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This question already has an answer here:

Why do we have two sets of teeth? Why do earlier shed and new teeth replace them? In humans, kids do not have any need for replacement of the teeth. Why don't the milk teeth grow themselves? Or why don't we have a set of small permanent teeth as our first teeth?

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marked as duplicate by John, David, kmm, AliceD Nov 21 '18 at 8:48

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    $\begingroup$ Teeth grow beneath the gums in the mandibles. They don't grow once erupted. "Baby teeth" are small to fit small mandibles; too small to be as efficient as needed for the longer life of an adult. Teleologically it makes sense to have small teeth for the time we're small, and exchange then for bigger teeth for the rest of out lives. $\endgroup$ – anongoodnurse Mar 28 '17 at 17:40
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    $\begingroup$ @anongoodnurse And since we do not grow as an adult, new teeth are not required as mandible size is fixed! Why don't you post it as an answer? $\endgroup$ – YAHB Mar 28 '17 at 17:54