Physics Makes Aging Inevitable, Not Biology (2016)
nautil.us
nautil.us
https://en.wikipedia.org/wiki/Negligible_senescence
That strongly points toward a biological reason for aging. I mean, sure, the heat death of the universe is inevitable, but that is not even remotely on the same time scale as the life span of complex organisms.
Are we then not saying 'not aging is impossible, here are some examples of things not aging' ?
If you live long enough, you will accumulate damage even in the absence of aging. The hope for humans is thst we develop means to both halt aging, and repair accumulated damage.
> The constant risk is environmental (accidents, infectious disease), but much of the exponentially increasing risk is due to internal wear.
... but as examples of negligible senescence show, that is simply not the case. This "exponentially increasing risk" is just not a given.
http://experimentalbiology.org/getattachment/2014/About-EB/P...
"Naked mole rats also appear to remain spry and healthy even in the final years of their long lives"
Sounds like they changed the shape of the curve, not merely stretching it out.
As I posted in a comment below somewhere, I think it is the complexity of the nervous system that drives a tradeoff with repairable bodies. A brain does not want to have to retrain itself, but the history it stores makes it entropically very energy intensive to maintain. I think this drives most of what we see here: things without complex nervous systems (pyramidal neurons and such) have no disincentive in persisting indefinitely, because their relatively simple nervous system does not take increasing amounts of energy to maintain, and thus they can budget their energy for growing and other evolutionary benefits.
Most mammals have to budget their energy such that they can maintain the state of their brain, and thus have a disincentive for eternal aging: we either need to start forgetting earlier memories, which could potentially be bad, or we have to start paying the energy price of maintaining a very complex system. Energy usage tends to get optimized by evolution, so then, we end up trading off longevity.
Mole rats, however, being eusocial, have a different evolutionary strategy than most mammals. Since the entire colony effectively behaves as a single metaorganism, losing its experienced members is particularly painful if they have valuable institutional memory. Thus, there is stronger evolutionary pressure to pay the cost of maintaining these systems.
(I realize that isn't really what you were commenting about, but it does give a thermodynamic reason for their longevity, but not one simply based on temperature.)
... which implies that it is biologically possible to maintain them, which implies in turn that aging is a biological trade-off and not a physical inevitability.
The only assumption is that no warm-blooded animal is among those with the lowest internal temperature. The existence of some "cold-blooded" animals that are warmer does not change that.
And, less importantly, I already knew that.
The author mentions in a comment that he didn’t write the headline (as is so often the case). His original headline (“Aging: where physics meets biology”) is much less controversial.
Let us hope that through his HN comments ashelmire can fight the tyranny of linkbait titles.
We actually do the opposite!
Entropy always increase in closed systems, but we are not talking about a closed system here.
My understanding is that, from the point of view of physics, there is not a reason for not being possible to decrease entropy in the body increasing entropy outside (expending energy).
Also, this is untrue: "DNA can only be repaired when there is an intact replica to copy".
If you have, for instance, three incorrect copies of a DNA sequence you can compare them in order to get the original sequence. The probability that the copies are damaged at the same point being very small.
> Either way it'd be an unqualified success.
Certainly. I'm just picking nits on the math.
That is, when the protons are gone, so is chemistry, electronics, and so forth.
Of course lots of protons -- likely the vast majority of them -- will end up in stellar collapse remnants anyway, and in the black hole cases you're also not getting the protons back, whether or not they would eventually have decayed. The rest will be in an extremely tenuous gas and dust far outside the black-hole-dominated remnants of no-longer-star-forming galaxies, and most of that will be neutral hydrogen (and in that case if the proton decays, the residual positive charge will annihilate the electron, so all you have left is light flashing off in several random directions).
"Mining" for new components in the extreme future looks like a losing proposition even without proton decay; with proton decay, you run out of usable material all the sooner.
> no reason to assume that you can't replace parts with decayed protons with new parts
with reasons.
This is an argument of christian apologists against evolution as well: "evolution can't be true because it violates the 2nd law of thermodynamics". And this superficially apparent conflict has the same resolution: if energy can be put into a system, it absolutely can have less entropy (or more organization, or more specialization) over time.
That's the point. If we would fall for the authors interpretation of the second law of thermodynamics humans couldn't exists, as they wouldn't be able to develop in the first place.
