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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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    $\begingroup$ 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