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Almost all organs in the human body have a rather large threshold within which the organ or tissue is capable of repairing itself using materials supplied by the body, whether it's made from organic tissue or structural proteins. Wounds and minor damage to the skin or flesh for example heal by themselves, skin, nails and body hair can grow back if damaged or removed, even most bones can be repaired autonomously by a biological body. Cutting off a significant chunk from the human flesh or removing over 90% of a hair or a nail would cause no permanent injury, and the body is capable of repairing the damage over time with ease. The vast majority of the human body (not counting critical organs such as the heart or the brain) can be "grown back" to an extent by itself, because such parts have a rather large damage threshold.

On the contrary, teeth do not. There is a certain threshold in which tooth enamel can be supplied and "repaired" by the body to an minimal degree, but past that point virtually any damage caused to teeth are permanently irreparable by natural means. Which is quite strange, given how fragile a set of human teeth is, be it chemical or physical damage. Especially if you take more "artificial" causes into account. A moderately heavy object aimed towards a human mouth, for example, can easily render ten to fifteen teeth permanently damaged past the point of recovery within a heartbeat, also causing the said person's digestive process to be heavily impaired.

Another strange fact worth noting, is that the human body is capable of producing two separate set of teeth due to biological growth and size differences, but not more than that. Wouldn't it be beneficial for a biological body to be able to supply new teeth in the place of ones that have been damaged or removed? One could argue that such an action would be too complex and impossible, similarly to growing back an entire limb or a spare heart, but given the material that makes up teeth and its fragile nature, it would be safe to assume that it is biologically possible, especially given the fact that the human body DOES produce two separate set of teeth over its lifespan.

However, even stranger still, is that the only element capable of damaging or deteriorating teeth by natural means, is the consumed food itself, and the acidic bacteria found within. Several types of edible food consumable by human means are capable of damaging the enamel to critical levels within relatively short periods of time, even non-artificial ones. What happened before the invention of the toothbrush and various dental cleaning methods? Why haven't humans evolved in the past tens of thousands of years to the point where repairing or regrowing teeth by natural means is feasible? Simply put, why don't teeth grow back contrary to hair, nails or skin?

What is the biological and evolutionary reason behind teeth being so fragile and indisposable?

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Regarding the replacement of teeth, sharks do this continuously. –  Rory M May 11 '13 at 22:42
    
I think your premise is wrong - teeth are not fragile and enamel is supposed to be the hardest part of the human body. Also, strictly speaking, humans do not produce two sets of teeth independently. Both sets are produced at the same time (with a very cool looking result): The mouth of a child is a terrifying thing to behold –  fileunderwater Jun 10 '13 at 11:03
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2 Answers

Human teeth are strongly attached to a socket, which is not true for many other animals. Our teeth are designed to last our lifetime (specifically up until the point we pass on our genes). Most people's teeth last until they are in their 50s, when the majority of people have passed their genes on.

As for food attacking our teeth, this would be fruit juice, sugar etc. These were not the diet of people years ago, in fact refining sugar is more of a recent discovery. And since that time we have been plagued with bad teeth. I think it was queen Victoria that was first introduced to refined sugar and suffered deleterious consequences quite rapidly.

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The amount of food (especially more fiber?) and water consumed might also be significant (in this sugar might provide a double-whammy, dense calories as well as more directly encouraging tooth decay). Chewing on (tougher) low-sugar foods might have some direct tooth-cleaning effect and might encourage more salivation from the food being in the mouth longer (which might also help "wash" teeth). Just "reasonable" speculation. –  Paul A. Clayton Jun 9 '13 at 23:38
    
Adding to your answer; for most of our evolutionary history we lived shorter lives and reproduced at an earlier age. Therefore, our teeth have probably served us fine, and there haven't been strong selection for even more durable teeth. I can also imagine a trade off with other parts of the body for the minerals needed to build teeth, especially in a sometimes poor hunter/gatherer diet. –  fileunderwater Jun 10 '13 at 11:15
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Human teeth and animal teeth are not fragile. It is meant to last a life time, barring physical injury. If anything makes it fragile it is ourselves. The main two causes of tooth loss is dental decay and gum disease. Both are cause by the soft tenacious bacteria filled biofilm called 'Plaque". If there is no plaque then there is no dental caries(cavities) and no periodontal disease(gum disease). Wild animals don't have both and eskimos did not have these diseases till they were exposed to modern civilization. Raw fibrous food have a terrific cleaning effect and effectively removes plaque. And even if there is plaque, since there is no refined carbohydrates there is no decay. If you want to prevent dental decay remove plaque completely, by brushing ,flossing and sealing all the pits and fissures where plaque can lodge. Follow this and one can eat all the sugary things without worrying about their teeth if not anything else.

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"If there is no plaque then there is no dental caries". I can agree with that, as there must be a method of decay, but is bacteria really the cause? Can a tooth be "healthy" enough to resist the actions of bacteria (which I would think would be present in everyone's mouth, yet some people's teeth don't decay even with no brushing ie eskimos)? An analogy would be that dead animals decay because they're dead. If they decayed because of bacteria, then live animals would decay as well (unless they cleaned the bacteria off 3 times a day, which they don't). I question the accepted cause of caries. –  Randy Aug 23 '13 at 4:54
    
I think the reason for caries is more chemical than biological (ie plaque is a symptom, not the cause). Also, caries are common in all cultures and are present in animals. The diet seems to be the single most prominent factor in the formation of dental caries. On the Eskimo thing, is that from Price? –  fredsbend Jan 10 at 19:17
    
The sequence is like this: Plaque harbours and protects the acid producing (acidogenic) micro organisms. Acid producing microorganisms metabolise refined carbohydrates to pretty strong acid. This acid easily decalcifies the enamel which is 96% minerals. However the quantity of the acid priduced is minute and its action lasts for 20 minutes to 40 minutes and tapers off. The role of the plaque is to harbour these Microbes in high concentrations close to the enamel or tooth structure and to prevent dilution of –  Ram Manohar M Jan 12 at 3:37
    
the acid by saliva. The effect of acid in enamel is like acid on marble. Enamel gets damaged even by mild acids like carbonated drinks. These are well established facts readily available in standard text books in Dentistry. In short no plaque no caries. –  Ram Manohar M Jan 12 at 3:49
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