What are you comparing them to? Sure, we humans can do this in a lab, but in a nucleus, there is only the anti-copy of the other side of the nucleotides. Most eukaryotes just use one of their chromosomes and both are needed for mytosis/myosis. Cells have no way of knowing which copy is the 'correct' one. If there is more than one copy of a DNA strand floating about, cells really do not like this and, in many eukaryotes, will deal with it in a very complicated dance. Prokaryotes/Archea are a wildly different story however.
But I think that Medawar's argument was that all animals are subject to random accidents, predation, disease, etc. This will produce an exponential distribution of lifetimes. As a consequence, beyond a few half-lifes, there will be exponentially few members of the species. So if there is some mutation in the DNA which causes the animal to die beyond that time, there will be so few members of the species around to exhibit it that there will be effectively no selection pressure to keep this mutation out of the population. As a consequence, even if you started out with a population that could in theory live forever, it will very quickly become riddled with mutations which start to kill the animals after a couple of half-lifes. From the outside, it would appear as though the animals were "programmed" to die at a certain time.
George Williams presented a similar argument, but also claimed that some mutations can increase an animal's fitness early in life at the expense of decreased fitness much later in life. If the animal will likely have died of some accident by that time anyway, this trait effectively has no downside and will be selected for.
More oxidation, more metabolism, and more turnover gives you more chances to break down/accumulate mutations/shorten telomers, but that’s an artefact of cells running out of their means for rapid growth. These can be modified. Look, for example, at the countless immortalized cell cultures.
It is a biological problem with roots in statistical and biomolecular physics, like literally everything else in biology.
I imagine the internal temperature of the Great Basin bristlecone isn't that high either. However, see another thread on this article where someone mentions Naked Mole Rats, warm-blooded animals with long lifespans.
That's an entirely backwards way to look at the problem: selection works with living organism and the only reason an organism lives or performs a particular function is that it was evolutionary useful to do so for it's genes. Aging therefore is something natural selection can't discern on: at some point, the number of additional offspring that can be produced becomes negligible compared to the exponentially increasing probability of death from environmental causes (just like the test tube example); so organisms that quickly reproduce young are much more successful than long lived organisms.
It's easy to see then, why the long term accumulating damage, irrelevant for a young organism, can't be evolved out of. We are essentially "single use" from an evolutionary standpoint.
Therefore, advanced age (anything exceeding useful reproductive life) is something outside evolution. Anything happening at senescence time is simply a byproduct of the mechanism selected for ensuring success at young age, a system that continues to work by momentum and not design or selection, up to the inescapable moment when entropy catches up to it.
But there is no fundamental physical reason for it, and it stands to reason we could create biological machines that maintain the power to self repair indefinitely, given suficient outside energy. It's just enormously more complex than "curing aging", it means artificially redoing billions of years of evolutionary choices that were good enough for reproduction. It's a God level task.
I don't know evolutionary biology enough to estimate how big of a selection pressure that is, though. Maybe much lower than first-order effects, maybe not so much.
> But there is no fundamental physical reason for it, and it stands to reason we could create biological machines that maintain the power to self repair indefinitely, given suficient outside energy.
Agreed.
Per Google, our closest living relatives, chimps and gorillas, very rarely display grandparenting behavior and have comparable lifespans.
To me this is the main problem with this article. The (ever more common) notion that aging is a disease should be combated with philosophical arguments, not shaky physics such.
And you're not the person I was asking anyway.
For battling decay. It can be easier to grow a new body from a single cell, than to keep patching up a running one.
A desire for immortality is the individual point of view. In the big picture of things, there is little to no selection pressure for immortality. There is plenty of selection pressure for adaptation and change, and death and rebirth are effective tools for that.
And we do the same in IT. Sometimes the old system has piled on so many hotfixes and customizations that in order to upgrade, you can only build up a new system next to it and transfer your data to it. Then you shutdown the new system. That's procreation, learning, and death.
Yes there is... an individual that does not age will have many more offspring than one that does.
And viruses and parasites, probably a very hard problem to crack otherwise. Reboot from a fresh image.
For me eternal youth will happen sooner or later, probably 100 years in the future,50 years,200, but it will happen when we understand biology and could repair tissues, something that we could not do today. Today we don't heal a bone fracture, we let nature do it, because we really do not control or understand the process deeply. The body knows how to kill viruses, we do not. Only bacteria after copying nature with antibiotics is under our control but we do not control or understand the process again.
