Tooth issues - what gives?
April 14, 2011 7:44 AM   Subscribe

What is up with human teeth? Why do they so often come in crooked, in the wrong place, or not at all?

This question is not about dental hygiene; it’s about how teeth grow in.

Just about every person I know has some measure of teeth issues: crooked teeth, impacted teeth, overbite, underbite, or a combination thereof. I don’t mean that everyone I know has horrible teeth – it’s just that everyone has SOMETHING. For example I have mostly straight teeth but I also have a couple of crooked ones and my wisdom teeth were impacted. My husband has one tooth that’s higher up on his gums than the rest. My daughter has a tooth growing out of her palate. WTF, evolution? It seems to me that in general, other parts of the body grow in as they’re supposed to: the bones in our fingers or toes don’t tend to grow crooked; our ribs don’t poke out of our abdomens, spines generally grow straight, etc. Why then are teeth so messed up? I understand that physically interfering with the growth process (thumb-sucking etc) will alter the shape of the teeth. But barring those extenuating circumstances, why don’t our teeth just do what they’re supposed to do?
posted by yawper to Health & Fitness (29 answers total) 14 users marked this as a favorite
Best answer: Evolution is about efficiency, not appearance (assuming we're not using our teeth as camouflage). They don't always look pretty but they do what they're supposed to do, for the most part.

We're evolving, not evolved. As the human jaw gets smaller, teeth are running out of space and they get compressed more. I'd disagree with you about the spine, too. Your spine may be straight but there's a ton of variation, and our evolution from four-legs-bad to two-legs-good has introduced all kinds of back and neck problems.
posted by headnsouth at 7:58 AM on April 14, 2011

Best answer: Our teeth originally evolved when when we had a much larger jaw. Our jaw has since rapidly (in evolutionary time) evolved to be smaller, probably because jaw size is controlled by a relatively smaller number of genes than the amount and type of teeth we have. Now that we have modern dentistry, I wouldn't expect there to be any further evolutionary pressure to reduce the number of teeth we have, but maybe we can genetically engineer wisdom teeth away at some point.
posted by empath at 7:59 AM on April 14, 2011

No expert, but I remember reading something to the effect that human evolution has positively selected for large brains and language-related mouth parts, but the evolution of the skull around them is struggling to keep up. As long as the teeth keep chewing food until the individual reproduces, evolution isn't too picky about how effective they are.
posted by AzraelBrown at 7:59 AM on April 14, 2011

Well, they mostly at least come in through the gums, yes? I think part of the problem is perception - teeth, as a group, move together; their relative alignment doesn't change day-to-day. Also, teeth are in your *face*, so you're looking at or near them all the time. Fingers and toes *do* come in crooked. But motion both works them into proper, or at least more correct, configurations and hides the fact that they're crooked in the first place.
posted by notsnot at 7:59 AM on April 14, 2011 [1 favorite]

Hmm, I wonder how many animals have this problem. Can anyone think of an example?
posted by you're a kitty! at 8:06 AM on April 14, 2011

Domesticated dogs have all kinds of similar problems because we've warped parts of their body structure without giving evolution a chance to catch up and adapt everything else to work with it.
posted by empath at 8:09 AM on April 14, 2011

Nah, it's not just people. My cats have horrible teeth too, and it's genetic. I have mommy cat and two of her kittens, and they all have stomatis (ok, thats a gum issue), teeth growing in sideways, crooked, and other issues. And kitty dental isn't covered by my work dental plan.
posted by cgg at 8:10 AM on April 14, 2011

Actually, spines don't "tend to grow in straight" - many people (like me) develop adolescent idiopathic scoliosis. In my experience there is a lot of diversity when it comes to all aspects of a human body - for example, my husband has a very odd rib cage and the last set of long rib bones poke out at an unusual angle.

Teeth are only anchored on one side, so they are less constrained then vertebra or armbones, and additionally they are much more visible than my spine, so we notice teeth deformities more than we notice spinal deformities.
posted by muddgirl at 8:15 AM on April 14, 2011

I think part of the problem is perception - teeth, as a group, move together; their relative alignment doesn't change day-to-day. Also, teeth are in your *face*, so you're looking at or near them all the time.


