Most of the body doesn't regenerate because there was not enough evolutionary pressure. Even with regeneration you could still easily die from some severe injury, blood loss or disease. While salamander regeneration is impressive, it has its limitations. For example, no known adult salamander can regenerate an entirely removed eye. Also if you chop a salamander, it will not regenerate like a Planarian worm, it will just die. And a fun fact, Axolotls can regrow a limb yet cannot regenerate an adult eye lens.
Also, although the article is mostly true, there are some inaccuracies:
>> Neurons and cardiac muscle cells don't reproduce after childhood
There is some evidence showing a slow cell turnover in cardiac muscle, estimated around ~1% per year in young adults, falling toward ~0.5% in older people. Most neurons don't regenerate, but there an evidence that some parts of the adult brain (such as hippocampus) still produce new neurons, although the number is very low.
Liver regeneration is not perfect either: <a href="https://www.youtube.com/watch?v=rOv7Sr3X-eo" rel="nofollow">https://www.youtube.com/watch?v=rOv7Sr3X-eo .
> Most of the body doesn't regenerate because there was not enough evolutionary pressure.
All basal metazoan branches (ctenophora, porifera, placozoa, cnidaria) have species that are capable of whole body regeneration, so this trait was likely lost on the bilaterian branch (which includes chordates like ourselves). If anything, there are evolutionary pressures to loose this trait.
For example, consider how frequently people lose a limb, and further consider the ratio of survival to death for injuries in that general ballpark. Then balance this quality of life improvement, for those who survive, for those injured in the first place, against the fact that all people with such a biological capability will almost certainly experience a significantly higher rate of cancer. Because in a nutshell cancer is uncontrolled proliferation while regeneration is controlled proliferation. (It would be very interesting to know the rate of incidence of various cancers in axolotls if they lived anywhere near as long as a human.)
Yes that's kind of (but not entirely) my point. It's actually quite difficult (for a human) to lose a limb in a sudden accident without immediately dying. So it's exceedingly rare (~all historical civilisations) to be injured in such a manner, it's at least somewhat rare to survive such an injury (historically just about impossible), and even if you had flawless regeneration you'd be at a serious physical disadvantage (ie from an evolutionary fitness perspective) in multiple ways for an extended period. Meanwhile carrying said trait is consistently killing people off the entire time.
I think the extended period is a point that needs consideration. I don't know how long it takes a salamander to regrow a limb, but given the sheer size of any given limb in an adult human, it would take quite some time to regrow it to anywhere near its former state.
I mean... think of Deadpool's baby hands. Jesus.
Yes, that was my point. Amputations are quite uncommon, they usually lead to death (blood loss / infection, starvation, vulnerability to predators, etc) For relatively intelligent species they are even more avoidable. Regeneration would add very little benefit from natural selection point of view. Most body parts are durable enough for survival and reproduction.
I have read somewhere that losing a limb naturally (e.g. tearing, biting) stretches and closes up blood vessels in a way that cutting with a scalpel doesn't, and that natural limb loss has less bleeding than surgery/stabbing/Hollywood suggests.
I have no idea of the truth of this, but there are anecdotes of farmers losing limbs and walking back to their tractors, driving back to the farm, then driving themselves to hospital, and the like.
" against the fact that all people with such a biological capability will almost certainly experience a significantly higher rate of cancer. Because in a nutshell cancer is uncontrolled proliferation while regeneration is controlled proliferation."
Almost certainly? Where do you get this certainty from? There is no obvious reason why we should expect regeneration to be potentially carcinogenic, only because both are proliferations.
There is an assumption at your end that controlled proliferation can easily switch over to uncontrolled one, but real natural phenomena don't seem to indicate that.
Axolotls live about as long as dogs and cats do, but unlike them, they rarely suffer from cancer.
We humans are capable of regrowing our fingernails and toenails for our entire life and we indeed regrow them all the time, proliferation taking place 24/7, nonstop. And yet cancer of the nailbed is quite rare, much rarer than other cancers, though not completely unheard of.
