Human life spans have been increasing for decades thanks to advances in treating and preventing diseases and improved social conditions.
In fact, longevity has increased so much in recent decades that some researchers began to wonder: What is the upper limit on human aging?
"We never had so many centenarians as we have now," says Jan Vijg, who studies molecular genetics at the Albert Einstein College of Medicine in New York. "Maybe we can actually live much longer than 100. Maybe this goes on and on and on."
So Vijg decided to try to find out if that's the case. His conclusion, published online Wednesday in the journal Nature: The seemingly inexorable rise in the human life span may have hit a ceiling of about 115 years.
"We cannot break through that ceiling," Vijg says. "The take-home message essentially is this whole ever-increasing life expectancy of humans cannot go on."
Vijg and his colleagues are basing their conclusions on an analysis of decades of longevity records from around the world, including the Human Mortality Database and the International Database on Longevity.
"Every year we looked at who was the one who died in that year and was the oldest human in existence," Vijg says.
The researchers found that the age of the oldest people dying had indeed increased steadily between the 1960s and 1990s, according to their report.
But beginning in the 1990s, "you no longer see that," Vijg says. "You see that it stays the same."
The absolute maximum human life span may be as high as 125, the researchers calculated. But the chances of anyone actually living that long are less than 1 in 10,000.
"If we would have 10,000 worlds like ours, only one individual across all these 10,000 worlds would reach 125 in any given year," Vijg says.
But, he added, "the take-home lesson from what we found is that the human species most likely has a maximum life span of about 115 and we cannot break through that ceiling, at least not as far as we now know."
Other experts say it's not surprising that human longevity may have hit a ceiling.
"Right now, all we're doing is we're combating one disease at a time: heart disease, cancer, stroke," says S. Jay Olshansky, who studies aging at the University of Illinois and wrote a commentary article accompanying the report
"It's like a game of whack-a-mole. You know: One disease goes down another comes up," he says.
Olshansky says the only way that could change is if scientists figure out a way to fight the underlying cause of aging, not just individual diseases.
"That would be a game changer," he says.
Scientists are conducting a range of research to try to do that, including studying the genes of families that seem to have a lot of members who live unusually long lives. Some are also trying to identify beneficial substances in the blood of young people that might improve their chances of having a long life, he says.
But no one expects scientists to discover some kind of pharmaceutical fountain of youth anytime soon.
AUDIE CORNISH, HOST:
For decades the maximum human lifespan has been getting longer and longer, but that run may have come to an end. As NPR health correspondent Rob Stein reports, new research suggests we've hit a limit for human longevity.
ROB STEIN, BYLINE: The idea of living forever has long fascinated us and been fodder for books, poems and movies...
(SOUNDBITE OF FILM, "THE FOUNTAIN")
UNIDENTIFIED ACTOR: (As character) A special tree grows hidden.
STEIN: ...Like this 2006 film "The Fountain" about a time-traveling scientist searching for a magic elixir to save his wife.
(SOUNDBITE OF FILM, "THE FOUNTAIN")
UNIDENTIFIED ACTOR: (As character) The tree of life - they say whoever drinks of its sap will live forever.
STEIN: Well, no one really think humans will ever be able to live forever, but the oldest of the old among us has been getting older and older.
JAN VIJG: We never had so many centenarians as we have now - so people over a hundred.
STEIN: Jan Vijg of the Albert Einstein College of Medicine says that made him wonder. How long could we live?
VIJG: Maybe we can actually live much longer than a hundred. Maybe this goes on and on and on, and maybe humans can get, like, 120, 130, maybe 140 or 150. Who can say?
STEIN: So Vijg decided to try to find out. He analyzed decades of longevity records from around the world.
VIJG: Every year we looked at who was the one who died in that year and was the oldest human in existence. You can clearly see that between, say, 1960 and 1990, every year, the record oldest human that died in that year was older than the previous one until in the early 1990s. Then you know no longer see that. You see that it stays the same.
STEIN: A French woman died in 1997 at age 122, making her the oldest known person ever. But she was unique. Overall, the human lifespan seems to have plateaued well younger than that.
VIJG: According to our calculations, 115 is the maximum lifespan. We calculated it - 115 is human maximum lifespan. That's what it is.
STEIN: One hundred and fifteen - that's as long as Vijg says most people can probably ever expect to live if we're one of the lucky few who make it that far. There's a possibility that someone might make it to 125, but Vijg calculates that chances of that are less than 1 in 10,000 in any given year.
Other experts say it's not surprising that we may have hit a ceiling and aren't living radically longer lives. S. Jay Olshansky studies aging at the University of Illinois.
S JAY OLSHANSKY: Right now all we're doing is we're combating one disease at a time - heart disease, cancer, stroke. It's like a game of Whack-A-Mole, you know? One disease goes down. Another one comes up.
STEIN: Olshansky says the only way that could change is if scientists figure out a way to fight the underlying cause of aging, not just individual diseases.
OLSHANSKY: Advances in biomedical technology may allow us to slow the biological process of aging itself. Scientists are very excited about the prospects that we are eventually going to have the ability to slow down the biological process of aging. And that would be a game-changer.
STEIN: Perhaps by studying the genes of families that tend to live long lives, perhaps by identifying substances in the blood of young people that sustain youth. But we're unlikely to find some kind of fountain of youth any time soon. Rob Stein, NPR News. Transcript provided by NPR, Copyright NPR.