Five Questions on Human Sexuality

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    In his book "The Third Chimpanzee" Jared Diamond offers an interesting challenge. At the beginning of Chapter Three he presents five questions and challenges the reader to answer them, noting that biologists have not come up with convincing answers to them as of 1992. In this essay I accept that challenge and offer my answers to those questions. I wonít claim that I have done anything original here, because I donít have the ability to carry out a proper search of the biological literature to see whether others have proposed these solutions before me. Nonetheless, I would think that answers to questions as important as Dr. Diamond has indicated would have made their way into the popular literature by now (June 2009) and I have not seen them.

    Let me start by stating that my solution of Diamondís Problems emerges simply from the fact that our ancestors evolved from creatures that walk on all fours to creatures that walk exclusively on two legs. We know that chimpanzees can walk for a time on their hind legs, so our common ancestor with the chimpanzees likely could do so as well. As East Africa dried out at the beginning of the Pliocene Epoch (5,332,000 years ago to 2,600,000 years ago), those chimp-like ancestors evolved into genus Australopithecus, the first proto-humans, by way of genus Ardipithecus. By 1,800,000 years ago some of those creatures had evolved into the first species of genus Homo. We know from the fossil called Lucy that Australopithecines walked upright, so the features that Diamond describes in his question likely applied to that genus of hominids as well as they do to our species.

    We also know that a species of chimpanzee known as a bonobo split off from the other chimpanzee line two million years after our common ancestor did. We see what makes bonobos relevant in the fact that they have a more slender build than does the common chimpanzee and have an improved ability to walk upright; indeed they have body proportions similar to those of the australopithecines. They also have sex face to face, as we do, and unlike the other apes. It seems to be a female preference due to the forward shift of the clitoris.

    Now letís consider Diamondís five questions:

1. The Invisible Ćstrus:

    "Why donít women resemble almost all other female mammals in having easily recognized days of fertility, with sexual receptivity confined to those days?"

    I must confess here that I got the hint to the solution of this puzzle when I was thumbing through a pornographic magazine. I saw a series of photographs of two women dressed as cheerleaders in a gymnasium with more of the costumes disappearing as the series progressed. The last picture showed the women, completely naked, sitting side by side on the polished-wood floor with their legs spread for the camera and I noticed that their vaginal openings came to different heights above the floor and that one womanís vulva was larger than the other womanís. That provided the "Aha!" moment by reminding me of what Darwinís concept of descent with modification means. Sometimes we need these little reminders of what the theory of evolution will tell us if only we pay proper attention to the details upon which natural selection will operate.

    In other apes the ústrus involves a gross swelling of the femaleís external genitalia, basically the structures that in human women comprise the labia. In an ape that walks upright most of the time, that swelling would cause serious discomfort. In chimpanzees, gorillas, and orangutans, which walk on all fours, the swelling does not become a problem because the swollen flesh rides between the femaleís buttocks and outside her crotch. In humans, though, a woman in an ústrus that includes that kind of display would have to walk bowlegged to avoid chafing herself and thereby exposing herself to infection: running would become nearly impossible.

    So imagine the world of Proconsul, our common ancestor with the chimpanzees, as East Africa began to dry out. The extensive, continuous forest in which those creatures lived began to break into fragments as fingers of grassland penetrated the drier areas. Those fingers grew wider as the land became drier and, thus, the islands of forest shrank. Slowly at first, and then more frequently, bands of Proconsul had to cross those grasslands from one forest patch to another in order to find food. Those who could make those crossings walking upright had an advantage over those who could not: avoiding predators, seeing more clearly whither they were going, and so on. Thus natural selection would have favored those individuals who had longer legs and a wider angle between their thighs and their spines. As the mutations that produced those changes spread throughout the population they would have promoted other mutations as well, especially those pertaining to the display of ústrus in the females.