Not "pressure to lose this trait" as much as "no pressure to improve it"?
As anatomy gets more complex, the process of getting it from "arbitrary heavily damaged state" to "functioning state" gets more complex too. And mammals are a bit more anatomically complex than placozoa.
If your entire body is a hollow sphere 4 cells thick, "repairing arbitrary damage" is very simple and natural. When you have bones, blood vessels, nerves, muscles and tendons, all wrapped in skin - all of which have to be restored correctly for a lost limb to function well? The gap between "just plug the holes" and "restore the function" grows, and the complexity of implementing usable regeneration goes up massively.
Humans can repair most of simple tissue-level damage well enough. The complexity equivalent of placozoan regeneration is in place. Rebuilding complex anatomy is what's often unimplemented. Seems like that is the part that requires some novel adaptations rather than simply not deactivating the mechanisms that are already there.
I read a book on epidemics where some modern theories on death were essentially its selected for to force potential infecting organisms to keep starting over on mastering a given genetic environment. Ones grandchildren are more likely to survive the successor plague than you are, hence a population with excessively long life would have died out. I think the evidence was from long lived trees? (Pandemic by Sonia Shah).
senfiaj · · focus · HN ↗
Also, although the article is mostly true, there are some inaccuracies:
>> Neurons and cardiac muscle cells don't reproduce after childhood
There is some evidence showing a slow cell turnover in cardiac muscle, estimated around ~1% per year in young adults, falling toward ~0.5% in older people. Most neurons don't regenerate, but there an evidence that some parts of the adult brain (such as hippocampus) still produce new neurons, although the number is very low.
Liver regeneration is not perfect either: <a href="https://www.youtube.com/watch?v=rOv7Sr3X-eo" rel="nofollow">https://www.youtube.com/watch?v=rOv7Sr3X-eo .
burning_hamster · · focus · HN ↗
All basal metazoan branches (ctenophora, porifera, placozoa, cnidaria) have species that are capable of whole body regeneration, so this trait was likely lost on the bilaterian branch (which includes chordates like ourselves). If anything, there are evolutionary pressures to loose this trait.
fc417fc802 · · focus · HN ↗
bondarchuk · · focus · HN ↗
fc417fc802 · · focus · HN ↗
shmeeed · · focus · HN ↗
I mean... think of Deadpool's baby hands. Jesus.
senfiaj · · focus · HN ↗
jodrellblank · · focus · HN ↗
I have no idea of the truth of this, but there are anecdotes of farmers losing limbs and walking back to their tractors, driving back to the farm, then driving themselves to hospital, and the like.
inglor_cz · · focus · HN ↗
Almost certainly? Where do you get this certainty from? There is no obvious reason why we should expect regeneration to be potentially carcinogenic, only because both are proliferations.
There is an assumption at your end that controlled proliferation can easily switch over to uncontrolled one, but real natural phenomena don't seem to indicate that.
Axolotls live about as long as dogs and cats do, but unlike them, they rarely suffer from cancer.
We humans are capable of regrowing our fingernails and toenails for our entire life and we indeed regrow them all the time, proliferation taking place 24/7, nonstop. And yet cancer of the nailbed is quite rare, much rarer than other cancers, though not completely unheard of.
ACCount39 · · focus · HN ↗
As anatomy gets more complex, the process of getting it from "arbitrary heavily damaged state" to "functioning state" gets more complex too. And mammals are a bit more anatomically complex than placozoa.
If your entire body is a hollow sphere 4 cells thick, "repairing arbitrary damage" is very simple and natural. When you have bones, blood vessels, nerves, muscles and tendons, all wrapped in skin - all of which have to be restored correctly for a lost limb to function well? The gap between "just plug the holes" and "restore the function" grows, and the complexity of implementing usable regeneration goes up massively.
Humans can repair most of simple tissue-level damage well enough. The complexity equivalent of placozoan regeneration is in place. Rebuilding complex anatomy is what's often unimplemented. Seems like that is the part that requires some novel adaptations rather than simply not deactivating the mechanisms that are already there.
lanstin · · focus · HN ↗