The oldest person ever that we know about was Jeanne Calmet, a French woman who lived to the age of 122.

That’s really impressive! For a human, at least, since there are some animals like tortoises or the occasional whale that live even longer.

Some species of tree can live for thousands of years. Ooo, and, actually, there’s even a jellyfish that seems to be kind of immortal. So… What’s stopping humans from just living longer?

Like, 122 is great, but with all we know now about medicine and genetics, is that really the limit? Basically, do human beings have to die? In this episode, we’ll learn what the maximum lifespan of a human seems to be, what causes that limit, and how we’re trying to push beyond. [intro] Now, by and large, the question of whether humans should live longer is best left up to philosophers and priests, though we’ll touch on it a bit later.

For the most part, though, we’re just going to play in the space of what’s possible. Do humans have the capacity to keep on ticking, or is there a big ol’ stop sign we can’t pass? So this video will happen in a couple of parts.

In this first part, let’s just figure out what  the limit of human aging even seems to be. Not just the average lifespan, but the maximum lifespan. And for that, let’s start by asking demographers – people who look at the statistics of how people live out their lives and how that changes over time.

And, according to them, human life expectancy has been changing. Over the course of the last century, more people have been living longer on average, and the maximum lifespan seems to have been rising too. At least until recently.

Take this one paper from 2016, for instance. In it, three scientists got records from France, Japan, the UK, and the US – the countries with the highest number of people over the age of 110 – and plotted out the oldest reported death in each country per year. They found that the record for the oldest person actually seems to have plateaued in the late 90s.

Another study from 2022 tried to come up with some sort of mathematical formula to predict when your chances of living to some certain age that chance goes down to zero. Both research teams arrived at a maximum age around 125, not too far off from Jeanne’s record. It’s still heavily debated, though.

Other math-heavy approaches suggest there it’s not a hard limit so much as just a matter of luck. Like, you win a coin flip and live to 122, win another coin flip and live to 123, and so on. Getting that lucky might be rare, but enough people are living longer now that one paper from 2019 concluded that there was about an 80% chance that someone would beat the current record by 2050.

It’s hard to draw very concrete answers, though. Part of that might be because, technically, demographers aren’t really asking about why there’s a limit, just trying to figure out what that number is. To figure out the biology  of what’s really going on, well, we need to talk to the biologists… So this is part two, where we get into the question of why there’s a lifespan in the first place.

Because even if we’re able to avoid things like cancer or big, fatal accidents, there does seem to be a point at which our bodies seem to just kind of stop working. In the past, we might have described this as simply dying of old age. But, technically, if you ask a scientist or doctor, we don’t use that term anymore – not officially.

When we really look, it seems like there’s always a cause – a heart stopping or an organ failing or something. In general, when we’re younger and more resilient, we can bounce back from many injuries. But as we get older, we start to lose that ability to recover.

On a cellular level, we think it’s related to cells  either losing the ability to do their jobs efficiently, or becoming actively problematic. Some cells may become senescent, meaning they go into a kind of “quiet mode” when they can’t divide anymore. They can’t heal and, what’s worse, can damage tissue around them.

Eventually the damage is too much and something fails. In a sense, though, that’s just moving the question around. What we really need to ask is: what causes cells to stop working?

We don’t know for sure yet, but we think it’s the  build-up of molecular changes in the cells. What kind of changes? A lot of changes.

Accumulated mutations in our DNA is one popular answer. This could cause proteins within  the cell to not work right, or not get made correctly. And without proteins, a cell can’t run.

Or maybe it’s not mutated genes, but the inability to fix those mutations. Or changes in tiny flags called methyl groups that our cells use to ramp up or down the expression of genes. Many diseases of early aging involve flawed DNA repair pathways, and studies have found relationships between DNA methylation and a person’s age.

Telomeres are another popular scapegoat. These are part of our DNA, but they are not genes. They’re more like caps on  the end of our chromosomes.

Kind of like the aglet on  the end of your shoelaces that keeps them from unravelling. Because of the way DNA divides, telomeres tend to get shorter over time, until, eventually, they’re too short and the cell stops dividing. In other words, there are a lot of different ways in which DNA can be implicated in aging.

But there are other ideas too. One paper proposed that in addition to our circadian  and yearly biological clocks, we have a lifetime clock that governs events like primary skeletal growth, puberty, menopause… Their argument seems to be that death results from the  clock itself breaking down and failing to signal what should come next. But those are just some of the ideas.

Other ideas include inflammation, proteins like mTOR or hormones like IGF1… we could go on all day. Most scientists think these changes – and the whole aging and maximum lifespan thing – are essentially an evolutionary accident. There’s no overall benefit to aging, evolutionarily speaking, and it’s not programmed in, it’s just that whatever defenses we have against it only last so long.

Then again, maybe aging is a kind of evolutionary oversight. Remember that natural selection doesn’t really care about you unless you can produce offspring. Like, take inflammation.

