The Longevity Revolution-What's Love Got to Do with It?
Roszak, Theodore, Aging Today
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.
Now, imagine a protohuman creature whose reproductive powers ceased while it was still healthy and active. Suppose this happened because the females of the species experienced menopause in midlife and produced rather fewer young than the sex-crazed rival mentioned above. By stopping reproduction early, this creature would have more time to invest in its young. Invest is a dollars-and-sense term demographers like to use, perhaps because it has a good, crisp, businesslike sound. Obviously, this creature's young, with the advantage of a longer infancy, would stand a better chance of evolving high intelligence.
THE GRANDMOTHER EFFECT
Now imagine that the creature just described also happens to be blessed with genes that contribute to longevity on the far side of menopause. It might then survive long enough to invest not only in its own offspring but also in its daughter's, and offer food, protection, learning and other forms of sustenance. Having that much parental support would yield what has been called a fitness bonanza for the young. Such behavior exists among dolphins, elephants, primates and humans. Demographers call this phenomenon the grandmother effect. (The chapters in Sarah Blaffer Hrdy's book Mother Nature: A History of Mothers, Infants and Natural Selection (New York City: Pantheon Books, 1999) offer an excellent survey of grandmothering around the world and through prehistory.)
So far, this stuff 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 us 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. …