This is not being immortal, people will die in wars, or accidents, or being killed, but they will be young all our lives.
For this people will probably have to go to a place that resembles the uterus and spend some time there as the body repairs itself:
https://pm1.narvii.com/6320/0940afb810563b4c4b67c4efb773adc0...
After that you will be 20 years younger. Not different from repairing a car.
(1) I haven't seen this article mentioned here:
https://gravityandlevity.wordpress.com/2009/07/08/your-body-...
I am not sure if the article addressed this model in particular, but I found it compelling.
(2) It occurred to me that most of us don't fear death as much as becoming increasingly useless as we become older due to ill health and physical weakness.
It would be not much fun living to be 200 if the last 120 years were spent in a bed or wheelchair.
So what we want is not to increase the lifespan but to prolong the amount of time we can spend in relatively physically prime condition. I am sure this is not an original thought, but something worth mentioning.
While I agree (and while I additionally think any plausible advancement of that degree would necessarily need to increase healthy lifespan), I'd also say that it'd be inherently preferable to dying, and it would give 120 more years to solve those problems. We can come up with a lot of medical advancements in 120 years.
A causation is the way to describe reality, not the fundamental feature of the reality. And so, the causation should follow the goals of researchers. While searching for ways to live forever, we will not benefit from evolutionary explanation (we cannot fix evolution to get longer lives), but thermodynamics explanation could be useful (if its true, of course). So, thermodynamics is a good candidate for the role of "fundamental cause".
Aging and reproduction feels like the same pattern. Sure, you may be attached to the legacy code, but sometimes it makes more sense to do the big rewrite instead. Legacy code performed well for its time, now it is time to retire it.
Comparing lifespan to the bonds of a molecule is also pretty cringey. :-/
Obligatory link to Aubrey de Grey: https://www.ted.com/talks/aubrey_de_grey_says_we_can_avoid_a...
People concerned about aging would really, really prefer there was a biological cause for aging. If it's biological, there's a chance there's a trivial fix (gene therapy, supplements, ...) for it, or at least a trivial incremental fix that will get you an extra ten or twenty years while you wait for the next incremental fix to be developed for the next problem down the line.
If the problem is physical, caused by the wear and tear of your metabolism, the problem becomes significantly harder. Not impossible (after all, cell lines can be immortal, trivially, we have an unbroken line of single celled organisms dating back billions of years), but you are potentially looking at rebuilding mammalian life from the ground up before you make even incremental progress. And the only maybe-possible-but-the-animal-models-are-mixed incremental fix we have is to lower your metabolism by living your entire life on a calorie-restricted diet, or maybe by being just a little too cold all the time.
I'm all for still devoting a healthy amount of resources to trying to find easy biological fixes to aging, because I'm old enough now that probably nothing else would happen fast enough to keep me, personally, alive, but if it turns out most of the problems are physical, we shouldn't shy away from acknowledging that just because it makes the work of beating death harder. We aren't trying to beat aging because it seemed like an easy thing to do.
The germline and hydra must be brought forward every time someone says that aging is an inevitable, universal consequence of thermodynamics:
http://www.demogr.mpg.de/en/news_press/press_releases_1916/f...
The common perception that the bodies of all living beings age, is wrong. This has now been proved by a long-term experiment with the freshwater polyp Hydra, a microscopic animal. In a unique long-term experiment researchers created artificial conditions for the tiny water animals with their flimsy tentacles, which were free of fatal natural threats like predators. For almost ten years they have cared for of about 1,800 of the Hydras. Overall, the team has counted 3.9 million observation days of individual Hydra. The number of natural deaths per year, however, can be counted on one hand. On average there have been only five. When a Hydra passed away it was mostly due to laboratory accidents, such as a polyp sticking to the lid of its bowl and then drying up or simply having been dropped on the floor. From of the few natural deaths that remained researchers calculated Hydra's mortality. It is so low that even several lifetimes of researchers would not suffice to observe the end of the lifecycle of the polyps. Even after 500 years five percent of a cohort will still be alive. For two out of twelve of the Hydra cohorts under investigation, the risk of death was actually so small, that it will take 3,000 years until only five percent of the polyps remained. "Hydra apparently manages to keep its body young because it does not senesce by accumulating damages and mutations, as most other living beings do. Hydra are probably able to follow a special self-preservation strategy, as its body and cellular processes are rather simple." For instance, Hydra are capable of completely replacing parts of the body that are damaged or are somehow lost. It can even fully regenerate if its body is destroyed almost completely thanks to a high number of stem cells. Stem cells are capable of developing into any part of the body at any time. Additionally, as Hydra replaces all of their cells within only four weeks, it regularly and quickly expels all cells that have been changed genetically by mutations. Thus, damages have little chance to accumulate.
