It began with a throwaway comment to a journalist, but it is now the most famous wager in science.

Back in 2000, Steven Austad, a biologist who studies ageing at the University of Alabama at Birmingham (UAB), told the magazine Scientific American that he believed that the first human being who would live to 150 "is probably alive right now".

His friend Jay Olshansky - a professor of public health in Chicago who specialises in gerontology - assumed he must have been misquoted.

When he telephoned to check, Austad stood by his prediction. Olshansky was flabbergasted - so the pair decided to bet on it.

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On 15 September 2000, they each put $150 into an investment fund, and signed a contract stating that the money and any returns would be paid to the winner (or, realistically, his descendants) in 2150.

In 2016, they doubled the stake to $600. 

The lucky great-grandchildren stand to scoop a projected prize fund $200 million (£154m) - but who is right?

Austad says he remains "more convinced than ever" that he is correct, and that drugs to slow the ageing process will make it possible.

Even more remarkably, the deal includes a clause that the 150-year-old - should they exist - must be "of sound mind" for Austad's side to win.

Perhaps, when he made the bet, he was buoyed by the record-breaking lifespan of Jeanne Calment, a French woman who died at the age of 122 years and 164 days in 1997.

Until then, scientists believed that 120 years must be the maximum human lifespan.

Yet, Ms Calment remains the oldest living person and is still the only person to have exceeded 120 years, so perhaps they were not so wrong after all?

The number of people living to 100 years or more has rapidly increased over recent decades.

In Scotland, the number of centenarians in the population has gone from 170 in 1981 to 1,040 by 2021.

Women are much more likely than men to live to 100Women are much more likely than men to live to 100 (Image: NRS) According to the latest analysis by the National Records of Scotland (NRS), published on Wednesday, by 2045 average life expectancy for males in Scotland will be 80.1 years, and 83.4 years for females.

If that proves correct, it means that advances in medicine and living conditions will have added 11 years to male lifespans and just over eight years to female lifespans over the course of a 65 year period, from the early 1980s.

Improvements to life expectancy are projected to continue, but at a much slower rate than during the 20th century (Image: NRS)

But the past decade has not been a good one for life expectancy.

After decades of steady increases, gains in life expectancy first stalled and then declined in both the UK and the US. Only now are there signs of a slight reversal.

Earlier this month, a paper published in the journal Nature Ageing (co-authored by Prof Olshansky, who as we know has good reason to be pessimistic) concluded that further "radical life extension" in humans was "implausible".

While life expectancy had improved rapidly during a 20th Century "first wave", thanks to advances in public health and medicine, the researchers found that "since 1990, improvements overall in life expectancy have decelerated".

This was true for every population included in the study: the US, Hong Kong, Australia, France, Italy, Japan, South Korea, Spain, Sweden and Switzerland.

This lends credence to the hypothesis, first put forward in 1990, that humanity was reaching its "upper limit".

In essence, the "early gains" from those advances in healthcare (mass vaccinations, sanitation, antibiotics, cancer screening, better drugs and so on) had been achieved; the only avenues left for extending life expectancy significantly, at a population level, would depend on a scientific breakthrough which actually slows the process of biological ageing itself.

Steven Austad (left) and Jay Olshansky placed a bet in the year 2000 on whether there was already a human being alive who would go on to live to 150 years old (Image: UAB/YouTube)

Achieving a "second wave" of radical life extension, whereby average life expectancy could reach 110 years, would also require "the complete cure or elimination of most major causes of death that exist today".

Speaking to the AP news agency, Prof Olshansky, said: “We’re squeezing less and less life out of these life-extending technologies. And the reason is, ageing gets in the way.”

But perhaps there is hope on the horizon that might yet vindicate Austad's wager.

In recent weeks, research published in the journal, Cell, found that the diabetes drug Metformin slows ageing in multiple organs, including the brain and skin, in monkeys.

There have also been suggestions that semaglutide-based drugs such as Ozempic and Wegovy can actually "slow down the ageing process" by reducing inflammation.

Not everyone wants to wait around for a breakthrough, however - at least, not in their natural lifespan.

To date, 650 people have signed up to payment plans with German-based Tomorrow Biostasis to be cryogenically frozen in liquid nitrogen after they die for as long as it takes until science can find a way to revive and rejuvenate them.

A team from Tomorrow Biostasis training in cryopreservation techniques using a dummyA team from Tomorrow Biostasis training in cryopreservation techniques using a dummy (Image: TomorrowBio) The company - the first of its kind outside of the US and Russia - has already frozen six clients and five pets.

Bodies are collected by ambulance, cooled, and then kept at a storage facility in Switzerland.

The full-body service costs €200,000 (£167,000), but customers can choose a brain-only package for the discount price of €75,000 (£63,000) in the hope that it could be transplanted into a younger donor body in future, or even a robot.

Those going for the full-body package are relying on the hope that scientific advances will make it possible not only to re-animate a corpse, but to restore them to health - possibly through processes such as "cell reprogramming" to turn back the body clock.

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Such things remain the stuff of science fiction for now, but the lure of a second chance at life - maybe centuries from now - seems to be worth the gamble for some.

Perhaps it feels reassuring in an era when humanity faces the triple existential crises of artificial intelligence, global warming, and nuclear weapons to be able to skip ahead and (potentially) discover that disaster never came.

In that case, Austad and Olshansky should probably update their loopholes: would a cryogenically frozen 90-year-old revived in 2110 who lives another 40-plus years count as 150 years old, or 40?

It's a shame we won't be around to find out.