You Should Live So Long

By Philip Elmer-DeWitt

Most everybody wants to live as long as possible. And given the enormous strides made in medicine and the health sciences during the past 150 years, people could be forgiven for hoping that someday human beings will live, if not quite forever, at least far longer than at present. Since the mid-19th century, average life expectancy at birth has nearly doubled: from 40 years to 75. Today many people live past 100, and the oldest individuals have reached either 115 or 120, depending on whom you believe.

So it comes as something of a jolt to be told by the experts that human beings have taken life about as far as it can go. That is the sobering conclusion of a report in Science magazine last week by demographer S. Jay Olshansky and gerontologist Christine Cassel of the University of Chicago and biostatistician Bruce Carnes of Argonne National Laboratory. Barring an unexpected breakthrough in basic science that would forestall the aging process, they say, the era of rapid increases in human longevity has come to an end -- at least in developed countries. Even if science could eliminate heart disease and cancer -- which account for nearly 50% of all deaths in the U.S. -- it is unlikely that the average life expectancy at birth would increase much beyond 85.

What makes the report so compelling is that it is based on simple mathematics. In the past, the upper limits of life have been extrapolated from actuarial tables by estimating how death rates would change if, say, the incidence of heart disease was halved. "We reversed the question," says Olshansky. Taking an "engineering approach," his team members asked themselves how much mortality rates would have to be reduced in order to increase average life expectancy to 120 years. What they discovered, after running the numbers through a computer, was that big hits in current death rates in the U.S. would give only small lifts to life expectancy. For example, if through some miracle of medicine and risk avoidance no one ever again died before reaching age 50 (thus eliminating more than 12% of all deaths), the increase in average life expectancy would be only 3 1/2 years.

% There seems to be a kind of built-in biological limit programmed into the cells of the human body. In laboratory experiments, human cells divide only about 50 times before they begin to fall apart like old jalopies. This planned obsolescence on nature's part makes a certain amount of evolutionary sense. Survival of the fittest, after all, rewards only those who reproduce, not necessarily those who reach old age. Once procreation is over, human bodies may as well be disposable goods, biologically speaking.

The best way to combat cellular aging is to postpone its effects at the molecular level. Basic research is now under way to understand the mechanisms that make human cells wear out and to try to find the genes that cause the major degenerative diseases of old age -- arthritis, osteoporosis, Alzheimer's disease. This work could have a double benefit: extending life expectancy and helping to make those extra years worth living. But researchers have no idea when, or if, breakthroughs will take place.

In the unlikely event that scientists do manage to unlock the secrets of aging, some experts believe tomorrow's children could reach 130, 150 and even 170. But the authors of the Science report are extremely dubious. Among the increasing numbers of aging baby boomers, contends Olshansky, "very few people are going to live past 110 or 120." And what about Methuselah, the grandfather of Noah, who lived 969 years before he died? Simple, says the researcher. Someone misplaced a decimal point.