Current thinking on why near all species fail to behave like hydra looks somewhat like this:
http://arxiv.org/abs/1103.4649
Understanding why we age is a long-lived open problem in evolutionary biology. Aging is prejudicial to the individual and evolutionary forces should prevent it, but many species show signs of senescence as individuals age. Here, I will propose a model for aging based on assumptions that are compatible with evolutionary theory: i) competition is between individuals; ii) there is some degree of locality, so quite often competition will between parents and their progeny; iii) optimal conditions are not stationary, mutation helps each species to keep competitive. When conditions change, a senescent species can drive immortal competitors to extinction. This counter-intuitive result arises from the pruning caused by the death of elder individuals. When there is change and mutation, each generation is slightly better adapted to the new conditions, but some older individuals survive by random chance. Senescence can eliminate those from the genetic pool. Even though individual selection forces always win over group selection ones, it is not exactly the individual that is selected, but its lineage. While senescence damages the individuals and has an evolutionary cost, it has a benefit of its own. It allows each lineage to adapt faster to changing conditions. We age because the world changes.
But even if you don't subscribe to that particular view of what constitutes a consistent identity, the issue is still fundamentally thermodynamic. It is not a task of just maintaining a particular dynamic system indefinitely, but maintaining one with memory. If the hydra is storing absolutely no information about its past states, then it isn't evolving, and it is completely at the mercy of its environment. This makes it more akin to fire or a piece of iron rusting than life: just a consumptive chemical process. This harmonizes exactly with your last paragraph, things need to store some amount of memory of past failure if they are to adapt. This storing process is exactly aging, but how it manifests in different organisms can obviously be very different. Landauer's principle, then, tells us that since information is thermodynamic, so too must be aging. However, I don't really think the author of the original article was arguing at that deep of a level. In humans, many of the processes that we consider to be the detrimental effects of aging do occur because the components that make us do have memory effects. One particular example would be the cross-linking of elastin, causing degradation of vascular system efficiency.
But aging truly is an inevitable consequence of thermodynamics, it's just that since everything is a consequence of thermodynamics, it's not a particularly illuminating argument.
Most of our cells in our bodies are renewed in a few months or something. Yet people don't assimilate this to death.
Now, you may object that neurons don't follow this rule, so our subjective identity is not concerned by this. To this I will say that when people are concerned about ageing, they are indeed concerned about what happens to their brain (that is, neuro-degenerative diseases), but certainly not only that. The state of all the other tissues is at stake : the heart, the bones, the skin and so on. Yet all of these are renewed on a regular basis. That renewal is not perfect though, and it's that imperfection they blame on ageing.
http://experimentalbiology.org/getattachment/2014/About-EB/P...
It might be harder but it cuts through the issues of aging and accidents.
Wonder what evolution in the simulation would come up with as the best lifespan.
Could be a fun weekend project.
Feynman's Lectures on Computation are a great read that explores the physical limits of computation and information.
If that is true, I wonder what the total live expectancy for our species is. I mean shouldn't stem cells be affected by that too?
other then that, stress causes aging. people who reduce stress keep their mind younger and often live longer and are less effected by diseases common among aging biological systems.
ofcourse, stress then isn't the root cause of aging... as stress takes many forms, what causes stress is. so the answer to what causes aging is probarbly not a single point or item, but a combination of factors which all cause some form of stress on the system.
From what I can tell, the reason why aging is so prevalent is that most species haven't had time to evolve the mechanisms that solve the problems that appear later in life. If there's some truth to this, you'd expect that the species that are better at this also tend to be the simpler ones, since for those simplicity also means smaller diversity of things going wrong.
We're so successful that the best strategy for a lot of other animals has been to be food or company. Cows are doing pretty well because of us.
Bees, for example, have evolved to have very strong but non-reproducing organisms in a community. There's still selection pressure on a hive to produce stronger and longer lived workers.