    We can conceive one mutation that we would not see favored by natural selection: we would not see the genitalia shift backward to continue riding on the buttocks. To achieve that change the female body would need a set of mutations that make the labia grow away from the vagina, around the anus, and up at least as far as the bottom of the coccyx. And if we could look into the past, we might actually see manifestations of mutations headed in that direction. But the mutations that would produce that kind of change have a much greater complexity than do the mutations that produce external genitalia that are smaller than normal and that donít swell as much during ústrus: because of their simplicity, those latter mutations would occur more frequently than the genitalia shifting mutations and, thus, would be favored by natural selection.

    If we look at the chimpanzees, we will indeed find that some females have smaller swellings of their genitalia than do others in the same group. Those differences reflect differences between the DNA in the gene sequences of individuals in the group. That fact would apply to our proto-human ancestors as well. Because they have a genetic basis, natural selection would play on those differences in the size of genital swelling as our ape ancestors began to evolve the leg structure for upright walking. Those females with the smallest genitalia would have had the greatest probability of reproducing themselves successfully: more of them means fewer of the females with the largest genitalia and over time that process leads to the complete elimination from the population of females with large genitalia. We thus obtain a species of ape that walks upright and whose females do not display a visible sign of ústrus.

    The invisible útrus promotes one other major change in humans, in the men. Without a clear signal that a woman has come into heat, the men have no trigger for sexual desire as other apes do. As the sexual signal faded, so that women could walk upright, those men who would have sex at any time, regardless of the signalís presence or absence, would have reproduced more than those who stuck to the programmed response. Thus evolved the human maleís infamous "one-track mind". Likewise, women who had the ability to respond to male attention at any time, and not only during ústrus, would have reproduced themselves (and thereby the genetic coding for that instinctual behavior) more than would have those women who didnít have that ability. Those facts, in themselves, wrought great changes in Humanity as it evolved out of Proconsul and into Ardipithecus, the precursor of Australopithecus.

    We need to take account of one other feature associated with upright walking and the large human brain. In humans the process of birth is one of the most difficult of any mammal. Even among australopithecines birth was likely more apelike (i.e. relatively easy and painless). Even so, the brain of a newborn human comes out still unfinished compared to that of a chimpanzee. So why didnít selection favor expansion of the birth canal while it was reshaping the pelvis? The flippant answer says that natural selection does not care whether the female suffers intense pain in giving birth: so long as proto-humans conceived no connection between sex and birth, that long would the females fail to avoid the pregnancies that caused them so much distress and the species would continue to propagate. The serious answer tells us the hips and pelvis would have to expand in order to allow the birth canal to widen. Such an expansion requires more food for the growing female, but it confers no reproductive advantage, so natural selection would eliminate the genetic coding for such expanded hips and pelvis from the population.

    So how did biologists miss this seemingly obvious change in the female anatomy due to upright walking? Many biologists refer to the human ústrus as a hidden ústrus and thereby subtly sabotage their efforts to understand how the phenomenon evolved from the blatant ústrus of the chimpanzee. To understand how that happens consider the Sapir-Whorf hypothesis as explained by one of its originators.

    Benjamin Lee Whorf, with a degree in chemical engineering from MIT, worked as a fire prevention inspector for the Hartford Fire Insurance Company. He also studied linguistics and pursued the study of language as an avocation. Combining those two disciplines enabled him to see how the way we use language can sometimes mislead our thinking. He wrote:

    "My analysis was directed toward purely physical conditions, such as defective wiring, presence of lack of air spaces between metal flues and woodwork, etc., and the results were presented in these terms. ... But in due course it became evident that not only a physical situation qua physics, but the meaning of that situation to people, was sometimes a factor, through the behavior of people, in the start of a fire. And this factor of meaning was clearest when it was a LINGUISTIC MEANING [Whorf's emphasis], residing in the name or the linguistic description commonly applied to this situation. Thus, around a storage of what are called 'gasoline drums,' behavior will tend to a certain type, that is, great care will be exercised; while around a storage of what are called 'empty gasoline drums,' it will tend to be different -- careless, with little repression of smoking or of tossing cigarette stubs about. Yet the 'empty' drums are perhaps the more dangerous, since they contain explosive vapor. Physically, the situation is hazardous, but the linguistic analysis according to regular analogy must employ the word 'empty,' which inevitably suggests a lack of hazard. The word 'empty' is used in two linguistic patterns: (1) as a virtual synonym for 'null and void, negative, inert,' (2) applied in analysis of physical situations without regard to, e.g., vapor, liquid vestiges, or stray rubbish, in the container." (Whorf, 1956, p. 135)

