Diamond, Jared. Why is Sex Fun? The Evolution of Human Sexuality.
The subject of sex preoccupies us. It’s the source of our most intense
pleasures. Often it’s also the cause of
misery, much of which arises from built-in conflicts between the evolved roles
of men and women. (ix)
This book is a speculative account of how human sexuality
came to be the way it now is. Most of us
don’t realize how unusual human sexual practices are, compared to those of all
other living animals. Scientists infer
that the sex life of even our recent apelike ancestors was very different from
ours today. Some distinctive
evolutionary forces must have operated on our ancestors to make us
different. What were those forces, and
what really is so bizarre about us? (ix)
Understanding how our sexuality evolved is fascinating not
only in its own right but also in order to understand other distinctively human
features. Those features include our
culture, speech, parent-child relations, and mastery of complex tools. While paleontologists usually attribute the
evolution of these features to our attainment of large brains and upright
posture, I argue that our bizarre sexuality was equally essential for their
evolution. (ix)
Among the unusual aspects of human sexuality that I discuss are female menopause, the role of men in human societies,
having sex in private, often having sex for fun rather than for procreation,
and the expansion of women’s breasts even before use in lactation. To the layperson, these features all seem
almost too natural to require explanation. On reflection, though, they prove
surprisingly difficult to account for.
I’ll also discuss the function of men’s penises and the reasons women
but not men nurse their babies. The
answers to these two questions seem utterly obvious. Within even these questions, though, lurk
baffling unsolved problems (ix-x)
> The Animal with the Weirdest Sex Life
As a beginning, let’s consider normal sexuality by the
standards of the world’s approximately 4,300 species of mammals, of which we
humans are just one. Most mammals do not
live as a nuclear family of a mated adult male and adult female, caring jointly
for their offspring. Instead, in many
mammal species both adult males and adult females are solitary, at least during
the breeding season, and meet only to copulate.
Hence, males do not provide paternal care; their sperm is their sole
contribution to their offspring and to their temporary mate. (2)
Even most social mammal species, such as lions, wolves,
chimpanzees, and many hoofed mammals, are not paired off within the
herd/pride/pack/band into male/female couples.
Within such herd/pride/ et cetera, each adult male shows no signs of
recognizing specific infants as his offspring by devoting himself to them at
the expense of other infants in the herd.
Indeed, it is only within the last few years that scientists studying
lions, wolves, and chimpanzees have begun to figure out, with the help of DNA
testing, which male sired which infant.
However, like all generalizations, these admit exceptions. (2-3)
Sex in social mammals is generally carried out in public,
before the gazes of other members of the troop.
For instance, a female
All these features of human sexuality – long-term sexual
partnerships, coparenting, proximity to the sexual
partnerships of others, private sex, concealed ovulation, extended female
receptivity, sex for fun, and female menopause – constitute what we humans
assume is normal sexuality. (6)
The key to understanding human sexuality is to recognize
that it is a problem in evolutionary biology.
(10)
Some sexual characteristics may be more advantageous for
survival and reproduction than others, depending on each species’ food supply,
exposure to predators, and other biological characteristics. (11)
We can thus redefine the problem posed by our strange sexuality. Within the last seven million years, our
sexual anatomy diverged somewhat, our sexual physiology further,
and our sexual behavior even more, from those of our closest relatives, the
chimpanzees. Those divergences must
reflect a divergence between humans and chimpanzees in environment and
lifestyle. But those divergences were
also limited by inherited constraints. (13)
For a long time, evolutionary biologists thought of natural
selection as somehow promoting “the good of the species.” In fact, natural selection operates initially
on individual animals and plants.
Natural selection is not just a struggle between species (entire
populations), nor is it just a struggle between individuals of different
species, nor just between conspecific individuals of
the same age and sex. Natural selection
can also be a struggle between parents and their offspring or a struggle
between mates, because the self-interests of parents and their offspring, or of
father and mother, may not coincide.
What makes individuals of one age and sex successful at transmitting
their genes may not increase the success of other classes of individuals. (18)