As I understand it, the general mechanism involved here is usually called kin selection, and has been discussed as far back as Darwin, both as a potential problem with the theory and as a fatal defect in the Lamarckian view.
Nature does not have an ageing problem.
Aging and dying is a necessary condition for life to even exist.
It's only humans who have a problem with it.
In other words, the organism's procreation could be more successful if the olds contribute to the youngs survival to reproductive age.
In other other words, you haven't really reproduced if your offspring don't reproduce.
The problems that occur later in life do not have any affect on that. On the other hand, for communities and 'tribes', not keeping old members around make them more responsive to change.
Why do species have a prime? Why don't they reach a certain "amount of reproductibility" and stay at that constant amount forever?
The fact that a prime exists is just one instance of one of those things that evolution hasn't figured out yet. Or maybe there's a different reason why it hasn't happened, but that reason is certainly not fundamental physics.
The background for this theory is that evolution is gene-centric, not individual-centric. What is fitter is what will better propagate the genes, which is not necessarily what's best for the individual.
So it can build "designs" that have a better chance of propagating genes earlier on, where features that help that (I'm making this example up, but say something like faster growth) may later have a deleterious affect on the individual. And since evolution is gene focused, there's not as much evolutionary pressure for a longer and healthier life for the individual.
In general you can do things significantly faster with a lower number of bits if you leave out the redundancy required for error detection or correction.
How many of these elders do you need to care for your children though? 2, 4, 8, 16, ...? Maybe grandmother hypothesis works, but evolution would not care much for a great-grandmother hypothesis.
This actually seems consistent with the current lifespan of human beings. Most people have grandparents when they are children. Very few people have great-grandparents.
I doubt it.
IMO, aging is just a way to ensure that dying occurs.
And dying is necessary for evolution, as it's easier to replace the old with a new generation that understand better the reality than changing the current generation.
Old habits literally die hard.
I know evolution doesn't work that way, but definitely sounds like an interesting thought.
> So that would be like killing a program and respawning a new process instead for some
> benefit like reclaiming memory or speed ...
>
> I know evolution doesn't work that way,
I do wonder whether there is a point where the lost experience of other individuals weighs heavier than physically better adapted individuals. In a way, culture serves as a way to preserve information that would be lost by aging.
Another way to interpret this would be “increased diversity.” Which is a selection advantage in an environment that experiences change. While organisms don’t have “clocks” they do have different rates of aging, implying a biological control. And species that live longer tend to hold on to longer genomes. This tends to mean that short lived species hold diversity in their population while Long lived species hold diversity in every genome. Both operate as fitness advantages, and seem to imply that aging is a biological quality.
All species that didn't die of old age (if there were any) have disappeared regardless.
Though you won't see any of them developing rockets and space-stations. To what extent that is an evolutionary advantage on our part I'll leave to general consensus.
By your same logic, you could pick any species at all and call their traits "the winning strategy for long-term survival" as long as you live contemporaneously with them.
There are known species that don't seem to exhibit planned senescence--the naked mole rat is a commonly discussed example. Check the Wikipedia page for biological immortality[1] for more examples and info.
The article essentially claims that ageing is likely a result of interacting thermodynamic processes. If so, then without specific preventative measures, organisms will "age".
The discussion at this level is pretty hand-wavy. So without introducing more rigor, the best we can say is probably something like this: there hasn't been strong selective pressure in the past to develop anti-ageing strategies.
Emphasis is not exactly "can't", rather it is "has not yet"...
All species that used the Time Travel flatulence method have disappeared.
That's not true. An previous generation might actually be better suited to a particular environment than the current generation.
Besides, if you look at all big society changes, you'll notice the old generation of people that lived before the change have a much harder time to accept / adapt to the new order while it's natural to the new generation.
The research 100% disagrees with you.
Aging even occurs in computers: every computer will eventually get in an unexpected state that it cannot recover from. This could even be caused by a stray UV ray that flips a bit in an important memory sector.
Its main point is that ageing follows a roughly Gompertz-Makeham distribution, the statistics of which also nicely describe "wear-and-tear" processes like protein breaking.
The final paragraph even explicitly addresses the concerns you raise about possible routes to intervention in ageing processes.
You still see lifetimes next to 'stable' elements because nothing is actually stable on long enough time scales.
Granted, 10^~45 years may seem like a long time, but I am sure it would still feel short if that was the norm. ;-)
Or evolution isn't interested in this outcome. It is probably benefitical for a species if members die once they get old, to make way for the new.
