When a piece of skin, muscle, bone, nails, or hair gets removed or damaged, those usually grow back like nothing happened. So, why isn’t it possible for hands, or even something smaller like a finger to regrow? Or would a piece of meat just regrow there instead of something useful?
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The genes for regeneration exist, and the evidence for this can be seen in species as different as a lobster and a salamander. So if the common ancestor of lobsters and salamanders was able to regenerate, and its safe to assume that we evolved from the same thing, or something closely related to it, why don’t we have regeneration?
The only conclusion we can draw here is that evolution shut it down for some reason. Which would imply that individuals who had regeneration were dying before they reproduced. Maybe the regeneration genes caused cancer, maybe they caused new, extra limbs to sprout when the body was simply nicked in the wrong way.
Whatever the specific reasoning, something about regeneration reduced the fitness of animals who were related to our ancestors, so much so it caused any ones who had regeneration to die off. Both cancer and extraneous limbs would be considered ‘neoplasmic’ and something about this neoplasm was killing their hosts rather than helping them.
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