18 November 2011


This is a child's skull (from a normal human) which clearly shows the "baby teeth" and the not-yet-emerged adult teeth.  (Just to clarify - the reason this particular skull looks so odd is that portions of the mandible and maxilla have been cut away in order to show the preemergent dentition in place).  It helps one understand how teeth can emerge crowded, and why there are orthodontists.

But I do wonder why this arrangement won via natural selection (or, for some of you, why God chose this method of growing teeth).  Why not just have a single set of teeth that gradually get larger?  Perhaps someone who went to dental school can tell us...

Photo credit: Stefan Schäfer (with a hat tip to Skipweasel for sending me the link).


  1. There's a good reason deciduous teeth are called "milk teeth".


  2. I always assumed it was because one would need two sets of teeth to make it to adulthood (i.e. breeding age).

    If tooth decay and subsequent loss limited your diet to softer foods that could be the difference between surviving a famine or not, between having the strength to fight off a competitor for a mate and not.

    If you get fresh new undecayed teeth before sexual maturity you stand a better chance of making it to sexual maturity and attracting a mate when you get there which is really all evolution cares about.

    Much better to have an arrangement like sharks do where the tooth bud is continuously producing new sets of teeth to replace the old.

  3. what nolandda said: because you're going to lose teeth through breakage and decay.

    My theory on orthodontia is related: that we get molars of the course of several years because evolution his programmed our mouths to have replacements for the adult teeth we lose through breakage and decay, also.

    Based on sample size of 1: I had a tooth bud in the wrong place and got two molars in one spot, both of which had to be surgically removed. So when my wisdom teeth came in on that side, there was plenty of room. My dentist has my wisdom teeth marked in his files as back molars and wrote that I have no wisdom teeth on that side. On the other side, though, with the full compliment of teeth, I have had major crowding problems and my top front and bottom tooth on that side stick out a bit.

    Our teeth naturally fill in the spaces as our teeth come in, probably because in caveman times (and even later) we were going to lose some teeth along the way.

  4. The reason I don't find the above explanations satisfactory is that I have never seen (or heard of) humans losing teeth by wearing them down. The closest situation would be tooth-grinders, who polish down the grinding surfaces, but I don't believe any modern human loses masticatory abilities through wear and tear. And the same with children - the "baby teeth" that are lost are quite functional, not worn-down stubs.

    I suppose it could have been different if protohumans were gnawin on lots of bones or eating maize that had been ground up on metates with sand and dust admixed, and perhaps breakage was more common then - but prior to the invention of refined sugar, I'll bet decay was frankly less common.

    So I'm still unconvinced and wondering.

  5. You're right, Minnesotasan. Tooth decay seems to have been much less common in prior ages. People should take a look at some of the early human skulls in the local museum -- usually their teeth are in fantastic shape.

    I have no idea 'why' we replace teeth ('why' is usually a wrong-headed and often incorrectly answered question in biology anyway), but most non-mammals have multiple sets of teeth in their lifetime. People may also in some cases. Third sets of teeth have been observed in very old people, suggesting that our 'adult' teeth are not really 'final'; we just don't live long enough to see the next set.

    Google this, and also the 'Osr2 gene', to read some interesting stuff.

  6. From Wikipedia -

    "Protein odd-skipped-related 2 is a protein that in humans is encoded by the OSR2 gene. In mice, it is involved in the development of the palate and in suppressing the formation of teeth after the eruption of adult teeth."

    Osr2 seems to be one development that distinguishes us from sharks...

  7. Replacing teeth is the plesiomorphic state. We, as mammals, just have fewer sets than most critters.
    The only time animals evolve teeth which don't stop growing is when they spend a lot of time eating or gnawing really tough things. Glires (rodents and lagomorphs) and grazing animals do it. I don't know about glires, but grazing animals have to counteract all the wear the phytoliths in the grass causes (imagine eating bits of quartz or glass). If you don't constantly eat things that wear your teeth down, there's no reason to evolve teeth which you would then have to work to maintain at an ideal length.