There doesn't need to exist some conspiracy in a species to explain death.
The new compete with the old for resources, mates and avoiding predators ( don't need to be faster than a bear, just faster than your friends ). The old have a lifetime of cellular damage, injuries, diseases etc having done damage to their bodies. These things make them more susceptible to predators and dying, being unable to mate do to body problems, being unable to compete for mates and being unable to compete for resources and dying.
Evolution can only select for that which leads to reproduction and survival of progeny. Whatever happens to an animals body after it passes on genetic material is of no matter to the new animal, excepting where interactions with the existing creature can be of benefit and detriment to the young. A mother crocodile protecting her young will have more surviving young than one that eats them, and the former will tend to out-compete the latter and continue existing.
Assuming a long-lived effectively-immortal species that only reproduces when resources are available for doing so exists, what would happen if a single gene for fast reproduction arose? Resources would run out, and animals would have to start competing with one another over them. Assuming the old and the young are both 50/50 for living or dying in any given fight, the creatures that reproduce slower would be at a disadvantage because there would always be more and more of the fast reproducers.
The only stay for the fast reproducers would be whether they can keep their young alive. So perhaps if they have young one or two at a time instead of twelve at a time, their one or two might be more likely to survive to adulthood whereas the twelve would tend to die well before then.
In the end a balance would emerge where members of the species would tend to reproduce as much as possible where they expect to be able to properly nuture the offspring. Those reproducing slower would fail to compete for resources. Those reproducing faster would fail to keep their offspring alive.
But certainly, those that are steadfastly not reproducing for "the good of the species"? These would be out-competed in short order by the rest.
By saying "evolution isn't interested in this outcome" you're not so subtly anthropomorphizing evolution, but evolution does not have to have an "interest".
3 things are required for evolution to manifest
1. Variation
2. Heritability
3. Selection
Evolution has no "interest" in things that are "beneficial for a species". Evolution is an epiphenomenon of the existence of the three factors of variation, heritability, and selection. [0]
[0] https://www.ncbi.nlm.nih.gov/books/NBK22508/
EDIT: Just a comment that I didn't see one of my sibling replies whose first sentence I practically duplicated. Clearly at least two of us do not like anthropomorphizations of evolution.
If someone else reads higher order intentionality to it than that, why should that stop me from using a convenient turn of phrase? Sure, if someone wants to think that I’m implying my computer is sentient then they’re odd and incorrect but I’m not going to stop saying it. It’s convenient.
Species is a nebulous conflation of genetics, behavior and habitat. Some like to say that ability to bear fertile offspring is the limit to determine speciation. This definition allows ignoring the mule offspring of horses and donkeys, strengthening the position that the latter are different species, but fails on the liger, which can be fertile with few people claiming tigers and lions to be of the same species. So breeding is insufficient to determine species.
Species is then more a measure of the human perception of things being different than any more specific quality.
The idea that a species could somehow convince its constituent individuals to act for the benefit of the species isn't useful. Individual members of the species will each act with self-interest in the pursuit of resources, mates and territory. Altruistic behaviors will propagate by out-competing "cheating" behaviors, and most often multiple simultaneous behavior strategies will exist within a population. Even within an individual, which might choose different actions in different situations.
If there was more benefit to living longer in the species, cheaters would certainly exploit it in the face of "dying for the good of the species". However, if the detriments of living longer cause the individual to be unable to compete with shorter living individuals, the species will continue to tend to a shorter lifespan.
Suggesting that the creatures will conspire to die and make room for their offspring suggests that the anthropomorphized idea of species is causing faulty assumptions about the nature of the subject to be held, and that the metaphor should be discarded as detrimental to understanding.
I wasn't suggesting that, nor was anyone else. I really think you're reading too much into it. I'm merely suggesting, the rather obvious thought that you and everyone else here probably understands, that a group of animals where the weak and old die to make way for the new (obviously not in a self-sacrofice way, but just how the animals are) will probnably be more successful, on average, that a group of animals where the old very slowly, or indeed even never, die.
Now, I apologise that the use of language "evolution is not interested in this outcome" caused such confusion.
If you analyze death in humans you will quickly see it's not as simple as you might think. Cells die in humans all the time, in fact millions of your cells will have died before you finish reading this post. Some organs, such as the skin and the colon, depend on killing (ie. triggering programmed cell death) large amounts of their own cells to function, and even internally it is used as a tool where more gentle means would probably work, such as bone growth and repair, which involves large amounts of cells dying.
