Chapter Five turns to another distinctive human life-cycle trait: how we select our sex partners, marital or otherwise. That problem scarcely arises for baboon troops, in which there is little selection: any male tries to mate with each female as she comes into heat. While common chimpanzees practise some selection of their sex partners, they are still much less selective and much more promiscuously baboon-like than are humans. Mate selection is a decision of major consequence in the human life-cycle, because married couples share parental responsibilities as well as sexual involvement. Precisely because care of human children demands such heavy and prolonged parental investment, we have to select our co-investor much more carefully than does a baboon. Nevertheless, Chapter Five will show that we can find animal precedents for our procedure in choosing sex partners, by going beyond primates to rats and birds. 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 tradeoffs 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 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-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.