  8. Sorry, I didn't mean to imply that the teeth were "worn down" in that sense.

    What I meant is that when a cavity in a tooth reaches the inner pulp (i.e. a dental abscess) it is very common for the infection to spread rapidly down the tooth and into the bone causing the portion of the jaw holding the tooth in place to become infected (i.e. periodontitis).

    The result is that the bone pulls away loosening and eventually ejecting the infected tooth.

    The rate of cavities varied tremendously amongst pre-modern people depending upon their genetics and diet amongst other factors. Wikipedia has a history and this anthropology paper discusses ancient societies that had cavity rates ranging from less than 2% to more than 90%.

    Prior to farming and ready access to carbohydrates tooth decay rates do seem to have been lower. However our ancestors never used a toothbrush and their teeth did decay.

  9. Actually, our ancestors did use toothbrushes, of a sort. When I spent some time in Mali years ago, I met almost no-one with any cavities. I originally thought this was because of their diet, until I saw how much cola and other soda people there drank! Their method of cleaning their teeth is this: after the main meal of the day (what we would call supper or dinner), everyone finds a twig then hunkers down around the fire to catch up on the day. They chew the end of the twig until it's abraded and "fuzzy", then over the course of half an hour or so, they use these twigs to carefully scrub their teeth and gums. It serves as toothbrush, toothpick and dental floss all in one, and the social aspect of it gives everyone a chance to keep in touch with each other. When I got home, I found it really odd to stand in front of my bathroom mirror, brushing my teeth by myself!

  10. And have you guys seen the video of monkeys flossing using human hair?


  11. It's actually so that the tooth fairy can bring the kiddies some coins.

  12. Perhaps our primitive ancestors might have been less generous with their food. Perhaps children gnawed on more foliage and the like found at their size and hunting/gathering ability.
    The pubescent then are gifted with hormone growth spurts, new teeth, and through that yummy meat.
    I believe the american indians wore their teath down thought leather work... at least I was told that in school but now I think of the phrase 'long in tooth' and remembet that was an american indian phrase (I was told in school) and realize there is some contradiction there....

  13. My uneducated guess would be that baby teeth are the right size to fill the mouth of a child, but are replaced by the larger adult teeth as the individual grows to adult size, increasing the size of the jaw.

    Baby teeth wouldn't be large enough to fill an adult jaw, and adult-sized teeth would be too large to fit in a child's mouth.

    So, my guess would be that humans drop their baby teeth in order to slide larger teeth into place that fit the jaw as they reach maturity.

    At some point or another, an expert will chime in and properly explain the question.

  14. But that's not bad logic. We'll see if any dentists chime in.

  15. I remember it mentioned in lecture (probably Vertebrate Morphology in 2003) that humans and other animals have deciduous teeth so that as our mouths reach adult size, we can maintain the inter-tooth relationships necessary for effective food processing. We spend a lot of time getting food and chewing it up, and having to do that every day with horribly mismatched teeth would be miserable, unhealthy, and time-consuming.

    Mammals in general are unique for their diversity of tooth shapes, both in a single mouth and between species, and I've read in several different places the idea that the flexibility of diet and efficiency of food processing this heterodonty allows is a significant part of the success of mammals as a group.

    There's more on this on meat-eaters in particular in David MacDonald's book The Velvet Claw: A Natural History of the Carnivores. He attributes modern Carnivora carnivores' dominance over earlier carnivores to their evolution of carnassial teeth, which are much more efficient meat-cutters.

  16. @anonymous 8:28
    Actually, if you do a little poking around, you'll realize the expression "long in the tooth" pertains to horses. Old horses have very long molars (and sometimes even incisors), thus a seasoned trader would look in a horse's mouth (hence that OTHER expression) to get some idea of its age.

