Last October, scientists made a splash when they determined that on average, people can only live for about 115 years. That was the magic age at which the human body and brain just petered out; it wasn’t designed to chug along much longer than that, they said.
That conclusion, published in the journal Nature, sparked hot debate among longevity researchers. Some felt the results vindicated what they felt to be the case, while others took issue with pinpointing a limit—and such a specific one, at that.
Now, in the new issue of Nature, the editors invited scientists who criticized the original authors’ methods to lay out their arguments for why there isn’t necessarily a limit to human aging. In the five resulting critiques, researchers tease apart the original authors’ methods, noting that they made assumptions that weren’t warranted and overreached in their conclusions. (The researchers who concluded that human lifespan maxes out at 115 years stand by their findings, and they responded to each of the current authors’ criticisms.)
The new papers don’t argue that human lifespan is limitless. But they note that it’s premature to accept that a maximum lifespan for humans exists. It’s equally possible, they say, that humans will continue to live longer, and therefore might survive beyond 115 years. “It was reasonable that when everybody lived to 50 that the very long lived, for whatever reason—genetics or luck—would make it to 80,” says Siegfried Hekimi, professor of genetics at McGill University in Canada and one of the authors of a criticism. “If people live on average to 80 or 90, like they do now, then the very long lived make it to 110 or 120. So if the average lifespan keeps expanding, that would mean the long-lived would live even longer, beyond 115 years.”
Overall, trends in longevity have been going up, and average lifespan has inched upward since even the 1990s. Back then, life expectancy in the U.S. was just around 50 years, while babies born today live to about 79 years on average. In any given year, however, if you look at the longest-lived, or the age at which the oldest person died, there may be considerable variation. There may be several years in which the maximum lifespan drops a bit, and other years in which it jumps.
The maximum lifespan in a population varies so much year to year that if you take the wrong snapshot of data—as Hekimi contends the original authors did—it may look like there is a flattening of the age at which the longest lived die. “If you throw a die several times every year that represents maximum lifespan, by chance alone you will see a lot of spread,” he says. “Sometimes it will be low, sometimes it will be high.”
For example, in coming up with the maximum lifespan of 115 years, the original paper’s researchers divided their population data into two groups: from 1968 to 1994 and 1995 to 2006. They determined that maximum lifespan peaked in the first era and started to plateau in the next. However, that coincides with the years in which Jeanne Calment, the oldest-lived human, was alive. She passed away in 1997 at age 122, so the plateau in maximum lifespan that the original researchers saw could be wholly attributed to her, Hekimi says. He and the other authors argue that the conclusion that human lifespan stops at 115 years was based on misinterpreting the data by seeing a plateau at 115 years where there was none.