General hypotheses
Some
hypotheses about the evolution of the human brain argue that it is a
sexually selected trait, as it would not confer enough fitness in itself
relative to its high maintenance costs (a quarter to a fifth of the
energy and oxygen consumed by a human).
Sexual selection's role in human evolution cannot be definitively
established, as features may result from an equilibrium among competing
selective pressures, some involving sexual selection, others natural selection, and others pleiotropy. In the words of Richard Dawkins:
- When you notice a characteristic of an animal and ask what its Darwinian survival value is, you may be asking the wrong question. It could be that the characteristic you have picked out is not the one that matters. It may have "come along for the ride", dragged along in evolution by some other characteristic to which it is pleiotropically linked.
Darwin's sexual selection hypothesis
Charles Darwin described sexual selection
as depending on "the advantage which certain individuals have over
others of the same sex and species, solely in respect of reproduction". Darwin noted that sexual selection is of two kinds and concluded that both kinds had operated on humans:
"The sexual struggle is of two kinds; in the one it is between the
individuals of the same sex, generally the male sex, in order to drive
away or kill their rivals, the females remaining passive; whilst in the
other, the struggle is likewise between the individuals of the same sex,
in order to excite or charm those of the opposite sex, generally the
females, which no longer remain passive, but select the more agreeable
partners."
Charles Darwin conjectured that the male beard, as well as the hairlessness of humans
compared to nearly all other mammals, were results of sexual selection.
He reasoned that since the bodies of females are more nearly hairless,
the loss of fur was due to sexual selection of females at a remote
prehistoric time when males had overwhelming selective power, and that
it nonetheless affected males due to genetic correlation between the
sexes. He also hypothesized that contrasts in sexual selection acting
along with natural selection were significant factors in the
geographical differentiation in human appearance of some isolated
groups, as he did not believe that natural selection alone provided a satisfactory answer. Although not explicit, his observation that in Khoisan women "the posterior part of the body projects in a most wonderful manner" (known as steatopygia) implies sexual selection for this characteristic. In The Descent of Man, and Selection in Relation to Sex, Darwin viewed many physical traits which vary around the world as being so trivial to survival
that he concluded some input from sexual selection was required to
account for their presence. He noted that variation in these features
among the various peoples of the world meant human mate-choice criteria
would also have to be quite different if the focus was similar, and he
himself doubted that, citing reports indicating that ideals of beauty did not, in fact, vary in this way around the world.
Sexual dimorphism
Men are generally hairier than women, and Darwin was of the opinion
that hairlessness was related to sexual selection; however, several
other explanations have been advanced to explain human hairlessness, a leading one is loss of body hair to facilitate sweating. This idea closely relates to that of the suggested need for increased photoprotection and is part of the most-commonly-accepted scientific explanation for the evolution of pigmentary traits.
Indicating that a trait is under sexual selection can be
difficult to prove through correlational methods, as characters may
result from different selective pressures, some involving sexual
selection, others natural selection, and some may be accidental and due
to pleiotropy.
For example, monogamous primates are known to typically exhibit little
sexual dimorphism such as particularly large males armed with huge
canines; however, powerful big-toothed males can provide protection
against predators and may be bigger for that reason, rather than in
order to win confrontations over females. Males and females differing in
size can specialize in, and more fully exploit, different food
resources while avoiding competing with each other; furthermore, body
size can be useful in avoiding predators and may also be of assistance
in securing a mate. This is further complicated by the consideration
that with larger body size, the skeleton of mammals becomes much more
robust and massive (relatively speaking).
Bearing these caveats in mind, levels of sexual dimorphism are
generally seen as a marker of sexual selection. Studies have shown the
earliest homininae
were highly dimorphic and that this tendency lessened over the course
of human evolution, suggesting humans have become more monogamous. In
contrast, gorillas living in harems exhibit a much stronger sexual dimorphism (see: homininae).
Sexual anatomy
The theory of sexual selection has been used to explain a number of human anatomical features. These include rounded breasts, facial hair, pubic hair
and penis size. The breasts of primates are flat, yet are able to
produce sufficient milk for feeding their young. The breasts of
non-lactating human females are filled with fatty tissue and not milk.
Thus it has been suggested the rounded female breasts are signals of
fertility. Richard Dawkins has speculated that the loss of the penis bone
in humans, when it is present in other primates, may be due to sexual
selection by females looking for a clear sign of good health in
prospective mates. Since a human erection relies on a hydraulic pumping system, erection failure is a sensitive early warning of certain kinds of physical and mental ill health.
Homo has a thicker penis than the other great apes, though it is no longer than the chimpanzee's. It has been suggested the evolution of the human penis towards larger size was the result of female choice rather than sperm competition, which generally favors large testicles.