Second if you analyze what do humans die from if they die "peacefully in their sleep" ? Well they die of "natural causes". That's awfully noninformative. What do you die from, really ? Well, you die from energy exhaustion in your blood, causing multiple organ failure, leading to poisoning, leading to more organ failure, leading to more organ failure, leading to a relatively slow and orderly shutdown of your body. At this point there are certainly things that your body could do to stay alive (all it takes is accelerating energy dumping in the blood), but it doesn't. Your body chooses to die at a certain point, and it very gently kills itself at a certain point. There's a feedback mechanism that makes this happen. It takes something like a few weeks to actually die. People often report that they can feel this happening and the timeframe seems to be such that you have plenty of time to say goodbye or do whatever needs doing.
Because this is how death works in animals, it does mean that if someone dies from natural causes, the organs are thoroughly poisoned and have had to take extreme measures to avoid dieing completely before the body dies. This usually includes sacrificing significant parts of the organs. This means you cannot safely transplant organs from people who die slowly. But this is another subject.
Thirdly while everybody focuses on death, there are pretty serious symptoms that occur, frankly they start happening before you're even born, but by the time you're 60 there will be very obvious external symptoms of aging. Most of the ones we focus on boil down to cell senescence: your cells choose to systematically become less active as they age, and at a certain cell age (measured in generations) they kill themselves, and they are mostly, but not entirely replaced (that's the function of stem cells: replace senescenced cells that have killed themselves, and that's why they're such a big focus for anti-aging research). This is a mechanism that, essentially, stores your age, and the total energy use of the environment of the cell, into every DNA chain in every cell, and responds to it. Your age gets too big, and it kills itself. You use a certain amount of energy over time, and your cells kill themselves. Long before they kill themselves, they will force themselves to use less energy, even when it means things go wrong (this is why you should -and will- systematically eat less as you age).
And of course there are exceptions to death. There is a continuous cell line from every living human (even clones and the like) to the first human pair, and probably even to the very first living organism. So specific cell types, most notably the procreation related cells, are exempt from death. They do age, as in they measure their age, in years and energy, and they do kill themselves (even a lot quicker than normal cells) but they reset their age every time procreation happens. So, assuming you've got children, not all of your cells will die when you die.
There is a clear evolution to death, as a mechanism. Early lifeforms did not age (some are still alive, so we should say "do not age" in a few cases. Some were alive the first time Eve proverbially smiled at Adam and may still be alive when the last human dies). They would die from disease, getting eaten, or simply by getting themselves into a situation where they could not survive (getting buried was pretty popular). Of all of life's "kingdoms", there's 5.
* The Monera (mostly bacteria, mostly single cellular organisms, with many interesting exceptions), do not die of old age. A curious exception is that a number of them have a built-in self destruct (ie. death) if they do not reproduce.
* The Protoctista and Fungi (2 kingdoms). Have chromosomes and a cell nucleus. Some living protocta are hundreds of thousands, maybe even millions, of years old. They are mostly single cellular organisms, but some of them are very big, for example some algae can be 40 meters or longer. The general rule seems to be that sexually reproducing protoctista die, but there are many interesting mechanisms of death found in these cells, rather than just the one we see in higher lifeforms.
Many can "pause" their death clocks. Only time spent eating causes the death countdown to tick and various forms of hibernation do not.
Quite a few are "optionally" sexually reproducing and they disable and reset the death counter when they switch to the asexual mode.
Some have inherited death clocks: organisms can switch to asexual reproduction but their death clocks do not reset upon reproduction. If they, and their offspring, do not switch back before a hundred or so generations, they age, grow weak, and die.
Keep in mind that when single cellular organisms sexually reproduce, they start with 2 cells and end with 4 cells. Mostly, there is no difference between parent and child. They change their own DNA and then divide. So "parents" don't die sooner than their children, and mostly you simply cannot tell the difference between parent and child at all. And all of the cells have their death clocks reset.
Despite being single cellular organisms, they actually have sexual "organs" (parts of the cell dedicated to reproduction alone). Both outer and inner ones : a way to connect the cell membranes together, and dedicated "micronuclei" that actually reproduce. The cell then "kills" (eats-sort of) the "old" cell macronucleus. Generally the new cells immediately recreate their "sexual organs", which are deactivated immediately after creation, and the newly formed cells go about their business under the direction of their new macronucleus.
