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Human Sexual Evolution

By Goran Burenhult

Our gender roles and sexual behaviour are largely a result of an evolutionary process that extends millions of years back in time. But this fact appears too rarely in the debate about gender equality and sexuality.

Man is, by definition, a mammal (Mammalia), class vertebrates, the order primates. Mammals are characterised by reproducing through sex between a male and a female – a man and a woman. With an outer mating organ – a penis – the sex act leads to an internal fertilisation of females and, in higher mammals, the entire foetal development happens inside the mother's uterus. The offspring are born alive and, in the early days after birth, all mammals feed their offspring entirely on the milk that the mother produces in their mammary glands. The lactation period varies between different mammalian species and is related to gestational age, timing of sexual maturation and longevity. Man is no exception. From this biological fact, our gender roles are also genetically inherited. Men and women have such completely different hormonal sets – men produce testosterone in their testes, women oestrogen in their ovaries. This means that we have completely different inherited behaviour patterns. It all began in earnest when, 6-7 million years ago, we gradually adapted ourselves to upright walking – we became bipedal. There are several theories about the reasons for this but, whatever the underlying causes of human bipedalism, it came to mean an anatomical and sexual revolution. The transition from tree-climbing primate to human with upright walking on two legs meant large anatomical changes, which are reflected in the development of the form of bones and muscles. As the head was balanced in the middle of the vertebral column, for example, the powerful neck muscles were no longer needed, as the great apes had to hold the head in place, and therefore they decreased in size. In return, we developed powerful skeletal muscles to support the spine and allow for the establishment of upright walking, especially the back-, abdominal- and chest muscles, along with the hips (the gluteus muscle) and the calf muscles. With the disappearance of the coat, these new muscles became fully visible under the bare skin. In this man radiated strength and security, thus signalling ‘good genes’ in the choice of partners, and the process was enhanced by sexual selection. The favoured successful reproduction was also sexually attractive. Even the woman's rounded shape was accentuated by the new muscle, not least in the hips, which also came to be a fat reserve, ensuring amongst other things the production of breast milk. The ‘hourglass shape’ of the woman of childbearing age is controlled by sex hormones (oestrogen) and shows a clear difference between male and female fat distribution, i.e., the relationship between the hip and waist – the so-called ‘waist-to-hip ratio’ (WHR). Then came the female forms that were sexually attractive because they radiated health, fitness and successful reproduction. Many studies from different cultural areas, including the U.S., China and Indonesia, shows that men generally find women with marked hips and slim waist most attractive. Bipedalism also came to completely change genitalia placement. The primate female’s greatly swollen and reddened buttocks during oestrus periods is powerfully exposed at the back, but with bipedalism the vagina of the woman came to be hidden between his legs. For the man, it was just the opposite. Men have, compared to primates, exceptionally large penises – far greater than needed to fertilise a woman. The cause of the man's change is unclear, but according to many evolutionary scientists it has resulted from sexual selection. During the evolutionary process towards bipedalism (emphasised by, among others, Jared Diamond in the study Why Is Sex Fun?The Evolution of Human Sexuality [1998], and Bo Gräslund in The First Steps. Primordial Man and His World [2001]) the primordial human with a longer penis had a reproductive advantage. The woman's vagina came to be shifted in position to a ninety degree angle to the cervix, but during the period when it was neither directed backward or forward, the size of the primordial human’s penis could have been decisive. The male primate’s very small penis is, during four-legged walking, hidden between the hind legs, covered with fur. When man began to walk on two legs, suddenly the penis became fully exposed, even in the non-erect state, and it became an important visual signal of individual gender. It is often said that "size does not matter", and from the reproductive point of view, as well as to satisfy a woman, this is absolutely right. But from a psychological point of view, it seems that size may still have played a role. In most societies in the world, there is a more or less pronounced fascination with the size of the man's penis and the list of methods to enlarge the organ is very long. In ancient art, there is evidence of how people over millennia have been fascinated with big penises. Although it is of course always women who preferred smaller male genitalia, apparently primordial women made a contrary choice, as evidenced by the fact that men – through the sexual selection – came to look as they do. On the part of primordial women, there was another change that soon would become a part of her sexual signalling that was also to be reinforced by sexual selection: she developed breasts. A visible chest is not needed for milk production. Chimpanzee females have no breasts, but still produce large quantities of high-quality milk for their young in the mammary glands. Sexual differences were visually quite crucial, which also applied to sexuality, which had previously been based on smell. But equally crucial for the Incarnation was that the woman lost the oestrus (i.e., the restricted periods when mammalian females wish to mate) and developed concealed ovulation, which fundamentally changed our sexual patterns.

