reproductive drive. This reduces competitive and aggressive behavior in younger males. Thus, an old man can invest in his offspring and grandchildren without interference from younger males.

You are totally wrong. There is also no correlation between body hair and the absence of head hair, (although many people think that bald men always have hairy bodies.)
And bald men do not have more testosterone. Androgenetic alopecia is caused by the hair follicle sensitivity to DHT and balding hair follicle dermal papilla cells contain higher levels of DHT receptors than those from non-balding scalp. So it is not the amount of testosterone circulating in the bloodstream that dictates baldness, it’s down to genetics. Several genes are thought to be involved, all resulting in hair follicles becoming particularly sensitive to tiny amounts of circulating testosterone.

“The fact that women "don't like" hairy men may be a public convention of sorts, but doesn't seem to translate into their *actual* sexual decisions. Anecdotally, the majority of men I know who are married to very hot women are hairy and balding.

The female preference for male hairless trunks is is empirical evidence and I dont know if that is subject to genetic or cultural factors and changes over time. Regarding baldness, maybe you are observing that pattern in your social circle but is is nothing more than your biased and spurious subjective perceptions. Baldness decreases physical attractiveness. And therefore in order to advance a similar argument (unified in a broad evolutionary synthesis) for vague (independent) theory of a supposedly greater appeal of bald males, you would have to show their basis in evolutionary success beyond a circular argument (ie. how did female bias for these seduction systems *evolve* – what advantages did they confer *before* they became correlated with male reproductive success).

“Scientifically, if women truly didn't like hairiness, it would have been weeded out of the gene pool.”

If your claim is that male pattern baldness has mainly a positive (or a neutral impact) upon a man's attractiveness to potential female mates, all other things being equal, you go down a bad road. As I said, hairloss in men don’t seem to affect reproductive fitness significantly and may not be the product of sexual selection. There may be something else directly selected for with baldness surviving merely by pleitropic association. Another possibility is that male patter balding has no particular advantage of its own, but it is genetically tied to a seemingly unrelated trait that is beneficial.

Bones and Behaviours said...

Tyrion, I also think the same about baldness. The MRCA for H.sapiens may be reconstructed as partly polyandrous so that more than one male would have access to a female. Isn't it curious that if girls go for older men, male fertility peaks just after the onset of adolescence? Though for WEIRD cultures the average age gap is supposedly 2-5 years with the male being older, this is scarcely supportive of young fertile females being attracted to older, balding males. I suspect that females would choose different males as most attractive according to ovulation.

As to the question, someone else posted about European body hair, the Ainu also have hairy bodies. In both cases the explanation is either admixture from cold climate hominins or something to do with Rensch’s law (animals in colder climates tend to have longer hair than those from warmer climates). With Mongoloids, EDAR v370 A may cancel this last law out.

Bones and Behaviours said...

I forgot to add (about hairiness) that the Murrayans are supposed to be hairier than either the Barrineans or the Carpentarians. I don't think many people would take the trihybrid theory all that that seriously now, but the phenotypic clines are real and the hairiest Australians therefore inhabit the south of Australia.

Sean said...

Anti male pattern baldness drug Propecia is a 5α-reductase inhibitors that reduces baldness and hairyness hormone DHT and it reportedly causes 'brain fog' and and memory loss. See here.

"research amongst medical students in America found that 45 per cent of male trainee doctors were "very hairy", compared with less than 10 per cent of men generally. In Kerala, southern India, research among medical and engineering students and manual labourers showed that both groups of students had more body hair on average than manual workers.

In addition, "When academic ranking amongst students was examined, the hairier men got better grades," said Dr Alias. The top six engineering graduates had more hair than the bottom eight.

And a study of 117 Mensa members (who have an IQ of at least 140) were also found to have a tendency to thick body hair. Some of the most intelligent men appeared to be those who had hair on their backs as well as on their chests."

"'I am fairly certain that the vast majority of hairy/hirsute men, compared to the respective 'much less' hirsute men of the same race and ethnic group, are strikingly more intelligent and/or educated, but only from a statistical point of view.'"

Nathan said...

You say that young bald men are more attractice and successful in mating. Then why pharmaceutical companies are making a lot of money selling minoxidil, finasteride, etc??.and much young guys are in psycological therapy...and what about hair transplants.??

