Jeanne Calment, the French woman who holds the record for the longest verified lifespan, died in 1997 at 122 years old.
Few people, of course, ever become supercentenarians — 110 years old or older — and even fewer hit 115.
So few people have exceeded that age, in fact, that a group of researchers published an analysisin the journal Nature last year arguing that the human species' lifespan plateaus around 115.
But a number of scientists are now rebutting that analysis with five separate commentaries published in Nature on June 28.
The authors of these pieces argue that the original analysis relied on statistics that were incomplete or analyzed in a way that led to a false conclusion. They suggest two alternatives: We either don't have enough data to know if the human lifespan has a limit, or the plateau is closer to 125 than 115.
"The available data are limited, there aren't that many supercentenarians," Maarten Pieter Rozing, a professor at the University of Copenhagen who co-authored one commentary, told The Scientist. "And I think there are no strong arguments that show there is a decline [in the rate at which lifespans are increasing]."
Why some think life ends at 115
Life expectancy has crept up fairly steadily over the past 150 years or so. But Xiao Dong, Brandon Milholland, and Jan Vijg, the authors of the original analysis, argue that comparing the life expectancy of supercentenarians to the age at which they died can reveal the natural limit of the human lifespan.
The scientists used data on maximum reported age at death split into two sets based on supercentenarians from the US, UK, Japan, and France. The first set covered deaths from 1968 to 1994 — a period when the maximum age was inching up. But by the time covered in the next dataset, from 1995 to 2006, the age seemed to plateau or even slightly be on the decline (exceptions like Calment aside).
Life expectancy, however, rose throughout both time periods. The scientists therefore concluded that because humans' maximum age didn't keep rising with life expectancy, it appeared a limit had been reached.
Even if we were to cure various diseases like cancer or Alzheimer's, those scientists still claimed that humans would probably be unlikely to live past 115. And they put the chances of a person live past 125 at less than 1 in 10,000.
Limit or illusion?
The authors of the recent rebuttals say that because there are so few supercentenarians out there, the number of deaths for this age group between 1995 and 2006 is too small to yield reliable conclusions. There just haven't been enough supercentenarians to really pinpoint a maximum age.
As people live longer, it's likely that more will push past that supposed limit, the authors of the rebuttals argue — it'll just take time to get there.
"[T]he idea of a set limit to human longevity is not strongly supported by what is being discovered about the biology of ageing," Rozing and his co-authors wrote in their commentary. "The continuing increase in human life expectancy that has occurred over recent decades was unforeseen. It provides evidence for greater malleability of human ageing than was originally thought."
Over the span of human history, many of the lifespan increases we've seen would have been unimaginable at some point. Those living 200 years ago, for example, would have thought it was crazy that people could regularly live to be 80. Yet here we are.
Rozing told The Scientist that there's an easy way to find out whose hypothesis is correct about the maximum lifespan.
"[W]e can just wait and see who's right," he said.