Bunions are caused by misshapen foot bones. Also, some people have toes that are too long -- I had surgery recently because my second and third toes were too long, and this was making it hard for me to walk. But, had you looked at my foot before the surgery, you wouldn't have thought it looked all that weird, just that my big toe slanted a little towards my other toes, and the second toe was longer than it. It was only when the doctor took an X-ray that you could see how out of order the bone structure was. Deformities like this are pretty common, but they're hidden by skin, socks, and shoes. You just don't have much of a chance to notice them.
posted by meese at 8:20 AM on April 14, 2011

Many teeth problems are caused by diet. Poor diet inhibits proper growth and development of the body, including the gums and the shape of the head/jaw. It also makes thorough brushing and flossing much more important, putting hard-to-reach teeth at risk. Wisdom teeth interfere with how we implement orthodontics (braces, etc.) so they are often removed for that reason.
posted by michaelh at 8:30 AM on April 14, 2011

Dad's teeth. Mom's jaws.
posted by Cuspidx at 8:36 AM on April 14, 2011 [3 favorites]

I'm sure if people could easily see our ribs or spines just by looking at us, we would all think ours were crooked and unattractive. We would go to a professional and get then arranged just so, just like we do with our teeth, our hair, our skin, and other visible parts.
posted by that's how you get ants at 8:50 AM on April 14, 2011

So, remember that for most of human evolution we didn't have toothbrushes or dentistry.

How many teeth would still be in your head by your mid 20's? 30's? Etc.? I think part of the crookedness is that our bodies were built with the plan that we'd probably be missing some teeth between tumbles and tooth decay as we got older. If you had less teeth, there'd be a lot more space for things to come in.

Crooked teeth suck, but at least we got'em!
posted by yeloson at 9:02 AM on April 14, 2011

"bodies were built with the plan" is also metaphorical, and not, an argument on intelligent design or anything.
posted by yeloson at 9:03 AM on April 14, 2011

Best answer: headsouth and michaelh both have it.

Many of our dental problems are the result of both primate evolution (retention of a diverse, multi-purpose set of teeth and a marked degree of prognathism (protrusion of the mandible and maxilla); as well as changes in human diets (for instance, the dietary changes associated with emergence of more sedentary agrarian lifestyles have distinct consequences for bone and tooth development, and is clearly evident in the archaeological record of most early agrarian societies).

We should perhaps also distinguish between functional and aesthetic issues with regards to teeth. Functional issues are usually those dealing with use and retention of teeth, while aesthetic ones are wholly arbitrary and particular to specific contexts (for instance, in a number of societies, a snaggle tooth is considered cute or attractive, regardless of its functional impact. In others, it is not).
posted by TheWhiteSkull at 9:13 AM on April 14, 2011

Dad's teeth. Mom's jaws.
posted by Cuspidx at 8:36 AM

Same here.

Also, people have weird anatomical malformations all the time that just aren't visible or life-threatening, so we just don't think about them; for instance, a friend of mine was born with an extra vertabrae in his back. It's annoying as hell for him -- he had low-level back pain issues for years (until he met his current girlfriend, who's a massage therapist) -- but it's not life-threatening, and so he's just one of a squillion other people who also just have "bad backs" for any number of reasons. And if you looked at him all you'd see is that he's kind of a tall dude. So there's no reason that evolutionarily he would be disadvantaged, so...there's no reason evolution would try to "breed that out."

People have bad teeth, but they also can have extra fingers, vestigal tails, mirror-image internal organs, retroverted uteruses, or any manner of quirks that aren't life-threatening but are just...weird. Most of these things either get operated on (in the case of tails or extra fingers) or just left alone. Most people usually fix teeth because that's cosmetic as well.

(Tangentially -- the guy with the extra vertebrae is the one person I know who was born with perfect teeth, and has never needed any kind of orthodontia in his life. My other friends and I are all insanely jealous.)
posted by EmpressCallipygos at 9:17 AM on April 14, 2011

Sorry, my comment should read "a marked reduction of prognathism"
posted by TheWhiteSkull at 9:20 AM on April 14, 2011

Best answer: Other people have basically already said this, but I still want to chip in because I like the question.

