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Most promising of all is the possibility that scientists may someday not only lengthen life-spans but improve them as well. Researchers are starting to talk about the likelihood of people living well into their second centuries with the smooth skin, firm muscles, clear vision, high energy and vigorous sexual capabilities they once could enjoy only in youth.
For human beings, the sea change in aging has been a long time in coming. In the past decade or two, there has been an explosion in new therapies designed to slow the senescence process--from melatonin to antioxidants to hormone-replacement therapy to the intriguing hormonal precursor DHEA.
Popular as some of these treatments are, what they promise is modest: a few years added here and there, and an increased likelihood that those years will be healthy ones. What the new wave of researchers is looking for is life extension that's not so much incremental as exponential. Not just a year here or there, but a doubling or tripling of human life expectancy.
History shows it's possible. In 1900 the life expectancy for a person born in the U.S. was 47 years. At mid-century, it was little better. After 1950, however, things started to stir. In a single year, subtle improvements in medical care caused the 47-year figure to jump 2%. The next year it jumped another 2%; then another. For four decades, that pattern has roughly continued, a compounding of existential interest that, according to U.S. figures, has pushed the average life expectancy to nearly 76, with many Americans living well beyond. Less conservative demographers are more optimistic still, believing a child born in America today can realistically look forward to living 100 years. Last week the sunnier scenarios got a boost when the National Cancer Institute reported that between 1990 and 1995, U.S. death rates from cancer fell more than 3%, the first sustained decline since 1900 (see box).
But what if the medical breakthroughs were more dramatic? If living to the century mark involves little more than riding the demographic wave, how much further than 100 is it possible to go? Is 150 reasonable? 200? What about 300? And if not, why not? The body, after all, is just a machine--albeit a wet, cranky, willful one--and, as with all machines, it should be possible to extend the warranty. "There is no evidence we know of that human life expectancy is anywhere close to its ultimate limit," says James Vaupel, a Duke University demographer and the soon-to-be director of the Max Planck Institute for Demographic Research in Germany, "if there is an ultimate limit."
In the same way that the modern era of genetics research began in 1953 when the DNA double helix was identified, the modern era of aging research is thought to have begun in 1961, when anatomist Leonard Hayflick made an equally significant discovery. Hayflick had been troubled by the question of where aging begins. Is it the cells themselves that falter, dragging the whole human organism down with them? Or could cells live on indefinitely were it not for some age-related deterioration in the higher tissues they make up?