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).

Conclusion

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.

References

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

http://link.springer.com/article/10.1007/BF02390470#

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

http://isteve.blogspot.com/2009/01/neanderthals.html 

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

http://darwin-online.org.uk/EditorialIntroductions/Freeman_TheDescentofMan.html 

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

http://link.springer.com/article/10.1007%2Fs10764-009-9381-y?LI=true# 

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

http://books.google.fr/books?hl=fr&lr=&id=nXuqgInMOXIC&oi=fnd&pg=PR10&dq=Desolate+Landscapes.+Ice-Age+Settlement+in+Eastern+Europe&ots=UmwmLxMDDG&sig=bXfiayx_z0DSMQcwCauN5_8NHEw#v=onepage&q&f=false 

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

http://onlinelibrary.wiley.com/doi/10.1111/j.1749-6632.1962.tb13653.x/abstract 

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.

http://onlinelibrary.wiley.com/doi/10.1002/ajpa.22333/abstract 

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

http://books.google.fr/books?hl=fr&lr=&id=up4_q8ooKO0C&oi=fnd&pg=PP8&ots=IWCywpZQvq&sig=SaJsUdUZMeVTCGFFecXvD9mNVew#v=onepage&q&f=false 

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.