Anonymous said...

In addition, "When academic ranking amongst students was examined, the hairier men got better grades," said Dr Alias. The top six engineering graduates had more hair than the bottom eight.
And a study of 117 Mensa members (who have an IQ of at least 140) were also found to have a tendency to thick body hair. Some of the most intelligent men appeared to be those who had hair on their backs as well as on their chests.
1) Women from New Zealand and U.S rate as the most attractive those male bodies lacking any trunk hair, with a steady decline in attractiveness as hirsutism became more pronounced.
Coincidence ;)?

Ken Fabian said...

Aside from my concerns that failure to understand the functions of body hair has skewed most scholarship so far - I argue that sensory function stands out as the most significant function it has - I still have problems with most of the hypotheses that attempt to explain furlessness in humans.

The sexual selection hypothesis relies too much on the transposing the attitudes of modern, furless humans to hairiness ie it's a form of anthropomorphism that attributes our modern, mostly socially acquired attitudes to body hair to our distant, furry ancestors.

Judith Rich Harris and Parental Selection I think suffers from the same innate problem; lack of clear reason why furry hominids, expecting a furry child would reject them out of a preference for less hairy babies. That's something that might happen at the tail end of the process of becoming furless, not the beginning.

I think it would require some other significant element for a clear preference for furlessness to develop. Such as furlessness coming to indicate some form of superior fitness ie they are more successful hunters due to better endurance or they are less susceptible to disease.

But I also think it is an important question whether there was a distinct mutation or combination of them that made a furless variant, from which we descended, or if it was a case of incremental change, such as would be expected from sexual selection. I think the furless juvenile form is suggestive of mutation that resulted in a clear and marked difference in those that carry it.

The furless juvenile form, to my thinking, makes it less likely that it arrived via sexual selection. The adult variability, yes. I think those differences across different ethnic groups can be used to show that sexual selection results in variability in hair patterns. But that distinct juvenile human form with hair on head and only fine vellus hair elsewhere is universal. I think it had to be a distinct mutation

Bones and Behaviours said...

Peter, I want to find out more about those ridges on the finger bones of chimpanzees and neanderthals but not ours.

Can you point me in the right direction by telling me the name of the ridges please?

Peter Fros_ said...


I tried to get permission to post those other photos but was refused. Permission is granted only for "educational purposes."

Anon and others,

Selection for female hairlessness was partly sexual selection and partly natural selection. This denudation of the skin may have helped ancestral women on the mate market, but the main advantage would have been after mating, as a means to modify male behavior in the direction of increased provisioning and protection.

We're also looking at a period farther back in time, perhaps between 500,000 and 100,000 years ago. The intense sexual selection of European women would have been between 25,000 and 10,000 years ago.


I agree with Judith in the sense that parental selection represented the first stage of this evolutionary process. With longer juvenile dependency, and increased maternal investment, this selection pressure came to encompass adult women as well.


Yes, I'm aware of the points you raised, but none of them explain the denudation of human skin. In fact, they would be an argument against denudation.


Maternal investment is generally greater than paternal investment, so women were the ones who needed provisioning and protection.

I've read many evolutionary explanations of male hair loss, but none of them seem convincing. I suspect male hair loss was useful in some way for male-male relations, i.e., as a way of signaling that one is an older, mated male. I don't believe that women find bald men sexually attractive, either now or in the past.

Bones and Behaviours,

The evidence for Neanderthal hairiness is suggestive and not conclusive. Ultimately, the Neanderthal genome may provide an answer.

The terms I've seen are either "finger bone ridges" or "knuckle ridges." At one time, they were thought to facilitate knuckle walking, but we see them in primates that don't knuckle-walk. It might be a good idea to go to the West Hunter blog to get more information.

Ken Fabian said...

Peter, you said -
"...none of them explain the denudation of human skin. In fact, they would be an argument against denudation."

Except we have not been 'denuded' and the observable reality is that humans have never lost their body hair - it is just, (pre-puberty), all fine vellus hair. Post puberty it's highly variable but it's not absent.

What our lineage shows is a change of the size of hairs and of their distribution. It is a mistake to treat that as the same as a true loss of body hair.

The change in size and distribution does affect the way they work as mechano-sensory detectors. ie finer, sparser hairs (all else equal) are more sensitive to smaller impulses and therefore are superior detectors of smaller insects and ectoparasites.

