So I fell down a google hole for a little personal project I started recently on life forms that can regrow and become rejuvenated after serious injury and loss of parts. It's something I got interested in ages ago through reviewing a James Elwick book which detailed the way regeneration was a topic of fascination for biologists in the 19th Century. Apparently Richard Owen, the founder of the Natural History Museum and head honcho biology man in the UK until a certain mr Darwin came along and dethroned him, was particularly enamoured of the problem of explaining why some creatures could grow new bits to replace ones that fell off, and others could not. For Owen the ability of a lobster to regrow a claw, or a lizard to replace its tail, was a central mystery of life and held the key to understanding not only how life works but how to prolong it.
Owen's research program fell into disfavour in the late 19th Century because he never got along very well with Darwinism. I still find his explanation for varying regenerative capacity fascinating though. He theorised that in higher life forms, the vital essence had become centralised in the nervous system, especially the brain, which is why higher forms can only reproduce if they're imbued with 'fecundating principle' A.K.A sperm. Simpler creatures have their life force spread out more evenly throughout their bodies, which is why if a fragment breaks off it can simply regrow. I don't know why this appeals to me so much.
The axolotl is a bit of a poster child for regeneration, because it regrows not only its tail but other limbs and even internal organs.Sea cucumbers get the prize for self-repairing not just comprehensively but rapidly, regrowing lost parts in as little as a week. Hydras outdo their mythological name sake, because any fragment larger than a few hundred epithelial cells that is isolated from the body has the ability to regenerate into a smaller version of the whole. They're also thought to be truly immortal, because they show no processes of senescence -aging. They can just renew indefinitely. A planarian flatworm can also grow back its entire body from a speck of tissue,
One interesting thing is that there is often a key area or body part that must remain intact, in order for regeneration to be viable. We all know (right?) that an earthworm's future depends on where you split it. But did you know that if you cut it after its 13th segment (counting from the head) then it can regrow its head, but not sexual organs? Whereas cutting it between segments 20 and 21 can create two whole new worms? but cut between the 23rd and 55th segments and you'll end up with no worm at all. Mind you, this is just 'red wrigglers', while 'blackworms' will actually self-amputate in response to temperature shifts. And one unfortunate species can be induced to develop a head at both ends.
Why can't humans regrow limbs and organs? The standard modern line would, I think, be that such regenerative powers in a complex multicellular would be too risky, because it would make cancers more dangerous. But this isn't really a complete explanation. Axolotls are pretty complex after all - why doesn't cancer destroy them? Why do some very complex organisms suffer little or no cancer at all? Variable regenerative power could be a product of inherited constraints, frozen into certain lineages by chance.
Makes me wonder if there is any cool scifi imagining humans with axolotl-like abilities. Some superheroes have it, sure, but what if everyone did - how would that change society? It wouold affect our attitude towards risk, presumably. And create interesting new possibilities for body modification.
Right i should really get back to work now.............