Last year, a trio of New York researchers claimed humans had already lived as long as they possibly could.

About 115 years, said Albert Einstein College of Medicine researchers, that's the maximum human lifespan. Now a group of Canadian researchers challenges that assertion, saying its possible human existence may be boundless.

"It does not mean that we know there is no limit. But because we can detect no limit it is possible that indeed there is no limit," said McGill University biologist Siegfried Hekimi. "Average human life span keeps increasing dramatically and maximum human lifespan seems to follow. I see no statistical or demonstrated biological reason how we would know that this must stop."

Indeed, people are living longer than they were a century ago, but American lives have leveled off in recent years. In 1900, the average lifespan was about 47 years, noted the Centers for Disease Control and Prevention. By 1970, American life expectancy was about 71 years old, increasing slightly year by year until 2015, when, for the first time in two decades, it dipped to 78.8.

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Senior author of the Einstein study Jan Vijg said human maximum lifespan peaked sometime in the 1990s. Population and mortality data, he explained, showed people at least 100 years old didn't live longer based on when they were born. Of those who lived to at least 110, their ages increased from the 1970s, but plateaued in the mid 1990s.

Using that data, the group calculated 115 years as the maximum human lifespan.

Of course, people have outlived 115, but not by much. The verified oldest person ever, 122-year-old Jeanne Calment of France, died in 1997. The former world's oldest person, Italian Emma Morano, died earlier this year at 117. The current title of world's oldest person belongs to 117-year-old Jamaican woman Violet Mosses Brown.

Vijg said just because people are living longer doesn't mean the oldest among us are living any longer.

"Further progress against infectious and chronic diseases may continue boosting average life expectancy, but not maximum lifespan," Vijg said when the Einstein study was released in October. "While it's conceivable that therapeutic breakthroughs might extend human longevity beyond the limits we've calculated, such advances would need to overwhelm the many genetic variants that appear to collectively determine the human lifespan."

Hekimi and fellow McGill biologist Bryan Hughes challenged the math of the Einstein researchers. Their finding was that the same data can be used to show a number of outcomes, including that maximum lifespan is decreasing. The Einstein researchers stand by their study.

Hekimi said the takeaway of the study, published Wednesday in the journal Nature, is, "that no plateau in the increase in maximum human lifespan can currently be observed."

"I am not saying that I believe humans will ever be able to live forever," he said. "However, in the absence of proof to the contrary as a scientist, I keep an open mind."

He added it's not possible to know what the future holds for humans or what his finding means for people, except that maybe there's a chance future generations could live longer than us.

"Three hundred years ago, many people lived short lives," Hekimi said. "If we would have told them that one day most might live up to 100, they would said we were crazy."

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