For centuries, it has been taken as an article of faith that the maximum human life span was close to 70 years, the biblical three score and 10.
The entire animal kingdom seemed vibrant proof that, whether dictated by God or by genes, whether measured in months or years, heartbeats or cell divisions, Earth's creatures seemed destined to live -- and die -- in highly consistent rhythms: Galapagos turtles, within 150 years; elephants within 70; horses in 45; mice in two or three.
But a radical new conclusion is emerging among some scientists: Far from being programmed to age and die at a certain point, humans could live to be 200, 300 or more.
It may take a hundred years or more, but with new drugs and genetic tinkering, the odds are good, some scientists say, that we could not merely live much longer, but do so in good health.
Even if they are only partly right, the implications are staggering.
What would marriage be like, for instance, if "until death do us part" meant 100 years together? Could the workplace employ people for 80 to 100 years?
How would a planet groaning with people absorb masses of elders who did not, as former Colorado Gov. Richard Lamm once put it, feel a "duty to die"?
"Culturally, it would be a disaster" if people lived to 150 or more, says Richard Sprott, associate director of the biology of aging program at the National Institute on Aging (NIA). "I know of no 80- or 90-year-olds who want to live to 150."
But if the extra decades were years of health and vigor, others suggest, those attitudes might change.
Merely boosting life expectancy -- the population's average survival time -- can be done with no fancy science at all. Already, with no special tricks, the number of American centenarians -- people over 100 -- is 36,000, double the 1980 figure.
If we simply reduce deaths from cancer and heart disease, life expectancy could jump from 75 to 85 by 2050, says medical demographer Jay Olshansky of the Argonne National Laboratories in Chicago.
Such a reduction would hugely impact Baby Boomers -- the largest population group, born between 1946 and 1964.
Today, there are 3 million Americans over 85, roughly 1 percent of the population. By 2050, the mere aging of Baby Boomers will swell this to 17 million, or about 6 percent of the population, equivalent to the entire population of Australia.
If, in addition, we substantially reduce deaths from cancer and heart disease, there could be 30 million people 85 and over by 2050, says Mr. Olshansky.
As soon as 2010, when Baby Boomers begin to hit 65 and retire, the already strained Social Security system, designed in the 1930s when few thought life expectancy would exceed 70, could begin to collapse.
Mr. Olshansky also is concerned that increasing life expectancy will add to the time we spend sick and disabled at the end of life, especially because, until recently, federal research was geared more to fighting fatal diseases than fighting non-fatal but serious problems such as severe arthritis, osteoporosis or dementia.
But others, among them gerontologist James Fries of Stanford University, feel this view is too gloomy. He predicts the same measures that can expand life expectancy will extend the "health span" as well, compressing disability into an ever-smaller segment at the end of life.
George Roth, chief of molecular physiology and genetics at the NIA, agrees, stressing that the real goal of research is "not to add years to life but life to years.
"We would not be able to extend the quantity of life without XTC extending quality as well," he says. "We would not have a bunch of senior citizens in nursing homes at 150. We'd probably have people making contributions to society for longer periods, not requiring health care until the late stages."
The point "is not so much to increase life span . . . but to have everyone achieve the life span," adds Dr. Lewis Lipsitz, medical research director at the Hebrew Rehabilitation Center for the Aged in Boston.
Despite qualms about the possible downside of life extension, the idea that there might be no built-in limits to the life span is galvanizing researchers across the country.
Some are working on drugs -- human growth hormone, for instance, which can temporarily turn back the physiological clock; nerve growth factor, which may revive aging brains; a compound called PBN, which appears to combat one of the most basic processes of aging, to name but a few. Other scientists are hotly pursuing versions of a natural hormone called DHEA, which in rodents combats cancer and can make aging immune systems young again. Still others are looking into the life-extending potential of a drug called Deprenyl, already on the market for Parkinson's disease.
Meanwhile, geneticists are searching for "longevity assurance genes" and "mortality genes," which, if enhanced or knocked out, respectively, could boost the life span.
At the Institute for Behavioral Genetics at the University of Colorado, for instance, geneticist Tom Johnson has found two probable mortality genes in tiny worms. When inactivated, these genes extend life 50 percent to 70 percent.
And the Geron Corp. in California is studying dozens of genes that may trigger aging, as well as compounds that may reverse these effects, says chief scientific officer Michael West.