Health and Medicine
PUBLISHED
The oldest human to have ever lived did so for an impressive 122 years and 164 days. If, that is, she is who she claimed to be, with one mathematician and a gerontologist raising a few doubts about that back in 2019.
But what age is it actually possible for humans to reach?
A new study tackles that question by looking at our individual organs and assessing how long they could keep going if we were able to eliminate all other reversible aging processes.
The team focused on how long each of our organs could survive if the only factor were somatic mutations, or mutations which occur after conception, and can include things like DNA copying errors during normal cell division.
"To date, no attempts have been made to estimate how long a future human, having undergone a treatment against reversible aging, could possibly live," the team explains in their study.
"What would be the maximum effect of radical life extension, excluding extravagant interventions like massive cell replacement, or transferring consciousness onto a silicon chip? In other words, what is all this strife for?"
First: assume we are all 30 years old and Swiss
Doing this required a lot of math, and a few assumptions. The first is to assume that everybody had the mortality rate of a 30-year-old Swiss human. That may sound out of the blue, but it is reasonable when you think about the task.
The team wanted to study expected lifespans of humans if you remove certain factors. To do this, they needed a baseline expected mortality rate for an average person.
If you remove aging, people will still die from accidents or viruses or falling into a hot spring at Yellowstone, for example.
The team needed to base their model on a population where age-related disease played as little of a role as possible in the mortality rate. They chose Switzerland for its low mortality rates, and 30-year-olds, as they were generally in good health, but still exposed to a decent amount of risk of death from things like disease, workplace accidents, and drug abuse.
In this idealized world, where an increase in age does not come with an increased risk of meeting your maker, humans could live for a very long time, according to the team.
"We adopt the age 30 mortality rate as our baseline, which corresponds to median and maximum remaining lifespans of 1,759 and 29,221 years," they write in their paper.
That's a lot of years, and goes some way to demonstrating how much aging increases your risk of mortality, until eventually (sorry to go all Fight Club on you) the survival rate for everyone drops to zero. Were they to use a modern 110-year-old as their baseline, for example, the median remaining lifespan for everyone would drop to a single year.
Blame your brain for aging
In their model, the team then turned on and off various aging processes and looked at the expected lifespan of our organs.
Mathematically modeling all those juicy, squelchy bits inside of us, and including somatic mutations only, the team found that our maximum possible lifespan is being held back by a few key organs. Though it's nice having a brain (a lot of the time), its components may stop us from living for much longer than we already do.
"The key finding of the study is the discovery of substantial differences between tissue types," Evgeny Efimov, author of the study, and research intern at the Skoltech Biomed Technologies Center and a researcher at AIRI, explained in a statement.
"Neurons and cardiomyocytes, which lack the ability to divide, turned out to be the main limiting factors: when all other causes of aging are eliminated, somatic mutations alone reduce the theoretical median lifespan from 1,759 years (for a hypothetical non-aging human organism) to 156 years."
"At the same time, tissues with high regenerative capacity—such as the liver—can maintain their function for thousands of years through continuous cell renewal, effectively neutralizing the negative impact of mutations," he added.
How to live to 150 years old
That is the median age, while the maximum lifespan reached 470-years-old. You may have noticed that we don't have many 400-year-olds walking about the place, and one interesting bit about the study is that it can help the team understand what factors contribute to aging at all.
"Our study shows that somatic mutations contribute significantly to aging, but they cannot by themselves explain the observed mortality," co-author Dmitrii Kriukov, a research scientist at the Skoltech Biomed Technologies Center and a senior research scientist at AIRI, added.
"This means that other aging mechanisms—such as loss of proteostasis, mitochondrial dysfunction or epigenetic changes—contribute comparably to limiting lifespan."
The team writes that the work suggests aging is not uniform within humans, as it affects different organs, such as the brain and heart, more quickly than other organs. This means that attempts to increase lifespans should be targeted towards key areas of the body.
"In this context, translational cell therapy aimed at neuronal replacement may represent our only viable path to 150+ year lifespans, even assuming all other hallmarks are mitigated," they write.
Though we have certainly made progress in extending the human lifespan, according to this work the maximum median human lifespan possible may fall in the 146–194 years range, and is limited by somatic mutations.
The study is published in NPJ Aging.
