Why Is Sex Fun__ The Evolution of Human Sexuality - Jared M. Diamond [51]
The evolutionary chain of reasoning rests on several cruel facts. One is the human child’s long period of parental dependence, longer than in any other animal species. A baby chimpanzee starts gathering its own food as it becomes weaned by its mother. It gathers the food mostly with its own hands. (Chimpanzee use of tools, such as fishing for termites with grass blades or cracking nuts with stones, is of great interest to human scientists but of only limited dietary significance to chimpanzees.) The baby chimpanzee also prepares its food with its own hands. But human hunter-gatherers acquire most of their food with tools, such as digging sticks, nets, spears, and baskets. Much human food is also prepared with tools (husked, pounded, cut up, et cetera) and then cooked in a fire. We do not protect ourselves against dangerous predators with our teeth and strong muscles, as do other prey animals, but, again, with our tools. Even to wield all those tools is completely beyond the manual dexterity of babies, and to make the tools is beyond the abilities of young children. Tool use and tool making are transmitted not just by imitation but by language, which takes over a decade for a child to master.
As a result, a human child in most societies does not become capable of economic independence or adult economic function until his or her teenage years or twenties. Until then, the child remains dependent on his or her parents, especially on the mother, because, as we saw in previous chapters, mothers tend to provide more child care than do fathers. Parents are important not only for gathering food and teaching tool making but also for providing protection and status within the tribe. In traditional societies, the early death of either the mother or the father prejudiced a child’s life even if the surviving parent remarried, because of possible conflicts with the stepparent’s genetic interests. A young orphan who was not adopted had even worse chances of surviving.
Hence a hunter-gatherer mother who already has several children risks losing some of her genetic investment in them if she does not survive until the youngest is at least a teenager. That one cruel fact underlying human female menopause becomes more ominous in the light of another cruel fact: the birth of each child immediately jeopardizes a mother’s previous children because of the mother’s risk of death in childbirth. In most other animal species, that risk is insignificant. For example, in one study encompassing 401 pregnant female rhesus macaques, only one died in childbirth. For humans in traditional societies, the risk was much higher and increased with age. Even in affluent, twentieth-century Western societies, the risk of dying in childbirth is seven times higher for a mother over the age of forty than for a twenty-year-old mother. But each new child puts the mother’s life at risk not only because of the immediate risk of death in childbirth but also because of the delayed risk of death related to exhaustion by lactation, carrying a young child, and working harder to feed more mouths.
Yet another cruel fact is that infants of older mothers are themselves increasingly unlikely to survive or be healthy because of age-related increases in the risks of abortion, stillbirth, low fetal weight, and genetic defects. For instance, the risk of a fetus carrying the genetic condition known as Down’s syndrome increases with the mother’s age, from one in two thousand births for a mother under thirty, one in three hundred for a mother between the ages of thirty-five and thirty-nine, and one in fifty for a fortythree-year-old mother, to the grim odds of one in ten for a mother in her late forties.
Thus,