If this were a mechanism that appeared under some kind of evolutionary pressure. What would that be? In which situations a normal mammal find its bones squished (but not crushed)? Or is it just a side effect of something else going on?
1. Mammals don't naturally exist in environments where exercise isn't mandatory. _Some_ system had to generate those compounds, and there was never any pressure to keep it from being tied to the other stress-related changes bones undergo (like density changes). Once it evolved once somewhere in some critter once via that machinery, the advantage was large enough that it was locked in.
2. Exercise directly contributes inflammatory factors. Tying the cure to those to a measure of exercise intensity (bone squishing) helps calibrate the dosage. Even if an imperfect measure, the proxy of bone loading might confer some advantage in, e.g., not wasting energy on unnecessary proteins.
3. Exercise indirectly suggests a resource-poor environment, leading to lower chance of offspring succeeding and needing longer lives to produce enough viable offspring. The compounds in question aren't free to produce, so there's an advantage in only providing them to organisms who will meaningfully benefit. Mind you, resource-poor environments often promote shorter lifespans and faster growth cycles as an evolutionary trait, _but_ without a mechanism to force your offspring to mature more quickly the osteocyte-mediated lifespan could allow for resource-dependent lifetime offspring counts.
4. The mechanism could have evolved for unrelated reasons, perhaps directly contributing to the bone density calibration osteocytes already mediate. Any impact on lifespan is incidental or otherwise produces less evolutionary pressure for or against the feature than the direct purpose of the compounds.
5. More than having evolved for unrelated reasons, the fact that the compounds have any effect at all might not be evolutionarily selected for. We produce plenty of unimportant metabolic byproducts that we don't particularly care about, and it could be that some more important osteocyte function incidentally produces these "waste" proteins.
It would seem plausible that there may be more than one mechanism via which resource scarcity creates pressure for increased longevity (to increase the odds of offspring).
Caloric restriction is probably exactly like exercise in that it was normal, so "scheduling" autophagy for the "famine" times meant it happened regularly and alongside similar systems.
What did it for me was that IF comes with muscle growth hormones increase.
I mean a 3-5 day hunt can look a lot like a marathon and a fast / IF mashed together.
Also, I think studies of poverty show generally show that resource scarcity is almost always worse for health outcomes. Resource over-abundance is maybe also damaging but the idea that being resource scarce is beneficial for longevity because you need longer to mate seems almost certainly false to me.
Similarly, explanations of aging that amount to "the body just for no reason fails to do something that it easily could even though it's obviously beneficial" are pretty much like explanations of physical processes that violate conservation of mass/energy. There had better be some possible evolutionary explanation, since otherwise it's very likely that the result is due to some experimental error.
If we're trying to piece together how the past actually happened based on the shadow it left on the present, yes, falsifiability is hard, sometimes possible to achieve. Most of the speculations I provided could be reasonably well tested though.
Take (5) as an example. If you find other important byproducts from the associated genes, those strengthen (5)'s argument. If you find significant negative effects from the new "miracle" proteins, those strengthen (5)'s argument. If you find an organism with an energetically intensive mechanism for removing these new "miracle" proteins, those strengthen (5)'s argument. If you apply a medicinal intervention to allow the gene to be enabled but those particular "miracle" proteins to be disabled in a wild population and observe the gene frequency dwindle, that strengthens (5)'s argument.
None of it's perfect, but there are a variety of experiments and observations you can do to strengthen or weaken your belief in specific causal mechanisms.
> used to justify unjustifiable positions
I go back and forth on this one. Should you hide information (in this case, not even very good information, just plausible mechanisms, provided because somebody was a bit curious and I thought scratching that itch might help them on whatever journey they're currently undertaking) just because it's possible to misunderstand it? Because it's prone to misuse?
Like, obviously if people are concluding from a list of guesses appropriately described as such that osteocytes evolved to extend life as a poorly calibrated part of the exercise process, that's not justified. What happens when people extend similar wild speculation to gender or racial differences? You're right, it's a touchy subject.
On the other hand, that's an argument against:
- Data-driven organizations -- a common cognitive bias is the spotlight effect, and applied to "data-driven" organizations it often leads to abandoning intuition and reason when the best proxy measurements you're able to perform incorrectly suggest a different course of action to be optimal.
- Giving correct estimates to your PM -- you know the organization will run more smoothly if you start unraveling the spaghetti sitting in fromt of you, but (in many orgs) it'll take longer to argue that point than it will to just do your job correctly, so you fudge ticket descriptions and estimates to make sure critical maintenance happens.
I try to evaluate that sort of thing on a case-by-case basis. I think it's fine here. I don't have a good justification for why I think that.
Repair pays off only when the skeleton is used -- we can invest in repair because we know the animal is active/foraging and we will get paid back on the investment.
Low loading is a sign that we should cut back on maintenance -- the animal is sick, injured, starving etc and it's better to conserve energy.
The optimal amount of exercise is what tells your body to invest in repair, without acquiring repair debt. Sedentary life is the misleading signal, because it tells the body you're declining when you aren't.
Here goes a terrible analogy: Maybe it’s like software. When I see issues and PRs pop up in my GitHub projects, I get the motivation to maintain, fix bugs, and upgrade them.
Running, exercising, etc.
Given two tribes, one where their elderly are contained at home, and another where they're wandering and learning and teaching, I guess I could see a hypothetical fitness function.
I can't imagine it's too strong, though. A simpler, more-mechanical explanation may make more sense: when you stress bones, you set off all kinds of repair processes, many of which have systemic effects.
What evolutionary pressure led to that? If I have to guess animals that were still able to hunt/move/... for longer were also able to mate for longer.
Maybe there is some evolutionary trade-off behind it?
There's a common theory that for mice, they're unlikely to survive for many years because of predation, so there's not much benefit to preventing aging in later years. Small benefit means any counterbalancing disadvantages (lower vigour?) could outweigh the anti-aging benefit, or that the force of selection is insufficient to overcome random mutations that degrade the anti-aging mechanism, but "that's good enough, no need for more" is not how evolution works.
But that argument doesn't prevent limited mechanisms, and conditional mechanisms in particular would still be rewarded. Conditional mechanisms provide much more short term adaptability to changing conditions, without damaging the long term species-wide adaptation.
There are intriguing parallels with centrally planned vs market economies.