I am afraid that the true answer will turn out to be: There are thousands of mechanisms that gradually kill you after hundred years or so. They are side effects of something else, or maybe trade-offs like “this will make your muscles 0.01% stronger or faster, but they will accumulate some irrepairable damage that will ruin them in hundred years”. They are not selected against by natural selection, because for each of them individually, the chance that this specific mechanism will be the only cause of your death is very small.
At least it seems to me that if a new mutation would arise, that would provide you some small advantage, in return for certainly killing you when you are 150, the evolution would happily spread it. And this probably kept happening since the first multicellular organisms have appeared. EDIT: Okay, probably not.
There are some reasons for optimism, although nothing that conclusively rules out a large number of mechanisms.
First, humans only have ~tens of thousands of genes, so thousands of independent mechanisms would mean that a substantial portion of all the molecular species in the organism are involved. On top of that, the vast majority of components—from molecules to organelles to cells—turn over on timescales many orders of magnitude faster than aging, which severely limits the number of possible root mechanisms. Just based on that, I’d be very surprised to see even hundreds of independent mechanisms. Tens of mechanisms would be plausible, but single-digits seems most likely. It’s just extremely rare for biological systems to operate on timescales that slow.
Another direction: there are animals which display negligible senescence (i.e. no aging). Most of them are not very close to humans on the evolutionary tree, but even distant animals have reasonably similar core mechanics, so there’s only so much room for divergence.
(Not the author, obviously.) Part of my personal intuition against this view is that even amongst mammals, lifespans and the way in which lives ends seems to vary quite a bit. See, for example, the biological immortality Wikipedia page, this article about sea sponges and bowhead whales, and this one about naked mole rats.
That said, it’s still possible we’re locked in a very tricky-to-get-out-of local optima in a high dimensional space that makes it very hard for us to make local improvements. But then I suspect OP’s response would be that the way to get out of local optima is to understand gears.
I agree with both replies, that if there are species, including mammals, which live very long, then the mechanisms responsible for killing us have not been here “since the first multicellular organisms have appeared”.
Was aging death reinvented independently by so many species, because statistically, at some moment most of them encountered a mutation that provided a Faustian deal of an advantage X, in return for death at age N (for various values of X and N)? And the few immortal ones are simply those who avoided this class of mutations (i.e. had the luck that any mutation causing death turned out to be a net disadvantage)?
Or are the mechanisms of aging death more general, and there are a few species which found out how to avoid them? In other words, are the immortal species descendants of mortal species? Is there a chance to find the answer in archaelogical record? (Whether the common ancestor of a mortal species and an immortal species was mortal or immortal itself.) I’d like to see the entire evolutionary tree, with colors, like these species are/were mortal, these were not.
Or are the mechanisms of aging death more general, and there are a few species which found out how to avoid them?
There’s a fun question. One of the main things which spurred the current wave of aging research was the discovery that certain interventions increase lifespan across a huge range of different species (e.g. calorie restricted diets). That strongly suggests conserved mechanisms, although it doesn’t rule out some species-specific mechanisms also operating in parallel.
I haven’t looked this up, but I’m fairly confident the negligibly-senescent species are descendents of species which age. Examples of negligibly-senescent species include someturtles, rougheye rockfish, and naked mole rats; I’m pretty sure the closest relatives of most of those species do age. Again, I don’t have numbers, but I have the general impression that negligible senescence is an unusual trait which occurs in a handful of isolated species scattered around the evolutionary tree.
Thanks for working on an important problem!
I am afraid that the true answer will turn out to be: There are thousands of mechanisms that gradually kill you after hundred years or so. They are side effects of something else, or maybe trade-offs like “this will make your muscles 0.01% stronger or faster, but they will accumulate some irrepairable damage that will ruin them in hundred years”. They are not selected against by natural selection, because for each of them individually, the chance that this specific mechanism will be the only cause of your death is very small.
At least it seems to me that if a new mutation would arise, that would provide you some small advantage, in return for certainly killing you when you are 150, the evolution would happily spread it. And this probably kept happening since the first multicellular organisms have appeared. EDIT: Okay, probably not.
There are some reasons for optimism, although nothing that conclusively rules out a large number of mechanisms.
First, humans only have ~tens of thousands of genes, so thousands of independent mechanisms would mean that a substantial portion of all the molecular species in the organism are involved. On top of that, the vast majority of components—from molecules to organelles to cells—turn over on timescales many orders of magnitude faster than aging, which severely limits the number of possible root mechanisms. Just based on that, I’d be very surprised to see even hundreds of independent mechanisms. Tens of mechanisms would be plausible, but single-digits seems most likely. It’s just extremely rare for biological systems to operate on timescales that slow.
Another direction: there are animals which display negligible senescence (i.e. no aging). Most of them are not very close to humans on the evolutionary tree, but even distant animals have reasonably similar core mechanics, so there’s only so much room for divergence.
which animals?
See Highlights of Comparative and Evolutionary Aging.
(Not the author, obviously.) Part of my personal intuition against this view is that even amongst mammals, lifespans and the way in which lives ends seems to vary quite a bit. See, for example, the biological immortality Wikipedia page, this article about sea sponges and bowhead whales, and this one about naked mole rats.
That said, it’s still possible we’re locked in a very tricky-to-get-out-of local optima in a high dimensional space that makes it very hard for us to make local improvements. But then I suspect OP’s response would be that the way to get out of local optima is to understand gears.
I agree with both replies, that if there are species, including mammals, which live very long, then the mechanisms responsible for killing us have not been here “since the first multicellular organisms have appeared”.
Was aging death reinvented independently by so many species, because statistically, at some moment most of them encountered a mutation that provided a Faustian deal of an advantage X, in return for death at age N (for various values of X and N)? And the few immortal ones are simply those who avoided this class of mutations (i.e. had the luck that any mutation causing death turned out to be a net disadvantage)?
Or are the mechanisms of aging death more general, and there are a few species which found out how to avoid them? In other words, are the immortal species descendants of mortal species? Is there a chance to find the answer in archaelogical record? (Whether the common ancestor of a mortal species and an immortal species was mortal or immortal itself.) I’d like to see the entire evolutionary tree, with colors, like these species are/were mortal, these were not.
There’s a fun question. One of the main things which spurred the current wave of aging research was the discovery that certain interventions increase lifespan across a huge range of different species (e.g. calorie restricted diets). That strongly suggests conserved mechanisms, although it doesn’t rule out some species-specific mechanisms also operating in parallel.
I haven’t looked this up, but I’m fairly confident the negligibly-senescent species are descendents of species which age. Examples of negligibly-senescent species include some turtles, rougheye rockfish, and naked mole rats; I’m pretty sure the closest relatives of most of those species do age. Again, I don’t have numbers, but I have the general impression that negligible senescence is an unusual trait which occurs in a handful of isolated species scattered around the evolutionary tree.