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I am not asking what the consequences of this would be, nor what kind of humanoid entity might evolve such a thing - simply how it would be possible.

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    $\begingroup$ Accepting the only answer you get after just two hours is not the best way to ensure you get a good pool of answers from where you can pick. We have users all around the world, and some of them were sleeping when you posted the question and accepted the answer. $\endgroup$
    – L.Dutch
    Jul 28 at 6:11
  • $\begingroup$ This is actually a thing that is actively being researched based on sharks' ability to regrow teeth. $\endgroup$
    – Plutian
    Jul 28 at 11:18
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    $\begingroup$ Please define 'teeth'. Do they have to be enamel teeth? $\endgroup$ Jul 28 at 13:48
  • $\begingroup$ @JustinThymetheSecond Yup - human teeth. $\endgroup$
    – KEY_ABRADE
    Jul 28 at 15:38
  • $\begingroup$ So crocodile and dinosaur teeth are out of scope? "Now, researchers at the University of Missouri have found that crocodiles — and even their plant-eating ancestors — had thin tooth enamel, a trait that is in stark contrast to humans and other hard-biting species" ..."“Enamel takes a long time to build, so it’s not something animals will do ‘off-the-cuff,’ so to speak,” " Unlike people, crocodiles ... get rid of them and replace them with new copies." munewsarchives.missouri.edu/news-releases/2019/… $\endgroup$ Jul 29 at 13:42
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Very easily. Instead of having a definitive set of teeth, one could have multiple dentitions, all deciduous.

In any given moment, every teeth would have a grown instance, without real roots, and a growing bud underneath. The bud could reach quiescence (stopped at the "half size" stage if you will) if blocked by the tooth above; once it drops or is removed, the bud would produce a full tooth in a relatively short time and erupt from the gum.

This sometimes already happens in humans (it's one of the forms of hyperdontia), but only for one or two teeth and usually the extra bud has to be removed. Occasionally, though, an unerupted third bud will grow and replace a non-deciduous tooth when it falls.

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    $\begingroup$ @KEY_ABRADE Most humans didn't lose that many teeth while our species was evolving, or they didn't live long enough o lose more. Anything requiring extra structure devolves unless it has a need to be present. $\endgroup$
    – DWKraus
    Jul 28 at 2:31
  • $\begingroup$ @DWKraus Incorrect. Unless there is a selective pressure against an existing part (as in, those who produce it are less successful than those that don't) whether it persists or not is random. Ex. five fingers is not particularly more useful than 4, it's simply ancestral for the 5th one to be there. $\endgroup$
    – Bennie
    Jul 28 at 18:50
  • $\begingroup$ @Bennie In evolutionary time, any trait requiring an organized structure is subject to continuous mutation. Traits devolve over evolutionary time, as genes and proteins are altered and have no negative consequences. So while you are not strictly wrong, retaining a complex structure over millennia without selective pressure is like rolling dice over and over again and never rolling a one. $\endgroup$
    – DWKraus
    Jul 28 at 20:58
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True to the Heraclitus'

No man ever steps in the same river twice, for it's not the same river and he's not the same man.

your humanoids could use an approach similar to rodents: ever-growing teeth.

The distinguishing feature of the rodents is their pairs of continuously growing, razor-sharp, open-rooted incisors. These incisors have thick layers of enamel on the front and little enamel on the back. Because they do not stop growing, the animal must continue to wear them down so that they do not reach and pierce the skull. As the incisors grind against each other, the softer dentine on the rear of the teeth wears away, leaving the sharp enamel edge shaped like the blade of a chisel

Due to the continuous growth, the teeth they have today won't be the same that they will have in 1 month and so on.

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Extremely easily. Humans, on average, are born with the primordia of one complete set of replacement teeth (+ up to 4 bonus molars). These grow into their complete form and erupt over time - however, there's no inherent reason for the root tissue that produces the primordial tooth buds to only make two; it could simply produce a new one every few years.

Rodent-like teeth are usually optimized for snipping/stripping behaviors. While elephants do have continual-growth grinding teeth iirc, some of our specialized dentition probably wears unevenly in a way that means simply extruding more from the base won't retain its function - unless your species has a radically different diet or food-processing behavior, full replacement is more likely.

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