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
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