The "death" of the old nucleus is the first form of death that is encountered in evolution.
The defining feature of death seems to be that DNA is split into two parts: DNA that is used exclusively for reproduction and DNA that is used for, well, anything else. The reason you die is that a human being falls into that second category.
The obvious variant that must have existed yet is missing is the variant we all want to exist: sexual reproduction where the death clock is reset in both parent and child, and the parent DNA is not destroyed. We do not know of any lifeforms that do this.
So there is a simple and inescapable conclusion : sex (the sharing of genetic information of multiple individuals in a species) does not work without death. That is why you die.
Extremely interesting book on the subject: "Sex and the origin of death" [1]
[1] https://www.amazon.com/Sex-Origins-Death-William-Clark/dp/01...
It isn't hard to keep any particular process 'alive'. Stars burn for a long time. The hard part comes with maintaining a process that also requires memory of the past. What would seem to be the difference between a star and biological life is that life maintains a record of some information of its past states in its DNA. However, even on top of this, many animals, especially birds and mammals, have to teach their young and train an incredibly complexly connected brain.
So I think what you end up with is an optimization that trades off between these two complexity drives: a mind and a body. An organism that has invested a lot of time into training its nervous system does not want to completely regenerate it and lose all of that information. However, from a bodily perspective, being able to regenerate would be quite useful. However, making processes more reversible comes at a price (notably, thermodynamically, it would take infinite time. computationally, reversibility requires n^2 steps for some irreversible computation done in n steps), one that is no longer worth the tradeoff if the mind cannot be usefully regenerated.
I guess my point overall is that the brain, since it wants to store information, is inherently destined for breakdown. Regenerating it might be possible, but the interconnections might be so complex that it would be energetically infeasible to maintain this complexity. In humans, the brain already utilizes 20% of our energy budget. As the organism as a whole is being optimized through evolution to do evolutionary favorable things with its energy budget, this means that energy spent maintaining itself physically indefinitely is not worth it, as compared to something like a lobster, which ostensibly doesn't have the sorts of pyramidal neurons needed for a truly, truly complex nervous system and thus might as well live forever.
I assume that people will disagree with the idea that the brain is destined to break down, but if you were 10000 years old today, would you still consider yourself the same person that used to hang out with the pharaohs, having learned and forgotten hundreds of languages, names, families? It just gets absurd at some point, and the energy cost of it all is not worth it compared to reboot(strapp)ing everything from the ground up by having a child.
It is interesting the the mole rats are so long lived, and that they are also eusocial. Since institutional memory may be very important to the colony's survival, having long-lived individuals may be very helpful for an animal incapable of writing things down for posterity.
I would still say that this is all very firmly grounded in thermodynamics.
Organisms age and die when their cells age and die. An organism can not age and die by either replacing its cells as fast as they die, perfectly, or by having cells that don't die.
Why do cells die? The article presents two views: either they are programmed to, for biological reasons, or they just break down over time, because engineering is hard and physics is a real mother.
It'd be real convenient for us if it was the former, because humans aren't built to have all our cells constantly replaced throughout our life, but this article makes the argument that programmed cell death is unlikely and physical wear and tear more plausible.
But they do. The article argues that molecule ensembles are damaged by thermal motion but there are other ways in which molecules are damaged over time due to chemical and physical stress. Large molecules in the cells are bombarded by caustic chemicals (oxidative stress) and ionising radiation, both of which break chemical bonds. Even without these, many molecules in the cell are only temporally stable and decay over time (e.g. single-stranded RNAs are unstable even ignoring active degradation).
It is altogether accurate (and common!) to describe these processes as ageing on the cellular level.
As you’ve correctly noted, cells constantly repair and replace damaged molecules.
'My original title was: "Aging: where physics meets biology". Nautilus changed the title to the above more provocative version, which seemed to have worked - judging from the number of responses!'
Comment URL: http://nautil.us/issue/36/aging/physics-makes-aging-inevitab...
Really, cancer cells are so wracked with errors and DNA damage, typically, that they really aren't the species that they grew up in anymore.
For one instance, some tumor metabolisms are radically different than what is 'normal', more different than even what is seen in even prokaryotes: https://en.wikipedia.org/wiki/Warburg_effect