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Chapter two traced our evolutionary history through the appearance of humans with fully modern anatomy and behavioural capabilities, but that chapter does not prepare us to go straight on to consider in more detail the development of human cultural hallmarks, such as language and art. That is because Chapter Two took up only the evidence of bones and tools. Yes, our evolution of large brains and upright posture was prerequisite to language and art, but that was not enough by itself. Human bones alone do not guarantee humanity. Instead, our rise to humanity also required drastic changes in our life-cycle, which will be the subject of Part Two of this book. For any species one can describe what biologists term its 'life-cycle'. That means traits such as the number of offspring produced per litter or birth; the interval between births; the parental care (if any) that offspring receive from the mother or father; social relations between adult individuals; how a male and female select each other to mate with; frequency of sexual relations; and longevity. We take the forms of these traits as they exist in humans for granted, as the norm, but our life-cycle is actually bizarre by animal standards. All the traits that I have just mentioned vary greatly between species, and we are extreme in most respects. To mention only some obvious examples, most animals produce litters much larger than one baby at a time, most animal fathers provide no parental care, and few other animal species live even a small fraction of three-score years and ten.

Of these exceptional features of ours, some are shared by apes, suggesting that we merely retained traits already acquired by our ape-like ancestors. For instance, apes too usually give birth to one baby at a time, have births spaced several years apart, and live for several decades. None of these things is true of the other animals most familiar (but less closely related) to us, such as cats, dogs, songbirds, and goldfish.

In others of these respects, we are greatly different even from apes. Here are some obvious differences whose functions are well understood. Human babies continue to have all food brought to them by their parents even after weaning, whereas weaned apes gather their own food. Most human fathers as well as mothers, but only chimpanzee mothers, are closely involved in caring for their young. Like seagulls but unlike apes or most other mammals, we live in dense breeding colonies of nominally monogamous couples, some of whom also pursue extramarital sex. All these traits are as essential as large brain-cases for the survival and education of human offspring. That is because our elaborate, tool-dependent methods of obtaining food make weaned human infants incompetent to feed themselves. They first require a long period of food-provisioning, training, and protectionan investment much more taxing than that facing the ape mother. Hence human fathers who want their offspring to survive to maturity have generally assisted their mate with more than just sperm, the sole parental input of an orangutan father. Our life-cycle also differs from that of wild apes in more subtle respects whose functioning is nevertheless still discernible. Many of us live longer than most wild apes: even hunter-gatherer tribes include some elderly individuals who are enormously important as repositories of experience. Men's testes are much larger than those of gorillas but smaller than those of chimps, for reasons that will become apparent in Chapter Three. We regard human female menopause as inevitable, and Chapter Seven will show why it makes good sense for humans, but it is almost unprecedented among other animals. The closest mammalian parallel is among some tiny mouselike marsupials in Australia, and it is their males, not their females, that undergo menopause. Our longevity, testis size, and menopause were thus also prerequisites to our humanity. Still other features of our life-cycle differ far more drastically from those of apes than do our testes, yet the functions of those remaining novel features of ours remain hotly debated. We are unusual in having sex mainly in private and for fun, rather than mainly in public and only when the female is able to conceive. Ape females advertise the time when they are ovulating; human females conceal it even to themselves. While anatomists understand why men's testes are the size that they are, an explanation for men's relatively enormous penis still escapes us. Whatever their explanation, all these-features, too, are part of what defines humanity. Certainly, it is hard to picture how fathers and mothers could cooperate harmoniously in rearing their children if human females resembled some primate females in having their genitalia turn bright red at the time of ovulation, becoming sexually receptive only at that time, flaunting their red badge of receptivity, and proceeding to have sex in public with any male in the vicinity. Human society and child-rearing rest therefore not only on the skeletal changes mentioned in

Chapter Two, but also on these remarkable new features of our life-cycle. Unlike the case with our skeletal changes, however, we cannot follow through our evolutionary history the timing of each of these life-cycle changes, because they leave no direct fossil imprint. As a result, they receive only brief attention in paleontology texts despite their importance. Archaeologists have recently discovered a Neanderthal hyoid bone, one of the key pieces of our speech-producing equipment, but as yet no trace of a Neanderthal penis. We do not know whether Homo erectus was already on the road to evolving a preference for having sex in private, in addition to having evolved his and her well-documented large brain. Our sole clues about the dating of these life-cycle changes are that something about longevity can be inferred from skeletons, and that size differences between fossil men and women may be indirect reflections of their mating system (more of that in Chapter Three). We cannot even prove through fossils, as we can for our large brain size, that we rather than living apes are the ones whose life-cycles diverged most from the ancestral condition. Instead, we have to be content with merely inferring that conclusion from the fact that our life-cycles are exceptional compared not just to living apes but also to other primates, suggesting that we were the ones who did more changing.

