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Humans and many animals have multiple teeth consisting of separate pieces of bone embedded in the jaw. For humans, this arrangement has some disadvantages:

  • teeth are quite fragile when impacted and break easily
  • gaps between teeth are difficult to clean and risk decay

Why don't we have a single piece of bone in our mouth (possibily with an uneven surface to aid crushing food)? Is there any evolutionary advantage to having multiple separate teeth, or does it simply not matter because by the time problems arise we have already reproduced?

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    $\begingroup$ teeth are used to crush bones,so they are significantly stronger than any of the bones in the body. $\endgroup$
    – trond hansen
    Commented Sep 29, 2021 at 12:45
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    $\begingroup$ When discussing evolution, it's important to remember that forces of selection work by affecting reproductive fitness, i.e., the viability and number of offspring. So yes, as you point out in your last sentence, likely our teeth work well enough for us because their potential drawbacks don't significantly affect reproductive fitness. Also, as a matter of theoretical exercise, there is likely a redundancy benefit to having multiple teeth - if one is diseased, the others are still usable. I also wonder if there are any animals that do have one single bone as you describe. $\endgroup$
    – natb
    Commented Sep 29, 2021 at 13:35
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    $\begingroup$ Consider that many animals (sharks, for instance), have teeth that are continuously replaced. So a better question might be to ask what caused humans (and I think most mammals) to lose this useful trait? $\endgroup$
    – jamesqf
    Commented Sep 29, 2021 at 16:53
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    $\begingroup$ One broken tooth doesn't really affect the strength of the others. A crack in a single long tooth would damage the structural integrity of the whole chewing surface. $\endgroup$
    – swbarnes2
    Commented Sep 29, 2021 at 22:46
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    $\begingroup$ You should check and see if an question has been asked before you post biology.stackexchange.com/questions/104682/… and biology.stackexchange.com/questions/8302/… $\endgroup$
    – John
    Commented Oct 2, 2021 at 21:21

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It also has some advantages, although some of these have been mostly lost in humans.

In sharks for example teeth get continuously replaced, so losing one is much less of deal for them. This true in reptiles too, e.g. a lowly gecko can have 1,000 new teeth their lifetime. (The more technical term is polyphyodont.)

It is somewhat of a mystery why mammals lost most of this ability, but it is thought to be related to the specialization in the shape of the teeth. Species that have little differentiation between teeth [types] are most prolific at replacing them. (I guess you're ware that humans get two sets, but this is driven by age rather than by a loss process.) There are few polyphyodont mammals as well: elephants, kangaroos, and "sea cows".

Being able to replace teeth in smaller increments (i.e. individual teeth) is probably an advantage as at any given time only a small proportion would be missing... although as with other things in evolution there are some exceptions. Piranhas actually have quadrant-fused teeth (which basically act like larger serrated knives)... and they replace them in whole quadrant increments. (They also grow the replacements in such a way that it takes only about a day for the new set to replace a lost quadrant.) However this is almost certainly energetically expensive, which is probably why few other species have this feature.

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i know there are differences. My question was maybe a bit unclear. I didn't want to ask why there are teeth instead of bones, but why there are multiple teeth instead of one large spanning the entire jaw. Besides, parts of teeth (dentin, cementum) are not that different from bones. I never questioned the presence of the harder parts (enamel).

For one thing, multiple teeth can adjusted individually for wear and alignment.

However that clarification in the comments conflicts with with points in your question which could be construed intended as the argument against teeth of any kind, singular or multiple, especially singular.

  • teeth are quite fragile when impacted and break easily
  • gaps between teeth are difficult to clean and risk decay

Can you imagine what would happen if your ONE tooth broke or decayed?

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