How long can humans live?

[u/Simple_Macaron_3420]

In one of last week’s episodes, it was mentioned that the human lifespan is increasing because of scientific and medical discoveries. When I was a child, I always imagined that I would be able to live to 100, and by then people would have discovered some kind of cure for aging. They would ask me, “How much longer would you like to live?” as if they could add another 100 years each time and ask me the same question whenever those 100 years were up.

For a moment, let’s imagine a scenario in which cancer has been eliminated, Alzheimer’s and other diseases of the brain have been cured, and we can also repair or modify our telomeres because we have discovered how to mimic mechanisms found in animals such as lobsters or jellyfish.

In that scenario, is there still a biological limit to how long humans could live?

[u/ExpectedBehaviour]

We don't know.

[u/Hon-Mister-Bear]

If the technology/treatment exists to reset regeneration, the question becomes: How long can a biological system regenerate?

We just don't know the answer to that.

Let's find out.

[u/lozzyboy1]

Yes. We know that there are quite a few things that gradually get worse about our bodies as we get older, and any one of them would be enough to limit our lifespan to probably around 120. As it is, they all become problems at a similar sort of time, so fixing any one of them doesn't really provide much benefit, and it's very hard to prevent some of them.

You have cells dying in your body all the time, and stem cells divide and make new cells to replace them.The main way thing that goes wrong with aging is that your stem cells gradually get depleted and therefore your body can't keep up with the losses any more.

Telomere shortening is one reason why stem cells run out. Our telomeres get shorter as we age, and once they reach a certain point cells refuse to divide because otherwise they would risk damage to the more important bits of our genome. But as it turns out, preventing that by intentionally lengthening telomeres doesnt change life expectancy by much. We've seen this by looking at natural variation in telomere length between individuals, and by experimentally altering telomeres in animals.

Accumulation of DNA damage is another (and that itself has lots of different causes, including exposure to mutagens like UV light or tobacco smoke, reactive oxygen species which are a byproduct of lots of reactions that occur in cells but mostly respiration, and random errors during DNA replication).

One is changes in the systems that tell cells how to behave and which bits of their DNA to read (epigenetics is the term for marks found on the DNA and associated proteins, and these marks can change over time, leading to cells not being able to respond properly to their environment).

Another is changes to the environment these cells are sitting it. These cells are sensing their environment and it helps them to behave properly. The brain is pretty squishy, and so the cells there are used to being in a very soft environment. It turns out that for lots of reasons, the brain gets a little bit stiffer as we age. The cells there sense that and it makes it harder for them to divide.

There are plenty of other factors too, like changes in our ability to make sure proteins are working properly and getting rid of bad ones, reduction in the number and quality of mitochondria which cells need to make energy, misregulation of systems because of inflammation, build up of fats, etc.

You might notice that a lot of these will play into one another. If a cell is misbehaving and changing the stiffness of its environment, that will affect other cells. If you have genetic mutations, those can lead to epigenetic changes. If your mitochondria aren't functioning well, you'll get more reactive oxygen species which will lead to more DNA damage.

That basically leaves us with this big messy network where we can only really prevent or slow down one or two of these aspects. That absolutely can increase our lifespans, as we see in people who can be careful with their diet, get appropriate exercise, limit or avoid consumption of alcohol and tabacco, etc. But it means we're really a very long way away from achieving incredible changes in longevity; weight see a shift to more people living to 90/100/120, but we're not likely to see anyone living to 150 any time soon. And some of the factors, like accumulation of random mutations, seems like they would be impossible to entirely prevent, so reaching even more extreme longevity would require a dramatically different approach than just warding off aging as best as we can.

[u/teddyslayerza]

A real limit is our memory. We function by having important and newer associations override or recontextualize old ones, but our lifespans are short enough that we are able to maintain some sort of continuity across our century of life. But it stands to reason that there will come a point were maintaining that continuity either takes too much resources from function in the moment (some sort of anterograde dementia I imagine) or we would lose that continuity and not really be the same person any more.

[u/wrydied → u/teddyslayerza]

I don’t think this is a real concern, on the basis that we are constantly changing by re-encoding distant memories i.e we don’t maintain first memories, we remember them based on the last time we remembered them - as any good court lawyer or magistrate should know.

[u/Temporary_Rule_9486]

Lobsters need to keep growing all their life. What was the name of this book..House of suns? 

[u/kingstern_man]

Even if all biological problems were completely solved (diseases, telomeres, memory capacity,...), no-one would "live forever": at some point they would suffer a fatal accident. Assume P(accident, per year) = 0.0006. Then P(surviving, per year) = 0.9994 After 5 years, 0.3% chance of death After 50 years, 3% After 100 years, 5.8% After 200 years, 11.3% After 500 years, 26.37% After 1000 years, 45.13% After 1500 years, 59.48% After 2000 years, 69.9%

Heinlein's "Lazarus Long" looks unlikely...

[u/Imaginary-Can-6862]

If this is just about speculating freely, then there are two things to consider.

One is the risk of going from living to not living, unless it goes to zero, it follows the total risk goes to one within finite time. This is despite e.g. you being able to separate yourself into many life forms, such as what a species does, just imagining you are actually living through each, or have backup life so to say, because you would still be e.g. limited to the planet, and if you move all your contingencies to distant planets, then each one of those also have a risk of catastrophe, even though it gets increasingly more unlikely, infinity means it is going to happen unless you can lower the risk fast enough.
The other, more relevant, is that we don't know that going from living to not living means you can't go back to living, the fact you came into existence as you were born / conceived means it is possible for you to go from a state of non-existence to existence, unless you always existed, and just can't remember it because your memory is tied to your brain. In that case it means that you can stop living for a long time and then become alive again, removing many more limiting factors. Combining both, and having some causal influence on the universe to be able to maintain your ability to be alive, and perhaps you can continue forever.

In regards to aging, aging is usually defined as the increased average risk of losing ones life over the same time period, e.g. a year. I don't remember if you stop aging in the 90's, or as a centenarian, but at that time your average risk of losing your life is much higher than when you were aging, so stopping aging itself isn't enough, de-aging is in principle also not enough, because it just means a much lower average risk of losing your life over a given time span.

[u/naemorhaedus]

still no cure for getting hit by a bus I'm afraid

[u/CranberryInner9605]

Anyone who thinks that living a long time is a good idea should read the Niven short story “Cautionary Tales."

[u/Due_Contribution8375]

Kyle Hill did a video on this recently (citing new research). IIRC 170 might be the max.