It’s an important bodily defense tactic, helping us fight off infection, but also contributes to a  ton of age-related problems. In theory, an organism that  dialed back inflammation over its evolution might have had an edge to be able to live longer. But, it would probably die of infection first, which negates any selective advantage.

On the other hand, some scientists do think there may be something actually programmed into our bodies and that there may be some  sort of evolutionary benefit to having a maximum lifespan. For example, if parents end up competing with their offspring too hard, it might be better for their  overall reproductive success for the parents to be selected out. Altogether, though, this raises a new question: If aging does seem to just be something molecular inside our bodies breaking down – and most scientists seem to think aging is really more of an accident  than something programmed – then, does aging really need to happen at all?

That one immortal jellyfish seems to be able to avoid it, after all. Listen, I would love to get the  secrets about how cnidarians are able to live this long and if someone’s able to figure it out, please tell me. And even if it’s not technically immortality, there are those animals with  longer life spans than us.

Because if there’s a limit on aging, these guys are getting around it, or at least pushing it our further. And it begs the question of  whether humans can do it too. You may have seen me and  several other SciShow hosts wearing this shirt.

Well it’s not just a swimming astronaut. It’s a reference to NASA’s Neutral Buoyancy Lab, where astronauts train in a pool that is so big, the water looks like, actually blue. You can get one of these shirts, as well as our Orca Bucket Hat, angry Wandering Womb pin, and tons of other SciShow goodies at complexly.store.

And now we’re at part three: Human longevity trials. A lot, and I mean a lot, of people want to figure out how to live longer. Dampening one of those hormones I hinted about earlier, IGF1, doubled the lifespan of nematode worms, and at time of filming there’s a clinical trial looking at IGF1 drugs to help dogs live longer.

But while more time with our  beloved pets is priceless, the same strategy won’t work for humans. It’s just the beginning as far as getting anything to work for us. And Part of that’s because there are some logistical roadblocks that make the science hard to do.

For one thing, we are still  looking for a good metric. We need some sort of shorthand marker of how “old” someone’s cells are beyond  just their age according to the calendar. A bunch of the things we mentioned before, like inflammation or those DNA  methyl tags, might work out.

DNA methylation can predict  how “old” your cells are, though with a caveat – if you’re especially healthy, your cells might seem younger, and if your body’s been through  a lot of stress and inflammation, they might seem older. So looking at DNA methylation gives you a pretty sensitive age, it just is one that might not match up with what day you were born. So researchers haven’t settled on anything yet.

Another roadblock to trying  to test out anything in humans is that it might be hard to  recruit participants in trials. Part of that is down to what other conditions someone might have as they age, what medications they’re on, and so forth. We can sort of control for that, but it makes the study design trickier.

But if you are lucky enough to live to 100, let’s be honest, you’re probably going to be at least taking pills for high blood pressure or something already. And there are very real arguments against trying to extend the human lifespan. Like I said earlier, tackling the  arguments both for and against is more than I have the time or expertise for, but there are a lot o f interesting papers for you to read if you’re interested.

And there are research groups  looking at different interventions. Limiting calories – or getting our bodies to run on less food – seems promising, as do drugs to mimic it, but whether that actually translates to slowing aging in humans is just starting to be investigated. There are also things like supplements with something called NAD, which a molecule our bodies use to process energy, but at least as of a few years ago, we were still kind of figuring out if it’s even safe for humans to take.

Plain old-fashioned exercise seems to have a modest age-reversing effect, and that one, at least, I feel comfortable recommending to anyone at home who’s interested. Meanwhile, other people are looking at things like stem cell reprogramming,  microbiome augmentation, and all kinds of stuff. But there’s still that problem of having a marker everyone agrees on.

Along with everything else, you’ve got to show that an improvement in that marker actually translates to living more years. And our interventions have to be feasible, safe to take, and so on. And Even then, there probably won’t be a single silver bullet.

Odds are that we’ll likely need a cocktail of many different carefully  balanced interventions. On the plus side, though, even if anti-aging therapies don’t actually end up extending the maximum lifespan, they may increase the proportion of our years which are spent in good health, our “healthspan” if you will I may not be able to live forever, but if scientists can bother enough jellyfish and be able to get me out and hiking at 90, then that’s pretty good too. This brings us to part four, do we have to die?

We think of aging as the breakdown of molecular processes in the body. Most scientists think it’s something that happens essentially by accident, but some think it may be programmed in us somehow. If there is a limit, it’s pretty interesting that evolution has come up with a way to go: Time’s up!

Let the next generation handle this. As to what the true cause of aging is, we know a lot of things that happen when we age, but it’s hard to say which are truly to blame and to what extent. If there was a simple answer,  we’d have found it already.

We have figured out what look  like some big contenders, and there have been some  promising experiments in animals, but humans are not lab mice. Designing and even figuring out  how to measure human interventions for aging is likely to be especially complicated, and overall we’re still in very early stages. So, do we have to die?

We might be decades away from a yes or no answer to that question, if we ever find one. But the search might inform ways to be healthier and happier in the time that we get. In my humble opinion, that much is worth looking into. [ OUTRO ]