    In studying the cause of a fire which had started under the conditions just described, Whorf concluded that it was thinking of the "empty" gasoline drums as "empty" in the meaning described in the first definition (1) above, that is as "inert," which led to a fire he investigated. His papers and lectures featured many other examples from his insurance work to support his belief that language shapes understanding.

    That analysis, applied to the use of "hidden" rather than "non-visible" to the human ústrus, tells us how our theorizing can go wrong. If we refer to a hidden ústrus, we subtly assume that something did the hiding and we spend our effort in trying to figure out why Nature needed to hide it. On the other hand if we refer to a non-visible ústrus, then we have no connotations subtly guiding our reasoning and we remain free to discern what actually happened.

2. The Longest Penis:

    "Among the various ape species and man, which has by far the biggest penis, and what for?"

    Of all the great apes, Homo sapiens has, at five inches on average, the longest erect penis. We find the next longest in the chimpanzee Ė three inches. The gorillaís penis, when fully erect, extends about one inch and a quarter. If the biggest ape can get by with a Jenny jabber the size of your little finger, then why and how did natural selection give Homo sapiens the biggest and longest pregnifier of all?

    Here, again, we have a side effect of the evolution of upright walking in an ape. In this case changes in the hominid female drive a change in the hominid male. We begin here by asking about the average depth of the femaleís vagina and hypothesize that changes in the hips would have promoted lengthening of the vagina.

    As the hip bone and the pubic bone evolved for upright walking, the vagina had to lengthen. That engages natural selection into promoting a longer penis, because the man who gets his stuff in farthest has the greatest chance of instigating a pregnancy. Recall that if a certain phenomenon produces more pregnancies, then natural selection will favor that phenomenon so long as it has a genetic basis.

    The vagina consists of a muscular tube that connects the vulva to the cervix, at the entrance to the uterus. In a human female standing upright the vagina runs upward and backward, making about a forty-five degree angle with the uterus. Because of that arrangement the vagina is longer in the posterior (rear) part than it is in the anterior (front) part: on average the anterior length ranges from 2-1/2 to 3 inches and the posterior length averages about 3-1/2 inches. During sexual arousal the vagina lengthens as the uterus rises, achieving an average length of roughly 4 inches. Under pressure it can lengthen even further, so males engaged in a semen race gain an advantage from deep penetration with an elongated penis. We must note that sperm can survive in a hominid vagina for as many as nine days, so length rather than timing more likely determines who instigates a pregnancy.

    We also have the fact that the human penis has a greater surface area than do the penes of other apes. That implies more nerves and, thus, more intense arousal from direct stimulation. This might have made for a tighter emotional bond with a woman, thereby giving her offspring more support and promoting jealousy and a need for privacy. We thus have another factor promoting the selection for men with longer penes: those menís offspring, having more dedicated fathers, have an enhanced probability of surviving to reproduce themselves.

3. Men Bigger Than Women:

    "Why should men be bigger than women?"

    We find human men standing about eight percent taller and weighing twenty percent heavier than human women on average. Since humans evolved from a creature that resembles a chimpanzee, we can assume reasonably that protohumans displayed the same sexual dimorphism that chimpanzees do.

    We see a small size difference in chimpanzees, which have an egalitarian family structure, and the greatest size difference in gorillas, in which the males control harems of females. The biggest gorilla gets the harem, so natural selection promotes growth of the male, but not of the female, who might otherwise not submit to the maleís intimidation. Note that we cannot apply this reasoning to the fact that civilized humans oppress women and thereby deduce from that fact a gorilla-like evolution. We must look to Paleolithic people to answer this question.

