Eli5: Why do we not regrow teeth and limbs?
In: Biology
It makes little sense for an animal to evolve an ability that would never be used. Regrowing a human limb (maybe 10-20kg worth of complicated bone, flesh, muscle, nerve, connective tissue, blood vessels etc) is too energy and time consuming. Say that an adult limb takes 3 months to grow. During that period the person is vulnerable. Also, there will not be enough energy and material to regrow the limb stored in the body so it will require a LOT of excess foods during this period. Basically (before modern times) most humans will die before the limbs can regrow. So the ability is useless for our ancestors of 20-30 thousand years ago.
Our body doesn’t evolve to survive to our imagination (ie we’re not evolved to live hundreds of years, fight off all diseases, regrow limbs). Rather it evolves to survive well enough to be able to reproduce. While any one individual benefits from these capabilities, the species would not be better off.
Limbs: at some point it became evolutionarily better to quickly heal with scare tissue than take longer to start growing a new limb properly, so that’s what happens.
Teeth: mammals evolved to have teeth that are specially shaped and fit together nicely, allowing for more efficient chewing. Reptiles have a load of the same kind of teeth that continually grow and replace. For mammals’ nicely arranged teeth they can’t keep losing and regrowing teeth because it would mess the arrangement up, so we just have one final set we keep.
I’d hasten to point out that, for mammals anyway, the ability to keep something alive after a limb has been torn off is relatively recent, largely due to medical intervention, and would have been incredibly rare in nature.
Most dismembered creatures would die of blood loss. Those that didn’t would die of infection. Those that didn’t would die of starvation by being unable to properly walk, hunt, or make tools.
It’s not evolutionarily advantageous to regrow limbs if losing one is a death sentence anyway.
Regrowing a limb may sounds useful but up until a couple thousand years ago losing a limb was basically a death sentence. You either die immediately from whatever cause you to lose a limb, bled out shortly afterwards, died from infection or in the incredibly rare likelihood, that you somehow survived, were no longer able to function. So even if some early humans did have some genetic mutation that allowed them to regrow limbs, that would not have been evolutionarily advantageous, and did not help these people survive and procreate.
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