Our mate selection criteria, explored in Chapter Five, are relevant to human racial variation, as will be discussed in Chapter Six. Humans native to different parts of the globe vary conspicuously in external appearance, as do gorillas, orangutans, and most other animal species occupying a sufficiently extensive geographic range. Our visible geographic variability has often been taken as a pretext for exercising a human hallmark to be discussed in Chapter Sixteen: genocidal killings. Some of the geographic variation in our appearance surely reflects natural selection moulding us to local climate, just as weasels in areas with winter snow develop white fur in winter for better camouflage and survival. But I shall argue in Chapter Six, as Darwin maintained, that our visible geographic variability arose mainly through sexual selection, as a result of those mate-choice procedures of ours discussed in Chapter Five.
Chapter Seven brings the discussion of our life-cycle to an end, by asking why our lives have to come to an end. Aging is another feature of our life-cycle so familiar that we take it for granted: of course we shall all grow old and eventually die. So will all individuals of all animal species, but different species age at very different rates. Among animals we are relatively long-lived and became even more so around the time that Cro-Magnons replaced Neanderthals. Our longevity has been important for our humanity, by permitting effective transmission of learned skills between generations. But even humans grow old. Why is aging inevitable, given our extensive capacity for biological self-repair?
Here, more than in any other chapter, the importance of thinking in terms of evolutionary trade-offs becomes clear. If measured by the ability to leave increased numbers of offspring, it just would not pay us to make the increased investment in self-repair mechanisms required to live longer. We shall see that the trade-off concept also illuminates the puzzle of menopause: a shutdown of child-bearing, paradoxically programmed by natural selection so that women can leave more surviving children.
THREE
THE EVOLUTION OF HUMAN SEXUALITY
Human sexuality seems normal to us but is bizarre by the standards of other animals. Our bizarre sex lives were as crucial to our rise to human status as were our large brains.
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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 far 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 trouble 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.
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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-fold – mechanical 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 hunter–gatherer 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, h
ad better have some confidence in his paternity – for 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.
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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?
MALES, AS FEMALES SEE THEM
Humans and great apes differ with respect to the relative body size of males and females, penis length, and testis size. The main circles represent the body size of the male of each species, relative to that of the female of the same species. Female body size is arbitrarily shown as the same for all species at upper right. Thus, chimps of both sexes weigh about the same; men are slightly larger than women; but male orangutans and gorillas are much bigger than females. The arrows on the male symbols are proportional to the length of the erect penis, while the twin circles represent testis weight relative to that of the body. Men have the longest penis, chimps the largest testes, and orangutans and gorillas the shortest penis and smallest testes.
It turns out that, among polygynous mammals, average harem size increases with the ratio of the male’s body size to the female’s body size. That is, the biggest harems are typical of species in which males are much larger than females. For example, males and females are the same size in gibbons, which are monogamous; male gorillas, with a typical harem of three to six females, weigh nearly double the weight of each female; but the average harem is forty-eight wives for the southern elephant seal, whose 3-ton male dwarfs his 700-pound wives. The explanation is that, in a monogamous species, every male can win a female, but in a very polygynous species most males languish without any mate, because a few dominant males have succeeded in rounding up all the females into their harems. Hence, the bigger the harem, the fiercer is the competition among males and the more important it is for a male to be big, since the bigger male usually wins the fights. We humans, with our slightly bigger males and slight polygyny, fit this pattern. (However, at some point in human evolution, male intelligence and personality came to count for more than size: male basketball players and sumo wrestlers don’t tend to have more wives than male jockeys or coxswains.)
FEMALES, AS MALES SEE THEM
Human females are unique in their breasts, which are considerably larger than those of apes even before the first pregnancy. The main circles represent female body size relative to male body size of the same species.
Because competition for mates is fiercer in polygynous than in monogamous species, the polygynous species also tend to have more marked differences between males and females in other respects besides body size. These differences are the secondary sexual characteristics that play a role in attracting mates. For instance, males and females of the monogamous gibbons look identical at a distance, while male gorillas (befitting their polygyny) are easily recognized by their crested heads and silver-haired backs. Here too, our anatomy reflects our mild polygyny. The external differences between men and women are not nearly as marked as sex-related differences in gorillas or orangutans, but the zoologist from outer space could probably still distinguish men and women by the body and facial hair of men, men’s unusually large penis, and the large breasts of women even before first pregnancy (in this we are unique among primates).
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Proceeding now to the genitalia themselves, the combined weight of the testes in the average man is about 1½ ounces. This may boost the macho man’s ego when he reflects on the slightly lower testis weight in a 450-pound male gorilla. But wait – our testes are dwarfed by the 4-ounce testes of a 100-pound male chimpanzee. Why is the gorilla so economical, and the chimp so well-endowed, compared to us?
The Theory of Testis Size is one of the triumphs of modern physical anthropology. By weighing the testes of thirty-three primate species, British scientists identified two trends: species that copulate more often need bigger testes; and promiscuous species in which several males routinely copulate in quick sequence with one female need especially big testes (because the male that injects the most semen has the best chance of being the one to fertilize the egg). When fertilization is a competitive lottery, large testes enable a male to enter more sperm-tickets in the lottery.
Here is how these considerations account for the differences in testis size among the great apes and humans. A female gorilla does not resume sexual activity until three or four years after giving birth, and she is receptive for only a couple of days a month until she becomes pregnant again. So even the successful male gorilla with a harem of several females experiences sex as a rare treat – if he is lucky, a few times a year. His relatively tiny testes are quite adequate for those modest demands. The sex life of a male orangutan may be somewhat more demanding, but not much. However, each male chimp in a promiscuous troop of many females lives in sexual nirvana, with nearly daily opportunities to copulate for a common chimp and several daily copulations for the average pygmy chimp. That, plus his need to outdo other male chimps in semen output if he is to fertilize the promiscuous female, explains his need for gigantic testes.
We humans make do with medium-sized testes because the average man copulates more often than gorillas or orangutans but less often than chimps. In addition, the typical woman in a typical menstrual cycle does not force several men into sperm competition to fertilize her.
Thus, primate testis design well illustrates the principles of trade-offs and evolutionary cost/benefit analyses explained on page 52. Each species has testes big enough to do their job, but not unnecessarily larger ones. Bigger testes would just entail more costs without proportional benefits, by diverting space and energy from other tissues and increasing the risk of testicular cancer.
From this triumph of scientific explanation we descend to a glaring failure: the inability of twentieth-century science to formulate an adequate Theory of Penis Length. The length of the erect penis averages 1¼ inches in a gorilla, 1½ inches in an orangutan, 3 inches in a chimp, and 5 inches in a man. Visual conspicuousness varies in the same sequence: a gorilla’s penis is inconspicuous even when erect because of its black colour, while the chimp’s pink erect penis stands out against the bare white skin behind it. The flaccid penis is not even visible in apes. Why does the human male need his relatively enormous, attention-getting penis, which is larger than that of any other primate? Since the male ape successfully propagates his kind with much less, does not the human penis represent largely wasted protoplasm that would be more valuable if devoted, say, to cerebral cortex or improved fingers?