The idea that the person who will live to be 1,000 years old has gained momentum recently, propelled by transhumanists and sci-fi enthusiasts. However, scientists have now contradicted this view by suggesting that lifespan is subjected to biological constraints that cannot be exceeded.
In 1997, Frenchwoman Jeanne Calment died at the age of 122 years old, becoming the oldest documented person to have ever lived. Though no one has matched her record, one thing is certain: throughout the last 150 years, life expectancy has risen around the globe.
The authors of the recent study, published in Nature, give the example of France, where life expectancy has risen from 50 to more than 80 years for women, between 1900 and 2000.
They also point out that while increases in life expectancy have traditionally been fuelled by reduction in childhood mortality, but more recently there has been evidence of a decline in late-life mortality.
These two trends have largely been cited as arguments that maximum lifespan of humans is not fixed, and could still increase in the future. Some also believe that technological advances that have yet to be invented may one day put humans on the path to immortality.
But analysing demographic data from around the globe, the study's authors show that progress in life expectancy made in the 20<sup>th century is unlikely to continue further, because of biological limits specific to the human species.
Demographic data vs futuristic technologies
The researchers hypothesised that if no limit to human lifespan exists, the age group experiencing the greatest increase in survival should shift to ever-older groups over time. Yet, this appears not to be the case: the greatest improvement in survival appeared to plateau around 1980.
The scientists then focused on data regarding maximum reported age at death (MRAD) in countries known to be home to many centenarians – France, Japan, the United Kingdom and the United States. Before 1995, MRAD appeared to increase by about 0.15 years every year, but it has stagnated and even decreased a bit since then.
The researchers then plotted the data in a statistical model to approximate the absolute limit of human lifespan: they found that the probability of an MRAD exceeding 125 in any given year is less than 1 in 10,000. These findings indicate diminishing gains in reduction of late-life mortality and a possible natural limit to human lifespan that is unlikely to be changed by futuristic technologies.
What limits human lifespan?
A bit of vocabulary
Life expectancy - A population-based estimate of average expected duration of life for individuals, at any age.
Human lifespan - How long human can live.
Maximum reported age at death - Maximum age one or more members of a population have been observed to survive to.
These findings raise an important question: why is human lifespan to be limited? The study suggests biological causes are to blame – the natural limit is an inadvertent byproduct of genetic programs linked to development, growth and reproduction.
The age at which we die is not inscribed in our genes, but the scientists believe that the combination of these different genetic programs influence how we age: "Limits to the duration of life could well be determined by a set of species-specific, longevity assurance systems encoded in the genome", they say.
We may have gained in life expectancy and increased maximum age at death over the 20th century, but completely removing these biological barriers in the future is another matter. Any hope to surpass Jeanne Calment's record seems a long way away.