In his 1871 book Descent of Man, Charles Darwin viewed human evolution as involving both natural selection and sexual selection. In seeking to understand sex and human evolution, there are two basic features to consider: (1) the physical aspect of sex, including both primary and secondary sexual characteristics; and (2) the fact that human sexual behavior is more cultural than physical.
Humans experience sexual intercourse in a fashion different from that of most primates. For most other animals, sexual intercourse is an action which is required for reproduction and has no symbolic meaning. While procreational sex is important to the continuation of the species, among humans procreational sex is only one of many forms of sexual behavior. With regard to procreational sex, zoologist Desmond Morris, in his book The Human Zoo, writes:
“It has sometimes been mistakenly argued that it is the only natural and therefore proper role.”
According to Morris, the viewpoint that procreational sex is the only “correct” form of sex is often expressed from a religious perspective.
Comparing human sexual behavior with that of other primates, such as our close relatives the chimpanzees, provides another perspective. In his book The Gap: The Science of What Separates Us from Other Animals, Thomas Suddendorf writes:
“Female chimpanzees advertise their sexual readiness through distinctive pink genital swelling and during that time may mate between five and an astounding fifty times a day. Mind you, intercourse usually only lasts seven seconds.”
Unlike many other primates, human females engage in sexual intercourse without reference to potential fertility. In his textbook The Human Species: An Introduction to Physical Anthropology, Frederick Hulse writes:
“We are among the relatively few animals without a mating season, so that adult males are more or less constantly attracted to females, and sexual activity continues throughout the year. This has had a profound effect on our mode of life.”
In an article in Scientific American, Cordelia Fine and Mark Elgar write:
“Unlike many other species, in which coitus is hormonally coordinated to a greater or lesser degree to ensure that sex results in conception, humans engage in vast amounts of nonreproductive sex.”
In his book The Accidental Species: Misunderstandings of Human Evolution, Henry Gee writes:
“Estrus in human females is concealed, even from the female herself. No external sign betrays when a human female is more or less likely to conceive.”
In his book Humans: From the Beginning, Christopher Seddon puts it this way:
“One feature of human reproductive biology that is highly uncommon among mammals is concealed ovulation. In most mammal species, females are only sexually receptive when ovulating and hence able to conceive.”
This means that human sexual activity is inefficient with regards to resulting in actual reproduction. In other words, sexual intercourse is about something more than conception. For humans, sexual intercourse is filled with symbolism and has social, emotional, and religious meanings that transcend simple reproduction. Sex may function, for example, to strengthen relationships.
Zoologist Desmond Morris sees the pair-formation aspect of sex important in humans:
“The human animal is basically and biologically a pair-forming species. As the emotional relationship develops between a pair of potential mates it is aided and abetted by the sexual activities they share.”
Sex is not only important in the initial process of pair-bonding, but in the maintenance of the relationship. Desmond Morris writes:
“Once a pair-bond has been successfully formed, sexual activities still function to maintain and reinforce the bond.”
It is interesting to note that among humans, estrus is concealed and sexual intercourse tends to take place in private, while among apes, both are public.
With regard to anatomy, one of the striking features of the human male is the large visible penis. Compared with apes, the erect human penis is about four times larger than that of a gorilla. This large penis may be the result of sexual selection in evolution. In his book Thumbs,Toes, and Tears and Other Traits that Make Us Human, Chip Walter writes:
“Our sex organs are unusual, for example. Take the human penis, which to be blunt, is enormous compared with its peers in the rest of the primate world.”
In noting the large size of the human penis, Chris Stringer, in his book Lone Survivors: How We Came to be the Only Humans on Earth, also reports:
“But the human penis is considerably thicker than any of the others and has a much more bulbous end.”
In human societies, the penis is often a symbol of manliness, sexuality, and fertility.
With regard to the genetics involved in the penis, there are several genes involved. Geneticist Svante Pääbo, in his book Neanderthal Man: In Search of Lost Genomes, writes:
“One of these [genes] encoded a protein expressed in penile spines, which are structures on the penises of apes that cause males to ejaculate very quickly. These spines are not present in humans, which enables us to enjoy prolonged intercourse.”
With regard to the lack of penile spines, Sam Kean, in his chapter in A Companion to Linguistic Anthropology, writes:
“Besides sparing vaginas, this loss decreases male sensation during sex and thereby prolongs copulation, which scientists suspect helps humans pair-bond and stay monogamous.”
While sexual intercourse among chimpanzees takes only seconds, among humans, sexual intercourse lasting only seconds is generally considered abnormal.
Bipedalism, by the way, made the penis more visible. This, in turn, enhanced the symbolic meaning of the penis.
In addition, with bipedalism the female vagina now faces forward which makes face-to-face copulation easier. With face-to-face copulation, the couple can maintain eye contact which makes mind and face reading possible and may enhance the emotional experience.
Sexual selection has also resulted in female breasts which are usually much larger than those of apes. Human female breasts are usually large and defined. The breasts have two biological functions: parental and sexual. Chip Walter writes:
“Their increased size isn’t, as you might expect, necessary for lactation and feeding.”
In his book The Naked Woman: A Study of the Female Body, Desmond Morris puts it this way:
“The inescapable conclusion is that the hemispherical shape of the breasts is not a parental development. It is concerned instead with sexual signaling.”
It has been suggested that breast shape and size may have been more than sex appeal but may have also signaled fertility potential. With regard to the male penis and the female breast, Donald Johanson and Blake Edgar, in their book From Lucy to Language, write:
“Perhaps penis and breast size were important sexual signaling devices that served to attract potential mates.”
In looking at human sex from the viewpoint of both anatomy and behavior in his chapter on humanness in The Oxford Handbook of Archaeology, Jonathan Marks asks:
“When did our penises lose their small bone, or os baculum, and become enlarged and pendulous? When did our breasts become enlarged, resembling those of apes only when the apes are lactating? And when did both of these become centres of erogeny and arousal, reflected in a far more lengthy and tactile copulation bout than is found in the apes? And under what circumstances did human sexuality assume such an extensively non-reproductive aspect?”
Another anatomical feature related to sex is the rounded and muscled human buttocks. Chip Walter writes:
“Our buttocks are a third, strangely human adaptation. We are the only primates that have a round, muscled bum. Arguably it developed when we stood up because we needed those muscles to help support and balance all the weight we were now carrying above our pelvises. But they may have evolved as a sign of health that made us more attractive to the opposite sex.”
In early humans, the big buttocks may have served as an indication of strong, healthy, and active females, important characteristics in early foraging societies.
One of the important features of humanness is the curiosity and the willingness to experiment with new concepts, which has been vital to cultural evolution. With regard to sex, Desmond Morris, in his book The Human Zoo, reports:
“Ancient sexual writings record in detail the great diversity of novel sexual movements, pressures, sounds, contacts, scents and copulatory positions that were the subject of erotic experimentation.”
In the era before writing, the archaeological record has examples of sexually explicit paintings, drawings, and carvings, as well as sexual symbols such as artifacts in the shapes of erect penis and female genitalia. These are sometimes seen as religious objects.
Overall, the evolution of sexual anatomy has been a key element in the physical evolution of humans as well as an important element in human cultural evolution. In culture, this has played a role in the development of many religious traditions.
More Human Origins
Human Origins: Domesticating Fire
Human Origins: Menopause
Human Origins: The Mind
Human Origins: Bipedalism
Human Origins: Symbolism
Human Origins: Cultural Evolution
Human Origins: The Human Hand
Human Origins: The Large Brain