  17. It's also pretty fascinating to see in an x-ray


  18. @CAC - "carnassial"is a new word for me. From Wiki for those interested:

    Carnassials are large teeth found in many carnivorous mammals, used for shearing flesh and bone in a scissor- or shear-like way. In the Carnivora, the carnassials are the modified last upper premolar and the first molar, but in the prehistoric creodonts, the carnassials were further back in the jaw–first upper and second lower or second upper and third lower molars. These teeth are also referred to as sectorial teeth.

    Wear and cracking of the carnassial teeth in a wild carnivore (e.g. wolves, lions) may result in the death of the individual due to starvation.

  19. I'm so excited that I get to leave a comment on a post about teeth. I'm a third year dental student and have spent a good amount of time studying tooth morphology.

    I will say based on what I read, user 'DirtPoor' had the best comment. Primary (or deciduous) teeth are essentially place holders for the permanent (succedaneous) teeth. It wouldn't make a whole lot of sense if large permanent teeth came in before the bone was finished growing. There wouldn't be room, and things would be crowded and hard to keep clean.

    I'm also going to give 'nolandda' a gold star for effort and wikipedia research, but unfortunately a lot of his second post is misguided. The term 'dental abscess' is not used by any dentist, endodontist, or oral surgeon that I know. It would be like calling a Ferrari a 'motorized vehicle.' I would be laughed all the way out of the clinic. It is misleading, because we have a lot more specific terms to describe the etiology of the disease. Also, he tries to reference periodontitis, but in the current scope of the discussion about dental decay, periodontitis (i.e. gingivitis so bad that the bone is being resorbed/destroyed by the body's immune response) is not really relevant. I don't mean to crucify nolandda on a comment on a blog post. I just wanted to clarify a few points.

    Now, going back to the original post, Minnesotastan asked "Why not just have a single set of teeth that gradually get larger?" The reason this isn't possible is because of how teeth are formed in the first place. The outer surface of the tooth is covered in enamel, the inner surface of a tooth is made up of dentin. A tooth actually begins it's formation at the junction of these two layers. Cells that produce enamel (called ameloblasts) move outward, leaving a trail of hydroxyapatite, Ca5(PO4)3(OH), behind. Similarly, cells that make dentin (called odontoblasts) move inwards. Dentin is made of of hydroxyapatite, too, but also has a protein matrix and higher water content than does enamel.

    Eventually, the ameloblasts die and no more enamel can be produced. On the other hand, odontoblasts stay alive and continue to produce dentin for as long as they live. So, while teeth might be done growing on the outside, they are still growing on the inside! This is beneficial because the continued deposition of dentin allows for teeth keep a buffer of nonvital tissue between the worn outer surface and the nerve containing pulp tissue inside the tooth. Odontoblasts will also lay down dentin when they sense decay moving closer to the pulp chamber. While they always lose the race, they help to add additional time to a tooth's life.

    This is a good picture, showing the enamel, dentin and pulp chambers of vital teeth:


    Anyone else have any questions?

  20. Another observation:

    If you look on the two lower incisors you'll see they have three bumps the incisal edge. These are called mammelons, and are remnants from three of the lobes that eventually fused together.

    Frequently, these are worn away by natural tooth wear. It's kind of funny, but most of my incisors still have mammelons. It's actually really funny, because I'll be talking to one of my classmates and out of the blue, they'll exclaim, "Holy Cow! You still have mammelons!" At which point, every dental student/dentist within earshot will be checking my smile out.

  21. I happened to be getting some dental work done today, and while we were waiting for something to harden/dry, I asked the dentist about this. He thought the deciduous teeth as "placeholders" theory was generally good, except for the fact that as the human grows, the skull grows the least compared to the body (babies have proportionally large skulls). But he didn't have any other explanation to offer.

  22. Teeth are more heavily mineral than any other tissue; they're the only body part that can decay in a living animal. Tooth enamel is basically dead, isn't it? So it can't grow. ?


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