Like with the other (also incorrectly named) 'hairless' animals - mole rats, hippos and elephants for example, we have hairs and use them for sensory purposes. In the absence of an insulation function they tend to show adaptation for sensory purposes.

Argue that the changes to this function have minimal evolutionary relevance if you like but please don't treat a function as useful as this one as inconsequential. It's saved me numerous times from Australian Paralysis Tick bits, because I felt them before they dug in. If you had experience of Paralysis Ticks you could not say that a means of avoiding them was insignificant. Very significant I would say, especially in the absence of modern medicine.

From Darwin to Jablonski, body hair is treated as effectively non-functional and it's sensory function has not merely been treated as incosequential but as Non-existent. Cut that any way you like, that is a serious mistake and a whole body of scholarship on this subject is tainted by that omission.

Bones and Behaviours said...

Ken, if in the absence of an insulation function hairs tend to show adaptation for sensory purposes, isn't it simply possible that body hair evolved for sensory reasons first, maybe in a fossorial therapsid, and the hairs have returned by default to their original function?

I simply can't think what humans or naked mole rats have in common with elephants that all of them would lose their coats of fur whilst evolving the same new sensory function for the hairs. Of course if this sensitivity is just a side effect of the reversal to furlessness, then surely hairless mammals really do count as being denudated.

Ken Fabian said...

B&B - I think the point is that this function has not been regained, because it has never been lost. However it appears to be subject to/capable of considerable modification and specialisation for a variety of sensory purposes, even within species that do use fur as insulation. A "reversal to furlessness' - in the sense of changes to hair size and density such that it no longer functions well for insulation has taken place, no question. Actual loss of body hairs - denuding - has NOT taken place.

This fundamental function of the hair/follicle package has been there since before there were mammals and is still there, I think because it IS so useful. And because it is so useful it's important to give it the full consideration it is due.

Thick fur suited to insulation still has a sensory function, but it doesn't work so well at fine sensitivity because hairs laid against each other dissipate the movements and vibrations that need to reach the nerves in follicle and skin. Like a lot of traits that represents a trade-off.

In "When? Why? and How? Some Speculations on the Evolution of the Vertebrate Integument", author PAUL F A Maderson ( ) suggests mammal hairs evolved from a mechano-sensory precursor, that subsequently acquired it's thermoregulatory role after mutations increased their density -

"It is suggested that hairs arose from highly specialized sensory appendages of mechanoreceptor function which facih tated thermoregulatory behavioral activity in early synapsids Specialization of cellular differentiation within these units led to the appearance of dermal papillae A chance mutation led to subsequent multiplication of the originally sparsely, but spatially arranged papillae, causing the induction of a sufficient density of “sensory hairs to constitute an lnsulatory body covering”. The lnsulatory properties of this “prolopelage were the subject of subsequent selection but the sensory function of mammalian hairs remains important”"

It's not that mole rats, elephants and humans uniquely evolved sensory functions for their hairs after the insulation function was lost. Or that we share similar evolutionary histories because we share furlessness; the sensory function was always there and it gets modified by the unique evolutionary story of each species.

Ken Fabian said...

I made a badly worded statement in comment before last - I said "In the absence of an insulation function they tend to show adaptation for sensory purposes." I didn't mean to imply the sensory function is a consequence of furlessness. In the hairs that are left and are not being used for insulation, adaptations for sensory purposes just stand out more obviously.

ben10 said...

Neandertals were furry, so they couldn't make Vit. D?
So, them too didn't need it?

Why then, do only moderns European seem to have evolve fair skin to make VitD when nobody before them (neandertals and early Europeans), or nobody north of them where light is even dimmer (inuits, eskimos) seem to have evolve fair skin for this purpose?
Oh, I know. Sexual selection, now it makes sense. So let me reformulate my question:

Why is the Vit. D hypothesis still the only one mentioned in official schoolbooks?

No, it's not by fear of hurting kid's feelings with the word 'sex' since kids are already fed a propaganda of tolerance for all sort of sexual practices, so why, do you think?

Sean said...