It didn't benefit our survival enough, over the generations, for perfectly aligned teeth to evolve into the standard. If you can still chew food, your teeth are, technically, doing what they're supposed to do. Crooked teeth didn't affect one's ability to survive long enough to reproduce (the only thing evolution cares about), so there was no pressure to change. They're not attractive, sure, but that's a fairly recent state of mind. Way back in the day, "attractive" meant something like wide childbearing hips, a healthy amount of fat, and no outer signs of disease. Being snaggletoothed likely didn't help, but if you were some poor Dark Ages farmer and your choice was between the girl with the mysterious open sores or the girl with bad teeth, you'd probably marry the girl with bad teeth.

Wisdom teeth originally showed up (I think) thanks to a tough, rugged prehistoric diet of meat and foraged plants, which required plenty of molar power to chew into something digestible. Those wisdom teeth showed up late as a backup because your other molars were wearing out after years of chewing this tough food. Keep in mind our earliest ancestors were lucky to live to 30, so some new molars appearing at age 18 would hopefully last you until you died. We transitioned to an easier diet only within the last 9,000 years (if dating it to the beginnings of agriculture) which is a short period of time when discussing evolution, not nearly long enough for something as minor as wisdom teeth to get ironed out.
posted by castlebravo at 9:24 AM on April 14, 2011

Well, my dentist mentioned that we wouldn't have so many cavities if we ate what cavemen ate (unprocessed grains and other hard things) because our molars would wear smooth, and there wouldn't be the grooves for stuff to get trapped in. So that's something. Also, evolution goes with 'whatever works'. If it keeps you alive long enough to have babies, it's good enough. (Also, I'm missing some cartilage in my elbow. It's apparently not uncommon, and nobody would ever know unless they took an x-ray. A missing incisor would be a lot more obvious.)
posted by Green Eyed Monster at 9:25 AM on April 14, 2011

Best answer: This is a nine-part discussion on the evolutionary history of malocclusion - it's pretty fascinating.
posted by restless_nomad at 9:39 AM on April 14, 2011 [3 favorites]

There are tradeoffs between teeth which are better at chewing and teeth which are easier to grow: humans have teeth which are better at chewing. Close packed so that food can't fall between them and with interlocking shapes above and below so that food is ground up evenly. But since chewing teeth are so complicated problems are really obvious and we can't just grow more teeth (they wouldn't match up).

If you compare to an animal which only uses teeth for ripping, they have much simpler if there's an uneven gap, no big. If there's a tooth at an odd angle, it gets ripped out eventually and they grow a new one.
posted by anaelith at 9:45 AM on April 14, 2011

1. The invention of cooking has reduced the importance of teeth.

2. A normal spine isn't straight. The cervical and lumbar portions normally have lordosis, and the thoracic portion has kyphosis.
posted by neuron at 10:19 AM on April 14, 2011

Kitty, re: animals, specifically horses.

Again, if an animal lives long enough to reproduce, it has performed its function, and beyond that, all of its teeth can fall out or it can die. However, there is some selection for good teeth in that an animal with a correct mouth will be able to eat better and maintain good condition. That animal will be most fitted for breeding and likely to get mates. Mares will be able to come to term and feed their foals in a more advantageous way that will allow their offspring to survive and thrive to carry on the good teeth gene. Animals that develop major problems will be culled earlier and have fewer offspring to pass on bad teeth.

Twice a year I work as the handler for an equine dentist, and we see some particular trends in horses. First, like dogs, selecting for looks and breed 'type' has ruined many a horse's mouth by breeding for tiny tea-cup muzzles which cause crowding and crooked teeth. Small mouthed Arabians and tiny mini ponies are extremely hard to work on and keep correct to avoid problems as they age. Second, breeders often don't even bother looking at the teeth and mouth, and even though over- and under-bite are considered to be faults, they get passed on fairly regularly. Malocclusions are frequent in many registered horses, but seldom do we get mustangs with these problems.

Horses shed their baby teeth, and often these caps don't come off as they should and cause major problems with irregular chewing surfaces. Horses teeth are hypsodontic in nature, meaning they have a limited growth period but continue to erupt throughout the lifetime until the tooth is lost. The majority of a horse's teeth are within the jawbone, and lengthen to compensate for wear, but with prolonged eruption throughout the life of the animal, any irregular eruption on the molars is magnified as the animal ages. Wear patterns are a reasonable indicator of the age of a horse. The phrase "long in the tooth' refers to an old horse.

