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Human sexuality seems normal to us but is bizarre by the stmdards of other animals. Our bizarre sex lives were as crucial to cur rise to human status as were our large brains.

No week passes without publication of yet another book about sex. Our desire to read about sex is surpassed only by our desire to practise it. You might suppose that the basic facts of human sexuality must be familiar to lay people and understood by scientists. Just test your own grasp of sex by trying to answer these five easy questions:

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

Why should men be bigger than women?

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

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

Why don't women resemble almost all other female mammals in having easily recognized days of fertility, with sexual receptivity confined to those days?

If your answer to the first question was 'the gorilla', put on a dunce's cap; the correct answer is man. If you gave any intelligent answers to the next four questions, publish them; scientists are still debating rival theories.

These five questions illustrate how hard it is to explain the most obvious facts of our sexual anatomy and physiology. Part of the problem is our hang-ups about sex: scientists did not even begin to study the subject seriously until recently, and they still have [rouble being objective.

Another difficulty is that scientists cannot do controlled experiments on the sexual practices of us humans, as they can on our cholesterol intake or tooth-brushing habits. Finally, sex organs do not exist in isolation: they are adapted to their owners' social habits and life-cycle, which are in turn adapted to food-gathering habits. In our own case that means, among other things, that evolution of human sex organs has been intertwined with that of human tool use, large brains, and child-rearing practices. Thus, our progress from being just another species of big mammal to being uniquely human depended on the remodelling not only of our pelvises and skulls, but also of our sexuality. Given knowledge of how an animal feeds, a biologist can often predict that animal's mating system and genital anatomy. If we want to understand how human sexuality came to be the way it is, we have to begin by understanding the evolution of our diet and our society. From the vegetarian diet of our ape ancestors, we diverged within the last several million years to become social carnivores as well as vegetarians. Yet our teeth and claws remained those of apes, not of tigers. Our hunting prowess depended instead on large brains: by using tools and operating in coordinated groups, our ancestors were able to hunt successfully despite their deficient anatomical equipment, and they regularly shared food with each other. Our ability to gather roots and berries also came to depend on tools and thus to require large brains.

As a result, human children took years to acquire the information and the practice needed to be an efficient hunter-gatherer, just as they still take years to learn how to be a farmer or computer programmer today. During those many years after weaning, our children are still too dumb and helpless to acquire their own food; they depend entirely on their parents to bring food to them. These habits are so natural to us that we forget that baby apes gather food as soon as they are weaned.

The reasons why human infants are totally incompetent at food-gathering are actually two-foldmechanical and mental. Firstly, making and wielding the tools used to obtain food requires fine finger coordination that children take years to develop. Just as my three-year-old sons still cannot tie their own shoelaces, three-year-old hunter-gatherer children cannot sharpen a stone axe, weave a net, or build a dugout canoe. Secondly, we depend on much more brainpower than do other animals in acquiring food, because we have a much more varied diet and more varied and complicated food-gathering techniques. For instance, New Guineans with whom I work typically have separate names for about a thousand different species of plants and animals living in the vicinity. For each of those species they know something about its distribution and life history, how to recognize it, whether it is edible or otherwise useful, and how best to capture or harvest it. All this information takes years to acquire.

Weaned human infants cannot support themselves because they lack these mechanical and mental skills. They need adults to teach them, and they also need adults to feed them for the decade or two that they are being taught. As is true of so many other human hallmarks, these problems of ours have animal precedents. In lions and many other species, the young must be trained to hunt by their parents. Chimpanzees too have a varied diet, employ varied foraging techniques, and assist their young in obtaining food, while common (but not pygmy) chimps make some use of tools. Our distinction is not absolute but one of degree: for us the necessary skills and hence the parental burden are far greater than for lions or chimpanzees.

The resulting parental burden makes care by the father as well as the mother important for a child's survival. Orangutan fathers provide their offspring with nothing beyond their initial donation of semen; gorilla, chimpanzee, and gibbon fathers go beyond that to offer protection; but huntergatherer human fathers provide some food and much teaching as well. Hence human food-gathering habits required a social system in which a male retained his relationship with a female after fertilizing her, in order to assist in rearing the resulting child. Otherwise, the child would be less likely to survive, and the father less likely to pass on his genes. The orangutan system, in which the father departs after copulation, would not work for us. The chimpanzee system, in which several adult males are likely to copulate with the same oestrus female, also would not work for us. The result of that system is that a chimpanzee father has no idea which infants in the troop he has sired. For the chimp father that is no loss, as his exertions on behalf of troop infants are modest. The human father, however, who will contribute significantly to the care of what he thinks is his child, had better have some confidence in his paternityfor example, through having been the exclusive sexual partner of the child's mother. Otherwise, his child-care contribution may help pass on some other man's genes.

Confidence in paternity would be no problem if humans, like gibbons, were scattered over the landscape as separate couples, so that each female would only rarely encounter a male other than her consort. But there are compelling reasons why almost all human populations have consisted of groups of adults, despite the paranoia about paternity that this causes. Among the reasons: much human hunting and gathering involves cooperative group efforts among men, women, or both; much of our wild food occurs in scattered but concentrated patches, able to sustain many people; and groups offer better protection against predators and aggressors, especially against other humans.

In short, the social system we evolved to accommodate our un-apelike food habits seems utterly normal to us, but is bizarre by ape standards and is virtually unique among mammals. Adult orangutans are solitary; adult gibbons live as separate monogamous male/female pairs; gorillas live in polygamous harems, each consisting of several adult females and usually one dominant adult male; common chimpanzees live in fairly promiscuous communities consisting of scattered females plus a group of males; and pygmy chimpanzees form even more promiscuous communities of both sexes. Our societies, like our food habits, resemble those of lions and wolves: we live in bands containing many adult males and many adult females. Furthermore, we diverge from even lions and wolves in how those societies are organized: our males and females are paired off with each other. In contrast, any male lion within a lion pride can and regularly does mate with any of the pride's lionesses, making paternity unidentifiable. Our peculiar societies instead have their closest parallels in colonies of seabirds, like gulls and penguins, which also consist of male/female pairs.

At least officially, human pairing is more or less monogamous in most modern political states, but is 'mildly polygynous' among most surviving hunter-gatherer bands, which are better models for how mankind lived over the last million years. (This description omits consideration of extramarital sex, through which we become effectively more polygamous and whose scientifically fascinating aspects I shall discuss in Chapter Four.) By 'mildly polygynous', I mean that most hunter-gatherer men can support only a single family, but a few powerful men have several wives. Polygyny on the scale of elephant seals, among which powerful males have dozens of wives, is impossible for hunter-gatherer men, because they differ from elephant seals in having to provide child care. The big harems for which some human potentates are famous didn't become possible until the rise of agriculture and centralized government let a few princes tax everyone else in order to feed the royal harem's babies.

Now let's see how this social organization shapes the bodies of men and women. Take first the fact that adult men are slightly bigger than similarly aged women (about eight per cent taller and twenty per cent heavier, on the average). A zoologist from outer space would take one look at my 5-foot 8-inch wife next to me (5 foot 10 inches), and would instantly guess that we belonged to a mildly polygynous species. How, you may ask, can one possibly guess mating practices from relative body size?

PART TWO AN ANIMAL WITH A STRANGE LIFE-CYCLE | The rise and fall of the third chimpanzee | MALES, AS FEMALES SEE THEM