Humans may be a more basal species that living apes. A chimp foetus has a foot made for walking, hair on its head and eyebrows, just like modern humans. Chimps ect are actually quite specialised, and, just as they seem to have lost feet adapted for running, as their ancestors possessed, they may be descended from a something that had lost fur, and then got hairy again. So now they have have skin darkening and scanty hair. It is noteworthy that the Bonobo retains a juvenile white anal tuft and yet its face darkens at a younger age than chimps' do.

Neanderthals seem to have lost feet adapted for running which their ancestors possesed. Presumably Neanderthals became furry through being specialised for cold weather (maybe they could no longer use persistence hunting with a coat of fur, as they would overheat).
The hair colour of Neanderthals may have been related to them having being camouflaged to ambush hunt. The post suggests another possibility: the Neanderthal may have altered the colour of their coats as they matured.

Neanderthals infants had to have thick fur and maybe they had special fur colour to elicit care. Neanderthals are supposed to have been polygynous (here) and I think adult Neanderthal hairless facial skin would have been very dark, like the mountain gorilla, which is the darkest skinned and most polygynous ape.

Bones and Behaviours said...

I don't want to sound like a race denier but the form of the foot is notoriously affected by the individual's life history as well as by heredity. For example in Forth's book about Austronesian wildmen he provides an image of Negrito feet splayed out as a result of the way they live.

Before saying that neanderthals lost the foot anatomy that was evolved by earlier species of Homo, I'd like to see their feet compared to different living populations round the world and especially LPA people and those living without footwear.

And how hairy are mountain gorillas compared to lowland gorillas and just how well does that hair keep them warm up there anyway?

Ken Fabian said...

Darwin's mistake was to think there was a trend towards hairlessness in humans. His incorrect observation that body hair is a functionally useless evolutionary leftover led to the conclusion that it's complete loss is an evolutionary inevitability. It isn't useless so it isn't inevitable and I dispute that there ever was such a thing as a 'trend towards hairlessness'.

Judith Rich Harris' mistake is to suggest parental selection via infanticide - which is historically used to reject those infants that don't conform to normal expectation - would be used to selectively choose death for the normal hairy infant and life for the abnormal less hairy ones.

Sandel's mistake is to try to attribute human 'hairlessness' to the primate ape characteristic of lower hair density than the average mammal. But what distinguishes the human coat from both most mammals and all other apes is smaller vellus type body hairs across the juvenile form, not reduced hair density. There is no evidence of hair loss, just smaller hair size.

My mistake was to fall into the same trap of thinking humans have both smaller hairs AND that they shifted further apart - ie changed distribution across our skin. I don't think there is evidence of significant change to the distribution of follicles across our skin; the hairs are just smaller, not further apart.

If there are other changes - such as to the density of follicular nerve endings that would indicate adaptation for more - or less - sensitivity, I'm not aware. But I'm not aware anyone has looked.

Anonymous said...

Sorry but social signaling could have evolved very simply by hair color mutations, without the loss of a critical structure such as the pelt. After all, hairlessness carries some significant disadvantages too. The trait would need to be offset by a very powerful advantage. So for now I'm sticking with the persistence hunting hypothesis for the explanation of hairlnessness... and a lot of the rest of our anatomy. (cf Born to Run, Chris MacDougall, Chap. 28)

As to the Neanderthal-Vitamin D issue... how does ANY furred critter make Vit. D? In the oils on the fur itself, of course, and then it's resorbed. Even a very dark animal can make Vit D this way, though some N-thals seem to have been lightly pigmented. For that matter, naked modern humans don't get much Vit D if they sunbathe for an hour, and then jump in the water -- rinsing away the D before it can be resorbed. Because it happens in the external oil... and that can happen in hair.

Ben10 said...

Would it be it possible to infer Neanderthal genetic sequence in modern humans by a 'subtracting technique'?
For example subtract the genomes: Australian aborigines(assuming it contains some Neanderthal) - African San(assuming 0% Neanderthal) = N sequences

Among these N sequences, anything older than say, 100 000 years, is potentially Neanderthal. Then try to relate these old sequences to any phenotypic differences, for example aborigines have blond hairs. Could that be from Neanderthals?

Anonymous said...

Ben10 -- both European blonds, and Australasian blonds, have been shown to be due to relatively recent (post-N-thal) mutations. The oldest colorphase mutation in modern man is the ginger (pale/redhead) gene, which occurred in Paleolithic Europe -- the first white men were freckled redheads.