However, penis size may have been subject to natural selection, rather
than sexual selection, due to a larger penis' efficiency in displacing
the sperm of rival males during sexual intercourse. A model study showed displacement of semen was directly proportional to the depth of pelvic thrusting, as an efficient semen displacement device.
Selection preferences and biological drivers
There
are a variety of factors that drive sexual selection in humans. Current
available research indicates that selection preferences are
biologically driven, that is, by the display of phenotypic traits that can be both consciously and unconsciously evaluated by the opposite sex to determine the health and fertility of a potential mate. This process can be affected, however, by social factors, including in cultures where arranged marriage is practiced, or psychosocial factors, such as valuing certain cultural traits of a mate, including a persons social status, or what is perceived to be an ideal partner in various cultures.
Selection preferences in females
Some
of the factors that affect how females select their potential mates for
reproduction include voice pitch, facial shape, muscular appearance,
and height.
Several studies suggest that there is a link between hormone levels and
partner selection among humans. In a study measuring female attraction
to males with varying levels of masculinity,
it was established that women had a general masculinity preferences for
men's voices, and that the preference for masculinity was greater in
the fertile phase of the menstrual cycle than in the non-fertile phase.
There is further evidence from the same study that in fertile stages of
the menstrual cycle, women also had a preference for other masculine
traits such as body size, facial shape, and dominant behavior, which are
indicators of both fertility and health. This study did not exclude males with feminine
traits from being selected, however, as feminine traits in men indicate
a higher probability of long-term relationship commitment, and may be one of several survival strategies. Further research also backs up the idea of using phenotypic traits as a means of assessing a potential mates fitness for reproduction as well as assessing whether a partner has high genetic quality.
Another factor affecting the selection process is the environment
which the person inhabits. In biological terms, certain environmental
conditions may bring about demands for or the disregarding of certain
traits. One such example is a preference for males whose facial
structure indicates certain hormonal ratios, such as testosterone-cortisol levels (sex and stress hormones). Research shows that, for example, in countries with varying Human Development Index
(HDI) levels, females have different preferences for sex-stress hormone
ratios, as expressed in the male's face. A Royal Society research
showed a significant correlation between a measure of societal
development and preferences for indication of higher testosterone
levels, as manifested in facial features, and the interaction between
preferences for testosterone and cortisol." It was concluded that societal-level ecological factors impact the valuation of traits by combinations of sex- and stress-hormones.
Selection preferences in males
Like
their female counterparts, males also use visual information about a
potential mate, as well as voice, body shape, and an assortment of other
factors in selecting a partner. Research shows that males tend to
prefer feminine women's faces and voices as opposed to women with
masculine features in these categories. Furthermore, males also evaluate skin coloration, symmetry, and apparent health, as a means by which the select a partner for reproductive purposes. Males are particularly attracted to femininity
in women's faces when their testosterone levels are at their highest,
and the level of attraction to femininity may fluctuate as hormone
levels fluctuate. Studies on men have also been done to show the effects of exogenous testosterone
and its effects on attraction to femininity, and the results concluded
that throughout several studies, men have shown decreased preference for
feminine female faces in the long-term context, when given exogenous
testosterone, but this difference did not occur with placebo.
Common preferences in either sex
Sexual selection preferences are general terms by which the mating
and reproductive process are understood. As one article states, sexual
selection is in essence a process which favors sexual displays for
attraction, aggressiveness, dominance, size, and strength, and the
ability to exclude competitors by force if necessary, or by using
resources to win. Both male and female use voice, face, and other physical characteristics to assess a potential mate's ability to reproduce, as well as their health. Together with visual and chemical signals, these crucial characteristics which are likely to enhance the ability to produce offspring, as well as long term survival prospects, can be assessed and selections made.
Phenotype
Sexual
selection has continued to be suggested as a possible explanation for
geographical variation in appearance within the human species; in modern
hypotheses, marriage practices are proposed as the main determinant of sexual selection. John Manning suggests that where polygyny
is common, men face intense competition for wives and are more likely
to be completely unsuccessful in reproducing, and the result is strong
selection of males for traits which are adaptive for successful
reproduction. He proposes a link to skin color through selection of
males for testosterone-mediated traits which confer an ability to
successfully compete for females. He suggests testosterone makes the
human immune system less competent to resist pathogens. In this view the antimicrobial properties of melanin
help mitigate the susceptibility to disease that polygyny induces by
increasing testosterone. According to this argument, the
anti-infective qualities of melanin were more important than protection
from ultraviolet light
in the evolution of the darkest skin types. Manning asserts that skin
color is more correlated with the occurrence of polygyny – explicable by
it having an antimicrobial function – than the latitudinal gradient in
intensity of ultraviolet radiation, and he points to the lack of very
dark skin at equatorial latitudes of the New World and the relatively light skin of Khoisan people in Africa.