Should the incisors have problems, this can cause problems in grazing, and thus body condition, as well as affecting molar wear. As horses chew, their jaw travels in an elliptical motion, moving both to the left and right, as well as forward and backwards. Their molar arcades are naturally angled, and the grinding causes sharp points to form and can cause sores on the cheeks and tongue which can also affect their eating patterns. Bad teeth can also trap a fair amount of food in between one another or in cheek pouches which causes gingivitis. Left untreated, there can be tooth decay, gum disease and even serious abscesses.

Changing the foods that horses eat affects their teeth and exacerbates any minor tooth problems. Most domestic horses are fed hay and no longer graze, which helps keep the incisors straight and in correct wear. Silicates in the grass and dust/dirt wears the molars and prevents abnormal eruption height. Animals that graze on sandy soil often have the opposite problem--their teeth wear out faster than they can erupt.

I was given a very expensive registered 8 year old Missouri Fox Trotter deemed 'vicious' and unrideable because he bucked several people off. I took him because he had a beautiful kind eye, and restarted him under saddle. When he went in to the dentist last fall, it turns out he had horrible wolf teeth, extra 'fangs' seen mostly in males. These teeth sit in front of the molars where the bit rests and are very sensitive. Viciousness cured!

Out of the five horses, I only purchased one--the one with the worst teeth, of course. He's ancient, but worth his weight in gold as a safe horse for my grandkids. His mouth was never cared for, and he has lost so many molars that his gums squeak when he chews, so he's on special senior feed.

Another kid's horse that was given to me because he was too old to work for a living as a cowhorse on has seven incisors instead of six. This affects his chewing pattern, and he's harder to keep weight on.

The most recent horse given to me was the gift horse that I should have looked in the mouth! I never thought to check a younger horse, then I finally did look, he had extensive wear on his incisors, indicating that he'd had the habit of chewing wood for a long time. That explains my deteriorating fences.

TL;DR--more than anyone ever wanted to know about a horse's teeth!
posted by BlueHorse at 1:06 PM on April 14, 2011 [4 favorites]

A long time ago I took a postgrad class that touched on a lot of what's been mentioned above--evolution causes bodies to develop in quick, hodge-podge ways that get the job done with what's at hand, not necessarily to an end product that looks perfect from a design perspective. Thus we end up with crooked teeth that still get the job done, not straight teeth that look and work perfectly, and other examples mentioned upthread.

In passing the lecturer mentioned that there were indications that infant/childhood diet (specifically the softness or toughness of the food) could influence the rate of development of the jaw bone itself. It was only mentioned in passing, it's possible that it was an example of something once believed and disproved or whatever. But if not, then it is possible modern man's early diet is leading to smaller jaws and more crooked teeth.
posted by K.P. at 2:26 PM on April 14, 2011

Best answer: Depending on how interested you are, you could read Nutrition and Physical Degeneration. It's old but really fascinating.
posted by Durin's Bane at 4:19 PM on April 14, 2011 [3 favorites]

Seonding restless_nomad and Durin's Bane's comments. Fascinating resources, they are.
posted by Earl the Polliwog at 5:55 PM on April 14, 2011

I was going to post the same link as Durin's Bane. It's been several years since I read it, but the main point I remember is that refined carbs have a direct correlation with crooked/misaligned teeth.
posted by MexicanYenta at 9:02 PM on April 14, 2011

Best answer: Teeth grow in crooked for all sorts of reasons referred to above, but the main reason that this keeps happening and getting worse through the generations is that we are humans, and can adapt our environment, so the crooked teeth are not resulting in death by starvation before we can reproduce.

In 'wild' animals, evolutionary pressure says that those that are poorly adapted (e.g. bad teeth) don't reproduce as successfully, as they have to live with the teeth they are given and the environment they live in. Humans can adapt our environment by making tools to process food to make it capable of being eaten by someone with poor teeth, or even change the shape/positioning of the teeth to make them more effective. This results in what you might think of as poorly adapted humans (e.g. with bad teeth) still being fit enough to compete for a mate and therefore reproduce.
posted by muckybob at 6:56 AM on April 15, 2011

Here's a nice lay-friendly summary of the work by Weston A. Price cited above by Durin's Bane.
posted by Franzilla at 9:49 PM on May 18, 2011

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