In conjunction with bipedalism, primitive human groups would later develop other sexual expressions and changing social systems. It is here that the woman’s concealed ovulation comes into the picture – for some reason she was out of oestrus and constantly sexually available. This came to have a profound impact on the transition from a largely polygamous lifestyle to a largely monogamous one, with a family of mother, father and children as the core of the social system – something that applies to almost all people on Earth. How many think that it could have obviously been evolutionarily advantageous that primordial man did not know when his woman could be fertilised thus staying with her permanently in order to ensure the offspring were really his own? Already in 1979, Richard Alexander and Kathleen Noonan claimed in the essay "Concealment of Ovulation, Parental Care, and Human Social Evolution" that the woman's constant sexual availability sought to get the man to remain in the home. This favoured the survival of children who received a more secure upbringing when both parents were constantly on hand. Human offspring, unlike most mammals, are very vulnerable during the first few years of life, the mother's mobility is limited, and it is necessary that the man helps to ensure security. This is related to the exceptional growth of the human brain, which would be a million years after we became bipedal. As a consequence of this, our ancestors later in our evolution lived in, if not reproductively, then at least socially monogamous relationships or perhaps small polygamous groups. This created the basis for a stricter division of labour between the sexes, and may have contributed to the reduction of conflict within the group. Having sex at any time – but not with just anyone – came to be a typical behaviour of the genus Homo. The lost oestrus is thus at the core of the development of human sex life, but this is also one of our biggest evolutionary enigmas. What evolutionary forces lay behind such a drastic change in our sexuality? The classic idea was that concealed ovulation evolved to force primordial man to stay at home to ensure that the offspring were his own. This hypothesis has sometimes been called the ‘father-at-home’ theory. The risk that a woman, with concealed ovulation, could amuse herself with other men, who also wanted to spread their genes, would thus eventually lead to monogamy, or at least to closer relationships with paternal care, than would be the case if the oestrus had remained. The fact that virtually every society in recorded history has lived in monogamous relationships does not contradict this. In the societies that allow polygamy, it is practiced as a rule solely by the leadership in the community. Another, directly opposite hypothesis, the ‘many-fathers theory’ explains the hidden ovulation as an evolutionary defence against infanticide. In many mammals, it is common for males to kill other males’ cubs in order to gain access to females, who then go into oestrus when the kid no longer suckles. Cats, for example, behave so, and it also occurs among primates, such as gorillas and chimpanzees. As many as a third of all deaths among young gorillas are the result of infanticide of rival males trying to take over a harem of females. Among other anthropologists, Sarah B. Hardy and physiologist Jared Diamond have pointed out (in The Woman That Never Evolved [1981], respectively Why is Sex Fun? [1997]), that the primordial women could – being constantly sexually active with concealed ovulation – have however many males without any of them having a clue as to who was the father of the child. In such a situation, the risk of infanticide was decimated because the father could accidentally kill his own offspring. Seen from a modern human perspective, this is very interesting – of all child murders occurring in the western world annually (about 650 per million families) it is almost 100 percent that are committed by stepparents and not by the biological parents; scary and thought-provoking but, from an evolutionary perspective, perhaps not surprising. But the situation is even more complicated than that. The woman's concealed ovulation is not unique among primates. Of the sixty-eight primates studied, almost half concealed ovulation – thirty-two, including orang-utans. Of the others, eighteen have a weakly pronounced rut, such as the gorilla, while the remaining eighteen have a strongly pronounced rut, such as chimpanzees and bonobo. This has implications for mating behaviours. Most primates live promiscuously, which means everyone mates with everyone, while almost all primates with concealed ovulation live monogamously. The problem is that some promiscuous species also have concealed ovulation, which means that the hidden ovulation may have a different function depending on the mating pattern that applies. Our common ancestors, before the species split, about nine million years ago, apparently had a weak or moderate oestrus and lived promiscuously. At the species split, a million years later, humans, chimpanzees and gorillas with this in their baggage went their separate ways –humans with concealed ovulation and principal monogamy, chimpanzees with clear oestrus and promiscuous behaviour, and gorillas with weakly pronounced oestrus and harem living.

About two and a half million years ago began an exceptional growth of the human brain. The evolutionary processes that were behind this are unclear, but given the major drawbacks they also entailed, they must have been very powerful ones. The big brain requires a very large energy intake from the mother. While brain volume represents only two percent of our body weight, not less than sixty percent of our food intake goes to brain function. The large head of the foetus also made birthing considerably more difficult and risky. Meanwhile, the newborn’s brain is very undeveloped, making it totally dependent on parental care for many years. This would have direct implications for human social organisation and family. The mutations that ruled brain development must therefore be the result of very powerful environmental influences. It has been suggested that the competition with living, rival groups of hominids would have favoured the group that had the greatest capacity for communication and social organisation. Our large brains are indeed a prerequisite for the emergence of, amongst other things, a language, but the brain continued to grow even after the other groups had died out. Many scholars believe that it was instead sexual selection that was the driving force. Advanced group life on the African savannah was without doubt one of the factors. With linguistic ability and superior intelligence, there also developed singing and music, art and other cultural manifestations, which probably made individuals with higher brain capacity more attractive than others. The emergence of the first manufacturing of tools at this time is an indication of this.