Research seems to contradict Manning's explanation about skin color. In 1978, NASA launched the Total Ozone Mapping Spectrometer, which was able to measure the ultraviolet radiation
reaching Earth's surface. Jablonski and Chaplin took the spectrometer's
global ultraviolet measurements and compared them with published data
on skin color in indigenous populations from more than 50 countries.
There was an unmistakable correlation: The weaker the ultraviolet light,
the fairer the skin. Rogers et al.
(2004) performed an examination of the variation in MC1R nucleotide
sequences for people of different ancestry and compared the sequences of
chimpanzees and humans from various regions of the Earth. Rogers
concluded that, at the time of the evolutionary separation of
chimpanzees and humans, the common ancestors of all humans had light
skin that was covered by dark hair. Additionally, our closest extant
relative, the chimpanzee, has light skin covered by thick body hair. Over time human hair disappeared to allow better heat dissipation through sweating and the skin tone grew darker to increase the epidermal permeability barrier and protect from folate depletion due to the increased exposure to sunlight. When humans started to migrate away from the tropics, there was less-intense sunlight, partly due to clothing to protect against cold weather. Under these conditions there was less photo-destruction of folate,
and so the evolutionary pressure stopping lighter-skinned gene variants
from surviving was reduced. In addition, lighter skin is able to
generate more vitamin D
(cholecalciferol) than darker skin, so it would have represented a
health benefit in reduced sunlight if there were limited sources of
vitamin D. The genetic mutations leading to light skin experienced selective pressure due to settlement in northern latitudes.
Anthropologist Peter Frost has proposed that sexual selection was responsible for the evolution of pigmentary traits of women in Northern and Eastern European
populations. He contends that the diversity of hair and eye color in
Northeast European populations originated as a consequence of intense
female-female competition, and is an adaptation for reproductive success
in women.
Geoffrey Miller hypothesis
Geoffrey Miller,
drawing on some of Darwin's largely neglected ideas about human
behavior, has hypothesized that many human behaviors not clearly tied to
survival benefits, such as humor, music, visual art, some forms of altruism, verbal creativity or the fact that most humans have a far greater vocabulary than that which is required for survival,
Miller (2000) has proposed that this apparent redundancy is due to
individuals using vocabulary to demonstrate their intelligence, and
consequently their "fitness", to potential mates. This has been tested
experimentally, and it appears that males do make greater use of
lower-frequency (more unusual) words when in a romantic mindset compared
to a non-romantic mindset, suggesting that vocabulary is likely to be
used as a sexual display (Rosenberg & Tunney, 2008). All these
qualities are considered courtship adaptations that have been favored
through sexual selection.
Miller is critical of theories that imply that human culture
arose as accidents or by-products of human evolution. He believes that
human culture arose through sexual selection for creative traits. In
that view, many human artifacts could be considered subject to sexual
selection as part of the extended phenotype, for instance clothing that
enhances sexually selected traits.
During human evolution, on at least two occasions, hominid brain size
increased rapidly over a short period of time followed by a period of
stasis. The first period of brain expansion occurred 2.5 million years
ago, when Homo habilis first began using stone tools. The second period occurred 500,000 years ago, with the emergence of archaic Homo sapiens. Miller argues that the rapid increases in brain size would have occurred by a positive feedback loop resulting in a Fisherian runaway selection for larger brains. Tor Nørretranders, in The Generous Man conjectures how intelligence, musicality, artistic and social skills, and language might have evolved as an example of the handicap principle, analogously with the peacock's tail, the standard example of that principle. Another hypothesis proposes that human intelligence is a courtship indicator of health and resistance against parasites and pathogens which are deleterious to human cognitive capabilities.
Opposing arguments
The
role of sexual selection in human evolution has been considered
controversial from the moment of publication of Darwin's book on sexual
selection (1871). Among his vocal critics were some of Darwin's
supporters, such as Alfred Wallace, who argued that animals and birds do not choose mates based on their beauty or fine plumages, and that the artistic faculties in humans belong to their spiritual nature and therefore cannot be connected to natural selection, which only affects the animal nature. Darwin was accused of looking to the evolution of early human ancestors through the moral codes of the 19th century Victorian society. Joan Roughgarden,
citing elements of sexual behavior in animals and humans that cannot be
explained by the sexual-selection model, suggested that the function of
sex in human evolution was primarily social.
Joseph Jordania
suggested in 2011 that in explaining such human morphological and
behavioral characteristics as singing, dancing, body painting, wearing
of clothes, Darwin and proponents of sexual selection neglect another
important evolutionary force, intimidation of predators and competitors with the ritualized forms of warning display,
which uses the same arsenal of visual, audio, olfactory and behavioral
features as sexual selection. According to Jordania, most of these
warning displays were incorrectly attributed to the forces of sexual
selection. Jordania proposed an aposematic
model of human evolution, where most of the human morphological and
behavioral features that had been considered by Darwin as the result of
sexual selection, via female choice, are explained by the aposematic
(intimidating) display.