    In the wild a human male can dominate one human female, but not an entire harem. We might think of the mating behavior of the Spartans, which resembled forcible rape, though the Spartan male didnít always get the woman he had chosen: Spartan women knew how to fight and, thus, had a very effective way of saying No! to men they didnít want.

    But we have made an assumption that men gained their size as a result of a mating style that involved dominating women. That assumption seems unlikely to explain the phenomenon, given that the Great Apes to which we are most closely related do not display such a mating style.

    Actually, the size disparity between the sexes in humans may have had little to nothing to do with sex. It may have come about due to the way in which our ancestors defended themselves from predators or other hominids. Again we seek to discern a phenomenon that will shift the genetic make-up of the group.

    In the forest, when danger appears, chimpanzees scramble into the trees. Once up in the trees, the chimpanzees are relatively safe, because even tree-climbing predators, such as leopards, stand at a disadvantage to the branch-grasping apes. However, in open woodland or, worse, on open savannah, reaching the safety of the trees gets a little more complicated. Upon spotting danger, our ancestors had to run for their lives, striving to reach the nearest trees before their attackers reached them. All too often they didnít make it and someone had to fight off the attackers to let the others escape. In any fight not involving projectile weapons, size counts.

    That fact brings us back to Diamondís question with an addendum. In addition to men being bigger than women, all humans are substantially larger and heavier than chimpanzees, our closest relatives and, presumably close in size to our common ancestor. Our whole genus has grown, what we really need to ask is why human males grew more than did human females. What was it about life in the woods and savannahs of Ancient East Africa that made the average man grow more than the average woman?

    A fairly straightforward answer comes to us when we note that a troop that loses too many females will not likely survive over the long term, but that a troop could lose most of its males and still recover and survive. Thus we infer that the hominid troops that survived and evolved into us were those in which the males did the fighting, protecting the females and the young as they escaped from danger. In those troops natural selection promoted the growth of the male over that of the female, although the need to run farther and faster also promoted the growth of the overall species. Thus we have men typically bigger than women.

4. Testes Smaller Than A Chimpanzeeís:

    "How can men get away with having much smaller testes than do chimpanzees?"

    But still slightly larger than an orangutanís or a gorillaís. Since we evolved from an ancestor that we had in common with the chimpanzees and which resembled a modern chimpanzee, we must assume that our remotest ancestors had large testes and we must then ask why the human testes shrank over time.

    Diamond notes that chimpanzees perform group sex, so that the chimpanzee who squirts enough gene juice to flush his rivalsí contributions out of the femaleís vagina stands a better chance of instigating a pregnancy. Natural selection will work on that difference to promote those males who produce the most semen.

    Among humans, who donít "gang bang", that factor would have gone away and the testes would have evolved to good enough. We can make that claim on the basis of simple "economics". If the guy with the cherries has as much chance of instigating a pregnancy as does the guy hauling apricots, then natural selection will promote the cherries and demote the apricots, simply because the apricots take more of the bodyís resources than cherries do.

5. Copulation In Private:

    "Why do humans copulate in private, while all other social animals do it in public?"

    Other social animals, especially apes, copulate publically. We assume that our common ancestor with the chimpanzees did the same. Thus, we need to discern a phenomenon that takes people with a certain shyness about copulating in public and makes them winners in the genetic sweepstakes. This change likely occurred during the Ardipithecine Age, when upright walking first evolved, so we canít attribute it to the male wanting to protect his investment (as some biologists put it): proto-humans did not make any connection between having sex and having babies. Again, this feature of human behavior must have a genetic basis. It must have emerged from some pre-existing instinct. And it cannot reflect any kind of teleological process; in other words, the feature cannot reflect the pursuit of a goal, but instead must reflect the relative fit between the organisms and their environment, even when that environment consists of each other. Here we get into sexual selection, evolution occurring due to individuals selecting their mates.

