of other groups, and in individual fights within a group. Also, in many species the males are bigger than the females, but studies of red deer and of New World blackbirds show that males are thereby more likely than females to die when food becomes scarce.
Correlated with this greater accidental death rate of men, men also age faster and have a higher non- accidental death rate than women. At present, women's life expectancy is about six years greater than that of men; some of this difference is because more men than women are smokers, but there is a sex-linked difference in life expectancy even among non-smokers. These differences suggest that evolution has programmed us so that women put more energy into self-repair, while men put more energy into fighting. Expressed another way, it just is not worth as much to repair a man as it is to repair a woman. I do not thereby mean to denigrate male fighting, which serves a useful evolutionary Purpose for a man: to gain wives and to secure resources for his children and his tribe, at the expense of other men and their children and tribe.
My remaining example of how some striking facts of aging become comprehensible only from an evolutionary perspective concerns the distinctively human phenomenon of survival past reproductive age, Specially past female menopause. Since transmitting one's genes to the next generation is what drives evolution, other animal species rarely survive past reproductive age. Instead, Nature programmes death to coincide with the end of fertility, because there is then no longer an evolutionary benefit to gain from keeping one's body in good repair. It is an exception in need of explanation to realize that women are programmed to live for decades after menopause, and that men are programmed to live to an age when most men are no longer busy siring babies. But the explanation becomes apparent on reflection. The intense phase of parental care is unusually protracted in the human species and lasts nearly two decades. Even those older people whose own children have reached adulthood are tremendously important to the survival of not just their children but of their whole tribe. Especially in the days before writing, they acted as the carriers of essential knowledge. Thus, Nature has programmed us with the capacity to keep the rest of our bodies in reasonable repair even at an age when the female reproductive system itself has fallen into disrepair.
Conversely, though, we have to wonder why natural selection programmed female menopause into us in the first place. It too, like aging, cannot be explained away as something physiologically inevitable. Most mammals, including human males plus chimps and gorillas of both sexes, merely experience a gradual decline and eventual cessation of fertility with age, rather than the abrupt shutdown of women's fertility. Why did that peculiar, seemingly counter-productive feature of ours evolve? Would not natural selection favour the woman who remained fertile until the bitter end?
Human female menopause probably resulted from two other distinctively human characteristics: the exceptional danger that childbirth poses to the mother, and the danger that a mother's death poses to her offspring. Recall from Chapter Three the enormous size of the human infant at birth relative to its mother: our big 7-pound babies emerging from 100-pound mothers, compared to little 4-pound gorilla babies emerging from a 200-pound gorilla mother. As a result, childbirth is dangerous to women. Especially before the advent of modern obstetrics, women often died in childbirth, whereas mother gorillas and chimps virtually never do.
Now recall also from Chapter Three the extreme dependence of human infants on their parents, especially on their mother. Because human infants develop so slowly and cannot even feed themselves after weaning (unlike young apes), the death of a hunter-gatherer mother would have been likely to be fatal to her offspring up to a later age in childhood than for any other primate. Hence a hunter-gatherer mother with several children was gambling the lives of those children at every subsequent childbirth. Since her investment in those prior children increased with their age, and since her own risk of death in childbirth also increased with her age, the odds of her gamble paying off got worse and worse as she got older. When you already have three children alive but still dependent on you, why risk those three for a fourth?
Those worsening odds probably led through natural selection to menopausal shutdown of human female fertility, in order to protect a mother's prior investment in children. Since childbirth carries no risk of death for fathers, men did not evolve menopause. Like aging, menopause illustrates how an evolutionary approach illuminates features of our life-cycle that would otherwise be counterintuitive. It is even possible that menopause evolved only within the past 40,000 years, when Cro-Magnons and other anatomically modern humans began frequently to survive to the age of sixty or more. Neanderthals and earlier humans usually died before the age of forty anyway, so that menopause would have brought their women no benefits if it were to occur at the same age as in modern
Thus, the longer lifespan of modern humans than of apes rests not only on cultural adaptations, such as tools to acquire food and deter predators. It also rests on the biological adaptations of menopause and increased investment in self-repair. Whether those biological adaptations developed especially at the time of the Great Leap Forward or earlier, they rank among the life history changes that permitted the rise of the third chimpanzee to humanity. The last conclusion that I wish to draw from an evolutionary approach to aging is that it undermines the approach which has long dominated the physiological study of aging. The gerontological literature is obsessed with a search for The Cause of Aging—preferably a single cause, certainly not more than a few major causes. Within my own lifetime as a biologist, hormonal changes, deterioration in the immune system, and neural degeneration have vied in popularity for the title of The Cause, without compelling support having been adduced to date for any of the candidates. But evolutionary reasoning suggests that this search will remain futile. There
The basis of this prediction is as follows. There is no point doing expensive maintenance on one piece of the body if other pieces are deteriorating more rapidly. Conversely, natural selection should not permit a few systems to deteriorate long before all the others, as the cost of extra repairs on just those few systems would then buy a big increase in life expectancy and would be worth it. By analogy, Mercedes-owners should not install cheap ball-bearings when they are lavishing expense on all other parts of the car. Had they been so foolish, they could have doubled the lifetime of their costly car just by spending a few more dollars for better ball-bearings. But it would not pay either to go to the expense of installing diamond ball-bearings, when all the rest of the car would have rusted away before those ball-bearings wore out. Thus, the optimal strategy for Mercedes-owners, and for us, is to repair all parts of our cars or bodies at such rates that everything finally collapses all at once. It seems to me that this depressing prediction is borne out, and that we come closer to this evolutionary ideal than to the physiologists' long-sought Cause of Aging. Signs of aging can be found wherever one looks for them. Already I am conscious in myself of tooth wear, considerable decreases in muscle performance, and significant losses in hearing, vision, smell, and taste. For all these senses, the acuity of women is greater than that of men of equal age, whatever the age group compared. Ahead of me lies the familiar litany: weakening of the heart, hardening of the arteries, increasing brittleness of bones, decreases in kidney filtration rates, lower resistance of the immune system, and loss of memory. The list could be extended almost indefinitely. Evolution seems indeed to have arranged things so that all our systems deteriorate, and that we invest in repair only as much as we are worth.
From a practical standpoint, this conclusion is disappointing. If there had been one dominant cause of aging, curing that cause would have provided us with a fountain of youth. This thought, operating at a time when aging was thought to be largely a hormonal phenomenon, inspired some attempts at miraculous rejuvenation of old people by hormonal injections or implantation of young gonads. Such an attempt was the subject of Sir Arthur Conan Doyle's story,
I could have warned Professor Presbury that his myopic obsession with proximate causation would lead him astray. Had he thought of ultimate evolutionary causation, he would have realized that natural selection would never permit us to deteriorate through a single mechanism with one simple cure. Perhaps it is just as well. Sherlock Holmes worried greatly about what would happen if such an elixir of life were found: There is danger there—a very real danger to humanity. Consider Watson, that the material, the sensual, the worldly would all prolong their worthless lives… It would be the survival of the least fit. What sort of cesspool may not our poor world become? Holmes would be relieved to know that his worries now appear unlikely to materialize.
