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Here are a few simple exercises in demographic model-making. Imagine a protohuman creature somewhat like the Australian marsupial mouse. The marsupial mouse indulges in what is called big-bang reproduction, meaning it copulates to the point of exhaustion, then rapidly ages and dies. It's easy to see why such a creature could never give rise to the big-brained human species. High intelligence requires a lengthy infancy with good parental care. A parent that was constantly involved in sex and produced far more young than it could cope with would never have any quality time to spare for good parenting. Thus, it would have no human future.
THE GRANDMOTHER EFFECT
So far, this is pretty much a matter of common sense. Now, though, weave some basic genetics into the scenario and it becomes an explanation for how and why people age. By giving their progeny a better chance of reaching reproductive maturity, older, postmenopausal parents and grandparents assist in passing along their genes, including the very genes for longevity that made it possible for them to make such an effective transfer of resources to the younger generation. Transfer is another term demographers have borrowed from economics--as in transfer payments via taxation. Here scientists have a way of accounting for aging on the basis of evolutionary principles. Social species evolve a longer lifespan by way of contributing to the reproductive fitness--another favorite phrase of social scientists--of the next generation down. The foregoing, in brief, is the intriguing new theory of aging that has been worked out by University of California, Berkeley, demographer Ronald V. Lee. Hailed as a major breakthrough in demographics, his paper "Rethinking the Evolutionary Theory of Aging: Transfers, Not Births, Shape Senescence in Social Species," was supported by the National Institute on Aging and appeared in the July 14 issue of the Proceedings of the National Academy of Sciences. As Lee puts it, "Successful reproduction often involves intergenerational transfers as well as fertility." His theory also explains why juvenile mortality tends to decline with age. By way of the same "flow of resources transferred to offspring," the young are better able to survive and so their death rate is lowered. Although the classic evolutionary theory of aging recognizes the grandmother effect, it has not seen the connection Lee has found between that effect and reproductive fitness. Lee's work can be construed to place a far more positive evaluation on the nurturing that older members of a species provide. Does Lee's model have any empirical support? He offers statistics from two human groups that illustrate his theory: the Ache hunters and gatherers of Paraguay and the Swedish of the 18th century.
THE RHETORICAL EFFECT
That may be because it is not the genes that shape the emotion, but the emotion that shapes the genes. What if love comes first, a motivating force that encourages the affectionate care of the young, thus giving them a better chance of survival? Think of love as the emotional environment, a state of heart that created the selective pressure for genetic change. Born into that environment, the young would inherit a greater opportunity of reaching reproductive maturity. At that point, those who happened to carry longevity-favoring genes would bequeath the trait to their kin, gradually building up an advantage along that line of inheritance until longevity became a dominant quality. How might the emotion arise in the first place? Grant, for a moment, that our ancestors were cultural animals, capable of forming ideas about themselves and their children. After all, at some point that must have happened, giving rise to all the lore and myth and symbolism that has come to surround childhood. What is it the older generation sees in the newborn? What is it grandparents see in their grandchildren? They see life renewing itself, a small antidote to mortality. All that we humans value most but see slipping away is reborn in the young--hope, vitality, wonder, innocence. Children are the delicate shields we raise against death. That is why we hold them dear.
MARKETPLACE METAPHOR
Scientists do like to come across as hard edged and objective, even when their tone may somewhat mislead. At one point in his paper, Lee oddly describes reproduction as "conversion of foraged foods into sexually mature offspring, a process requiring both fertility and investment per offspring." That description may help with his demographic analysis, but it is important to remember that, judging by the rites and rituals that universally surround childbirth, no parent ever thought of having a child in those terms. Certainly, human beings have never viewed caring for the young as a cost-benefit proposition or as a way of augmenting their genetic capital. How we think about things must have some basis in our evolutionary heritage. In any case, now that Lee has added to our understanding of how great a role the altruism of the old plays in evolutionary history, it might be time to find a vocabulary more in keeping with the emotional warmth of the matter. Theodore Roszak is author of Longevity Revolution (Berkeley, Calif.: Berkeley Hills, 2001). His latest book, a novel, is The Devil and Daniel Silverman (Wellfleet, Mass.: Leapfrog Press, 2002). He lives in Berkeley, where he spends as much time as possible with his granddaughter, Lucy.
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