The gender role and gender equality debates today are more intense and more controversial than ever before, but the discussion of equality is often far from what it might really be about – something which, among others, Steven Pinker has pointed out. We are not talking here of equal pay for equal work – everyone probably agrees that the exact same work should pay exactly the same salary, regardless of whether you are male or female. Yet it is perhaps here that conflicts may arise because the woman in her role as a mother has to be away for long periods in order to bear children and breastfeed them, which men cannot do. Let us follow one woman's path through life to illustrate this problem. When a girl comes up to the age of twelve, she starts her monthly, hormonally controlled ovulation period, menstruation, often with PMS – in itself an unnatural state, as biologically women during their reproductive period should either be pregnant or nursing, just as it still is in all indigenous communities worldwide. A woman in such societies has about fifteen periods in their lifetime, while a Western woman today often has over five hundred. Randolph Nesse and George C. Williams in Evolution and Healing. The New Science of Darwinian Medicine (1995), and Boyd Eaton in The Paleolithic Prescription. A Program of Diet & Exercise and a Design for Living (1989), advance this as an explanation for the high incidence of cancer of the female reproductive tract in today's society. Breast and cervical cancers are diseases that are completely unknown in traditional societies. In adulthood, the modern woman gives birth to her first child, followed perhaps by several – a very large investment, with nine months of pregnancy, followed by a lactation period that should be at least a year for the child to build up a full immunity, but which often under societal pressure becomes significantly shorter. This despite the fact that many experts, as well as both the World Health Organization (WHO) and the World Health Association (WHA) recommend exclusive breastfeeding for six months and continued breastfeeding with complementary foods for two years or longer. Over eighty percent of Swedish children do not get even half the breastfeeding that they need. The man's investment in all this is but a few seconds of ejaculation – he cannot be of help with the pregnancy, nor with breastfeeding. He can, during this long period, theoretically be a father to hundreds of children, while the mother's large investment is in but one. In addition, tensions arise between the mother's natural desire to be with her child and the requirement for her to go to work and leave the child for many hours of the day. All this is repeated with each new child. In indigenous communities, the women begin working with food gathering or growing the day after birth – but the child is always with the mother during the first few years, on the chest or the back. Interesting in this context is that the woman alone has total control over reproduction – who she wants to be the father of her child (obviously apart from rape situations). Almost all women have at least one child during their reproductive period, while many men are deselected and never become fathers. Attractive and successful men have more sexual partners and more children than attractive women. In Sweden, women reject about 25 percent of all men. So there are many more men than are needed for the species to reproduce, and even this has an evolutionary explanation, as pointed out by, among others, Louise Barrett, Robin Dunbar, and John Lycett in Human Evolutionary Psychology (2002). The woman's range of choice was crucial.

Life on the savannah with Stone Age technology was extremely dangerous, especially since big-game hunting became a part of the diet, so it must have taken a heavy toll on primordial men. A population’s survival depends on women's security, while young, strong men can be used as cannon fodder, partly because men are generally less important than women when it comes to producing offspring. This is also an important reason for our differing gender roles; a man can be a father to thousands of children, while a woman only a few tens (the record number of children born to a woman that is usually invoked is 69: 16 pairs of twins, seven sets of triplets and three sets of quadruplets, while Genghis Khan is estimated to have been the father to between six and eight thousand children). A similar relationship appears to apply to homosexual relationships: according to a study in the United States in 1978, before the AIDS epidemic, 75 percent of all white gay men had more than 100 sexual partners, 15 percent had between 500 and 1000, and a full 28 percent had had more than 1,000. Later studies done in the United States have shown that one in twenty gay men have had more than a thousand sexual partners, while not more than one in a hundred lesbians had more than a hundred. The evolutionary advantages of these different sexual behaviours are several: a woman is more careful in her choice of sexual partner because her investment in sexual intercourse is so much bigger in pregnancy and lactation. A human group’s survival is not dependent on the fact that women have many sexual partners, while the converse may apply to men, for example, in a situation where the number of sexually active men has greatly decreased as a result of war. A human group’s survival and competitive strength is also dependent on as many women as possible getting pregnant, which is easier if men generally are less scrupulous than women in the choice of sexual partner. The human group’s survival is, further, dependent on men being sacrificed in conflicts with other groups, or as a consequence of risky behaviours, such as searching for new hunting or grazing land, or hunting. It also requires that women be protected during the long periods of pregnancy and lactation, the length of which in turn is a consequence of man's evolutionary advantages of such a high-level of brainpower. The differences in sexual behaviour, which have emerged as a consequence of the evolutionary advantages that such differences entailed, may seem unimportant today and are considered by many in no way so deep that they cannot be modified or suppressed in many cultures. However, it would be wishful thinking to believe that they can be completely overcome. Maybe we should still be happy about this, given how uncertain the future of mankind on Earth still is.

NR 7 2012

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