Since 1900, average life expectancy around the globe has more than doubled, thanks to better public health, sanitation and food supplies. But a recent study of long-lived Italians indicates that we have yet to reach the upper bound of human longevity.

“If there’s a fixed biological limit, we are not close to it,” said Elisabetta Barbi, a demographer at the University of Rome. Barbi and her colleagues published their research Thursday in the journal Science.

The current record for the longest human life span was set 21 years ago, when Jeanne Calment, a Frenchwoman, died at the age of 122. No one has grown older since — as far as scientists know.

In 2016, a team of scientists at Albert Einstein College of Medicine in the Bronx made the bold claim that Calment was even more of an outlier than she seemed. They argued that humans have reached a fixed life span limit, which they estimated to be about 115 years.

A number of critics lambasted that research. “The data set was very poor, and the statistics were profoundly flawed,” said Siegfried Hekimi, a biologist at McGill University.

Anyone who studies the limits of longevity faces two major statistical challenges. There aren’t very many people who live to advanced ages, and people that old often lose track of how long they’ve actually lived. “At these ages, the problem is to make sure the age is real,” Barbi said.

Barbi and her colleagues combed through Italy’s records to find every citizen who had reached the age of 105 between 2009 and 2015. To validate their ages, the researchers tracked down their birth certificates.

The team ended up with a database of 3,836 elderly Italians. The researchers tracked down death certificates for those who died in the study period and determined the rate at which various age groups were dying.

It’s long been known that the death rate starts out somewhat high in infancy and falls during the early years of life. It climbs again among people in their 30s, finally skyrocketing among those in their 70s and 80s.

If the death rate kept exponentially climbing in extreme old age, then human life span really would have the sort of limit proposed by the Einstein team in 2016.

But that’s not what Barbi and her colleagues found. Among extremely old Italians, they discovered, the death rate stops rising — the curve abruptly flattens into a plateau.

The researchers also found people who were born in later years have a slightly lower mortality rate when they reach 105.

“The plateau is sinking over time,” said Kenneth W. Wachter, a demographer at the University of California, Berkeley, who co-authored the new study. “Improvements in mortality extend even to these extreme ages.”

“We’re not approaching any maximum life span for humans yet,” he said.

Brandon Milholland, a co-author of the study finding a limit to human life span, questioned the new paper. The research, he noted, was limited to just seven years in one country.

“You’re reducing yourself to a narrow slice of humanity,” he said.

Milholland also took issue with how the team analyzed their data. They only examined two possibilities: that the death rate continued its exponential climb, or that it turned into a flat plateau.

The truth might be somewhere in between, he said: “It seems rather far-fetched that after increasing exponentially, the chance of dying should suddenly stop in its tracks.”

The research doesn’t explain why death rates flatten out in the oldest of the old. One possibility is that some people have genes that make them more frail than others. Frail people die off sooner than more resilient ones, leaving behind a pool of tough seniors.

But Hekimi speculated that there might be other factors at play.

Throughout our lives, our cells become damaged. We only manage to partially repair them, and over time our bodies grow weak.

It’s possible that at the cellular level, very old people simply live at a slower rate. As a result, they accumulate less damage in their cells, which their bodies can repair.

“This is a reasonable theory for which there is no proof,” Hekimi said. “But we can find out if there is.”

A flat death rate doesn’t mean that centenarians have found a fountain of youth. From one year to the next, the new study suggests, they still have a much higher chance of dying than someone in her 90s.

Exactly how long centenarians live may simply be a roll of the dice each year.

But even if this turns out to be the case, Jeanne Calment’s age won’t be easily matched, said Tom Kirkwood, associate dean for aging at Newcastle University, who was not involved in the study.

“The higher the ceiling gets set as records are successively broken, the harder it gets to break it,” he said.