Why can I grow new toenails, but not a new leg?

Writing a kids book and need a good resource on this question. Any suggestions...?


MrLokiNight said…
This is a great 24-slide powerpoint that could be simplified

Rgrds Michael Fisher

MrLokiNight said…
Not sure it 'took' first time

Paul P. Mealing said…
Salamanders are the best known animals for regenerating limbs, and eyes apparently. I found this after a quick search, which provides a not-too-esoteric explanation.

As for toenails, like hair, they don't stop growing unless they fall out. Same as rodents' teeth, including beavers. If they don't wear them down by chewing timber they'd grow into their lower jaw.

So toenails are a completely different process to regeneration, which requires different types of cells: skin, bone, cartilege, blood vessels; to generate.

Regards, Paul.
jeremy said…
As far as I know, this question doesn't have a universally agreed-upon answer, but the closest I think anyone has come is the theory by Randolph Nesse and George Williams, in their book Evolution and Healing. (Annoyingly, the book has a different name in America: "Why We Get Sick".) See the short "Regeneration of Body Parts" section of the "Injury" chapter.

Fundamentally, their answer is that evolution won't maintain capacities whose average benefit is low. However, their argument is quite shrewd, and nuanced.

If you can't obtain the relevant section easily, I can send you a summary of their arguments, if you'd find it helpful. However, getting hold their (wonderful) book would be better, I think.
Eric Sotnak said…
Some information here:
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