N-thals had their own MCR1 mutation in some individuals, which suggests that some N-thals could have been redheads or otherwise lightly pigmented. But the N-thal MCR1 mutation did NOT introgress to modern populations --we have our own MCR1 mutations which are different from N-thal.

ben10 said...

Anonymous above,
That's right, I forgot about that.
But, as said above, a simple subtraction or sequence alignment, of two very different modern populations of which only one is suspected to carry archaic DNA, should provide interesting results, at least for anything older than 50-100 000 years genes.

I am sure it has been done and you guys know the result.

Sister Y said...

This is a valuable contribution to one of the most interesting evo bio questions - thanks!

What do you think of the pubic louse evidence - that human and gorilla pubic lice diverged around 3.3 MYA? When you mentioned in the main post that most people are looking for human-specific traits to justify hairlessness, I figured this is what you meant - human-specific stuff happens much later than 3.3 MYA. But then in a comment you said more like 100,000 YA. Do you think the pubic lice DNA evidence isn't good evidence of when denudation occurred, or something else?

Anonymous said...

"a simple subtraction or sequence alignment..should provide interesting results..I am sure it has been done and you guys know the result..."

I'm a chemical engineer, not an anthropologist! This is just a hobby interest for me. So, no, I don't know the result. Probably some real anthropologists do know.

".... The oldest colorphase mutation in modern man is the ginger (pale/redhead) gene..."

I should correct myself. The oldest known colorphase mutation in modern men **that affects the hair**, is the ginger gene. The lightening of the skin may well predate it. The parents of the first redhead, were probably Amerind-bronze, not tropical black.

Bones and Behaviours said...

Speaking of skin tone why do so many Celts go red instead of tanning if other depigmented people can tan?

I was surprised to hear that the Iberians are of a similar skin tone to northwestern Europeans when they're kept out of the sun for prolonged periods. But I've also noticed of Yezidi Kurds that they can be very light like Europeans, but that they can also be very dark if they work in the fields. And I don't think this is related to their castes because light and dark Yezidis can look alike.

Of course this verifies that the origins of depigmentation lie in sexual selection - lots of people with genes for light skin would only be depigmented if kept out of the sun. This may also be the origin of the practice of wearing the veil by Greek and Persian women in the ancient world - to retain the good looks of their northern, IE foremothers.

But did a gene for tanning evolve after depigmentation to allow pale skinned people to survive in Mediterranean climates?

Anonymous said...

But did a gene for tanning evolve after depigmentation

Well... the ginger (pale/redhead) gene seems to be older than the "white but tannable" complex. A tannable-white skin can change with the seasons -- ginger can't.

I suspect that the ginger gene represented an OVERCORRECTION to the selection pressures. And since it came first, perhaps it was once far more widespread. But after a genetic path for tannable whitness evolved, that replaced the less optimal ginger gene.

Interestingly, the oldest Chinese descriptions of whites, describe green-eyed redheads. The blonde/blue complex came later.

Anonymous said...

why do so many Celts go red instead of tanning if other depigmented people can tan?

They inhabited the foggiest part of Europe. And also, prior to the deforestation by the Anglo-Saxon occupiers, it very heavily forested.

Anonymous said...

"They inhabited the foggiest part of Europe. And also, prior to the deforestation by the Anglo-Saxon occupiers, it very heavily forested"

IIRC ancient writers seem to mention white redheads in lots of places e.g. Thrace, Russia etc.

I wonder if the white redhead thing came first but only survived in the remote NW.

Anonymous said...

IIRC ancient writers seem to mention white redheads in lots of places e.g. Thrace, Russia etc.

Kyrgyz of central Asia were described as a highly redheaded population until Islamic and Chinese introgression darkened them. Even so, redhead throwbacks occur.

Genghis Khan and many of his relatives are reported to have been redheads.

Redheads crop up sporadically in Kurdistan (northern Iraq). ]

Finally: a friend of mine who is pure Egyptian (but Christian), and whose husband is a white American, had a redhead child! Yet the recessive has to come from both parents, so she, the Eqyptian, must carry the gene too. And indeed ancient Egyptian history records redheads in their country, long ago.

So yes... it was once widespread, and was largely replaced EXCEPT in the foggy reaches of NW Europe.

Oh, and there are lots of redheaded Jews too. Can anyone explain that?