Comment by senfiaj | Hacker News Reader

Comment by senfiaj | Hacker News Reader

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: https://www.youtube.com/watch?v=rOv7Sr3X-eo .

I have an axolotl. They're really incredible creatures, with way more personality and interests than you would expect. They are really soft, and almost have the same density as water, so they can be neutrally buoyant and even swim rapidly like a fish. They're also very sensitive to sharp things; mine recently got a cut from an aquarium decoration, which healed fully over the course of a week. Her previous owner had neglected her and induced tail curl, which took longer to heal (a few months) but did fully heal. I have since adjusted her tank to give it all flat or smooth surfaces, with some variety.

https://ambystoma.uky.edu/teachers_materials/axolitbook/Axol...

Oh no! I read that one as a teenager. Hunting!

> 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.)

When you lose a limb you'd die of blood loss though, right, not the mere lack of a limb.

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. Claiming it as a fact is way, way beyond what science currently knows.

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.

Our own children proliferate their cells like hell, growing twentyfold in the process, and yet pediatric cancer is not very common, with only 1 of 300 to 500 kids getting cancer before their 18th birthday. It is much more common in elderly people whose cells don't proliferate that much anymore.

You have no real observations to ground your certainty in, it is purely speculative, and this is not maths, this is biology, with all its glorious mess. Even very smart people usually get their speculations wrong when it comes to biology.

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).

So even the animals we think of as regeneration champions have a very specific menu of things they can rebuild

Exactly, limbs are quite large portion of the body, that's why many think salamanders would have no problems with regenerating all other smaller body parts. And that's not the case.

Still, regenerating a limb sounds very useful. I'd sign up for that.

> Most of the body doesn't regenerate because there was not enough evolutionary pressure.

Maybe, but isnt this basically tautological? "You didnt evolve this way because of how evolution works". The more interesting question is why did certain parts evolve the way they did, and can we harness the knowledge?

Dr Michael Levin at Tufts has a lot of electro based theories about regeneration based on findings with Palnarians he adjusts the voltage states and gets two headed liniages amongst the many many other things his lab covers

Getting "old" is a recent and tiny sliver of human evolution. It's only been the last 100 or so years that we've really started to push the boundaries of what the body is capable of and our biology has never really had a chance to deal with it and "develop" a solution.

This isn’t correct. For example, when Edmond Halley created the world’s first life table in the 1690s, he estimated that approximately 5% of his city’s adult population was >70. Getting old is not new.

Even still, if we were able to continue on current aging trend, including food availability and whatnot, for another hundred thousand or million years; what evolutionary pressures exists that would make our biology able to live even longer than our current status quo?

Thanks! I liked the liver video. I was wondering.

Actually I would argue that the evolutionary pressures run against it: if one generation dominates then the species can't evolve.

Regeneration is unlikely to warrant eternal life. In the wild, most animals die before even showing serious signs of aging. Even if regeneration increased lifespan, poorly adapted individuals would still die.

evolution doesn't act on the species or even individuals, it acts on the genes which is why it looks like evolution acts on the individual and its close family unit.

people have a hard time grasping this concept for some reason.

my favorite example to illustrate this is the trait of being poisonous. if evolution cared about species and then every single one would be poisonous and all predators would have to hyper specialize in prey (arms race between a particular poison and its resistance). it's precisely because it's so difficult to elicit a benefit from this strategy for the lineage that it's so rare. almost always a mutation in coloration has to happen to set the lineage apart from the others first. and then the lineage has to survive the disadvantage long enough.

The current model is that evolution acts on the molecular, cellular, and genetic level, and most importantly, at the population level.

    > acts on the molecular, cellular

this is rare and even more difficult to understand.

https://en.wikipedia.org/wiki/Structural_inheritance

basically, the genes do not generally contain blueprints but rather processes utilizing existing cellular machinery. a "mutation" can occur in a machine which will allow a propagating difference.

and there is nothing special about populations. it's merely the only way to observe most changes. gene frequencies in gene pools.

We have on evolutionary scales lots of predation damage by the lizards. Thus a turtle with 300 can still give birth, while we can not.

Reply on news.ycombinator.com