Darwin established in the mid-Nineteenth Century that the anatomy of animals has evolved through natural selection. Within this century, biochemists have similarly traced how the chemical make-up of animals has evolved through natural selection. But so has the behaviour of animals, including reproductive biology and sexual habits in particular. Life-cycle traits have some genetic basis, as we shall see below, and vary quantitatively among individuals of the same species. For instance, some women are genetically predisposed to give birth to twins, while genes for long lifespan run in some families more than in others. Life-cycle traits affect our success in passing on our genes, through affecting our success in wooing mates, conceiving and rearing babies, and surviving as adults. Just as natural selection tends to adapt an animal's anatomy to its ecological niche and vice versa, so natural selection also tends to mould animals' life-cycles. Those individuals leaving the most numerous surviving offspring promote their genes for life-cycle traits as well as for bones and chemical make-up.

A difficulty with this reasoning is that it seems as if some of our traits, such as menopause and aging, would reduce (rather than enhance) our output of offspring and should not have resulted from natural selection. It often proves profitable to try to understand these paradoxes through the concept of'trade-offs'. In the animal world there is nothing that is free or pure good. Everything involves costs as well as benefits, by using space, time, or energy that could have been devoted to something else. You might otherwise have thought that women who never underwent menopause would leave more descendants than women who do. But consideration of the hidden costs of foregoing menopause (Chapter Seven) will help us understand why evolution did not design these strategies into us. The same considerations illuminate such painful questions as why we grow old and die (Chapter Seven), and whether we are better off (even in a narrow evolutionary sense) in being faithful to our spouse or in pursuing extramarital affairs (Chapter Four).

I have been assuming in this discussion that our distinctively human life-cycle traits have some genetic basis. The comments that I made in Chapter One about the function of genes in general apply here as well. Just as our height and most of our observable traits are not influenced by only a single gene, there surely is not a single gene specifying menopause, testis size, or monogamy. In fact, we know little about the genetic bases of human life-cycle traits, though selective breeding experiments in mice and sheep have illuminated the genetic control of their testis size. Enormous cultural influences obviously operate on our motivation for providing child care or seeking extramarital sex, and there is no reason to believe that genes contribute significantly to differences among individual people in these traits. However, genetic differences between humans and the other two chimpanzee species probably do contribute to the consistent differences in many life-cycle traits between all human populations and all chimpanzee populations. There is no human society, regardless of its cultural practices, whose men have chimpanzee-sized testes and whose women forego menopause. Among those 1.6% of our genes that differ between us and chimps and that have any function, a significant fraction is likely to be involved in specifying traits of our life-cycle.

The story of our uniquely human life-cycle occupies the five chapters of Part Two. Chapter Three begins by taking up the distinctive features of human social organization and of sexual anatomy, physiology, and behaviour. As already mentioned, features that make us strange among animals include our societies of nominally monogamous couples, our genital anatomy, and our constant and generally private pursuit of sex. Our sex lives are reflected not only in our genitalia but also in the relative sizes of men's and women's bodies (much more equal than are the bodies of male and female gorillas or orangutans). We shall see how some of these familiar and distinctive features have known functions, while others continue to defy understanding. No honest discussion of the human life-cycle could get away with noting that we are nominally monogamous and just leaving it at that. Pursuit of extramarital sex is obviously greatly influenced by each individual's particular upbringing and by the norms of the society in which the individual lives. Despite all that cultural influence, we are left with having to explain the facts that both the institution of marriage and the occurrence of extramarital sex have been reported from all human societies; but that extramarital sex is unknown in gibbons, although they do practise 'marriage' (that is, lasting male/female pairing to rear offspring); and that the question of extramarital sex is meaningless for chimpanzees because they do not practise 'marriage'. Hence an adequate discussion of our uniquely human life-cycle must account for our combination of marriage with extramarital sex. As Chapter Four will show, animal precedents exist to help us make evolutionary sense of our combination: men and women tend to differ in their attitudes towards extramarital sex much as geese and ganders do.

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.

WORLD CONQUEST | The rise and fall of the third chimpanzee | THREE THE EVOLUTION OF HUMAN SEXUALITY