    The fact that we copulate face-to-face doesnít explain this desire for privacy: bonobos do the same and they still do it in public. But some chimpanzees do copulate in private: low status males do so in order to avoid the hostile attention of their troopís alpha males. We know this about chimpanzees because of discoveries made after Diamond wrote his book.

    At the beginning of the Twenty-First Century, Emily Wroblewski conducted a DNA analysis of feces recovered from chimpanzees at the Gombe research facility in Tanzania. She took as her goal the determination of the paternity of the chimps in the area and she succeeded. By correlating DNA patterns and organizing the results into family trees, she and her colleagues discovered, as they expected, that high-ranking males father many of the chimps in a group. But they also found something less expected: some low-ranking males also fathered a significant fraction of the groupís offspring. To succeed in obtaining enough sex to produce that result they employ a strategy of engaging a female, usually a younger and/or less desirable one, in a consortship. The male invests his effort in a period of exclusivity, in which period he and his female remain together as a pair, grooming each other, traveling together, and mating.

    Here we see a perfect example of scientific reasoning. Some twenty years before Wroblewski did her work, Jane Goodall made a prediction based on her observational data: she said, "The male who successfully initiates and maintains a consortship with a fertile female probably has a better chance of fathering her child than he would in the group situation, even if he were alpha." Though she deduced that proposition from her observations of the chimps, she had no data indicating the actual paternity of any offspring, so she had no scientific proof of her hypothesis. Wroblewskiís work provided the scientifically required evidence, thereby proving Goodallís hypothesis and verifying it. The fact that the evidence might also have falsified Goodallís hypothesis makes the study a properly scientific one.

    So we assume that our remotest ancestors also practiced both the gang bang style of group sex and the consort style of private sex, just as chimpanzees do today. As the overt signs of ústrus faded over the centuries, the connection between sex and pregnancy became more haphazard. The chimpanzee "jump Ďní pump" no longer led to the instigation of as many pregnancies as it would have if the "do it" signal had not faded out. If no one could tell when a female was hot for sex, they would have group sex at random times. That effort would have resulted in proportionately fewer offspring.

    On the other hand, consort sex would have resulted in proportionately more offspring. Mutual grooming became a prelude to sex, what we now call foreplay. Women are notoriously distractable, while men tend to be more single-minded. It takes time to foment lust (as the Puritans so deliciously put it, though they spelled the verb as foament). In other words, the men would have had to learn how to romance women. And the greatest success came to those who gained an intimate knowledge of one particular woman, of her likes and dislikes, that enabled them to gain The Big Favor more or less on demand. Such delicate negotiations do not go well with an audience, much less a hooting Greek chorus or an enraged, screaming alpha male. Couples who sought privacy from such disturbances for their special moments thus reproduced more than couples who didnít.

    Finally, we have one more phenomenon worthy of comment. In humans the sex act typically takes about four minutes; in chimpanzees it takes a few seconds. Why would the time devoted to coitus lengthen? Males who get interrupted donít reproduce as effectively as those who donít get interrupted, so those who took their females into the bushes for their lovemaking sessions seemed to have an advantage in that respect. That fact seems to give us a genetic basis, but why would the pressure not promote quicker copulation rather than private copulation? In this case we can answer that prolonged copulation is an effect, not a cause of copulation in private.


    Thus we can construct a reasonable description of how natural selection gave an upright walking ape the five characteristics that Jared Diamond mentioned in his challenge. But a reasonable description does not necessarily equate with scientific knowledge: the scientific method obliges us to present evidence to support our reasonable descriptions and in this case we appear to have little to none. In this case the facts that stymie us consist of the fact that soft tissue very rarely appears in the fossil record (almost always as imprints) and the fact that behavior leaves no effect on the fossil record.


Diamond, Jared; "The Third Chimpanzee", HarperCollins Publishers Inc., New York, 1992. ISBN 0-06-018307-1.

Margulis, L & Sagan, D. (1991) Mystery Dance, On the Evolution of Human Sexuality: Summit Books, New York.


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