How long can humans live? Before answering, said Robert Mitchell, professor of biology at Penn State, "We have to distinguish between maximum lifespan, which is as long as any human has ever lived, and life expectancy, which is how long you could live predictably, based on insurance statistics, for example. As far as maximum lifespan goes, we suspect it’s around 125 years. There’s no evidence that humans can live any longer than that. But as far as life expectancy goes, decade by decade we see that going up and up and up. We’re at the point where it’s somewhere around 76, 77 years. And that will continue to rise."
Mitchell continues, "Most geneticists will tell us that about 30 percent of our longevity depends on our genetic make-up. That surprises a lot of people. That’s saying that 70 percent depends on environment, behavior, what you eat, how careful a driver you are, if you smoke, if you wear your seatbelt. These things have a profound impact."
When she died in 1997, Jeanne-Louise Calment of France, the longest-lived person so far on record, was 122 years and five months old. "This woman was quite a character, if you read about her," noted Mitchell. "She was bright and spry into her later years. She took up fencing at age 87. I hesitate to say this, but she smoked until she was 117. The only reason she quit is because she got tired of asking people to light her cigarettes for her. She couldn’t see well enough to do it herself."
The "magic number" of 125, he added, has been established over millennia. "The program, so to speak, within our system has evolved to the point that we just can’t get beyond that. It’s a matter of repair. We only have so many repair systems that have evolved over millions of years. The best an individual can do is keep repairing damage, up to a certain point, and then you just can’t do it anymore. Long-lived animals have much better repair systems than short-lived animals. That’s genetic and it’s evolved that way.
"I don’t suspect that we’ll see maximum lifespan increasing beyond 125, unless we start juggling people’s genetic codes," Mitchell added, "and I don’t think anybody’s ready to do that."
Still, he said, one of the most exciting things he’s seen over his 35 years of research in the field of aging was "the discovery, in the 1990s, of specific genes in simple animals — fruit flies and roundworms that we use to study the aging process — that have profound effects on rate of aging and natural lifespan of those organisms. Many people believe that even though the code within our cells is made up of millions of genes, it might only be several dozen that are really key genes as far as programming our rate of aging and longevity.
"A lot of people right now are looking for those genes."
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Robert B. Mitchell is professor of biology in the Eberly College of Science. He can be reached at rbm1@psu.edu. Dana Bauer is a former associate editor of Research/Penn State.
