ToothEvolutionFilter: Why are human teeth so poorly "designed"?

March 21, 2006 8:59 AM

ToothEvolutionFilter: Why are human teeth so poorly "designed"?

I've read the wikipedia page on teeth and tooth development, which discuss how our evolutionary diets determine tooth shape (e.g. molars) and how different types of animals have different tooth-replacement policies (sharks have new sets, rodents grow forever) and types (horses have a different form of enamel). But I am still puzzled about why humans have two sets of teeth, why they get replaced at a fairly early age and never again, and why they are so susceptible to decay. I understand that we have much different modern diets than our evolutionary ancestors which might induce more decay (more sugar, less fiber) but it still seems that our teeth would have decayed and fallen out! Is this just an attribute in evolution that's non-selective?

posted by beerbajay to science & nature (29 comments total)

Because by the time your teeth fall out, you've already passed on your genes?
posted by Leon at 9:03 AM on March 21, 2006

It may well just be not selective enough. If we managed to get along with not-so-durable teeth, the teeth obviously sufficed. Selection is a passive, not an active, process -- we look at what lives and what doesn't live and we call it "selection" as a convenience.

In other words, better teeth may have had less effect on the survivability of a vector of the species than, say, sharp vision; and so some hypothetical humans with more durable teeth but poorer vision didn't persist. It's not that the teeth are or aren't selective; they just weren't (if ever there were better teeth out there) attached to an ultimately successful branch.

Consider also that if humans used to live till maybe 20 or 30, those baby teeth wouldn't have been disappearing "at a fairly early age".
posted by cortex at 9:08 AM on March 21, 2006

Also, sharks and rodents are way more tooth-dependent than we are. Sharks, remember, have no arms. Rodents don't have thumbs. Sharks and rodents both use their teeth as their only "tool." For us, it's not even a primary tool.

Why expend the resources to keep growing new teeth when we can smash our food between rocks (or boil it into gruel, or chuck it in a food processor, or...)?
posted by nebulawindphone at 9:10 AM on March 21, 2006

Or died. Medical advances in the last few hundred years have pushed the human life span past the original design requirements for teeth. Thankfully we also evolved dentists.

On preview, cortex got there faster than I did.
posted by mbd1mbd1 at 9:10 AM on March 21, 2006

The decay thing is directly attributable to agriculture and specifically sugar consumption. We've always had the plaque bacteria, but until we started consuming massive (relatively speaking) amounts of sugar in everything we eat, they didn't produce enough acidic waste products to be a huge problem.
posted by Capn at 9:11 AM on March 21, 2006

cortex, nebula: Hmm yeah that's what I was musing about in my last sentence there. Sounds plausible enough.

On a related note; is there any sort of technology for replacing all your teeth (besides dentures) which will withstand the plague of sugar-happy bacterias in our mouths? Like replacement enamel or something?
posted by beerbajay at 9:15 AM on March 21, 2006

The decay thing is directly attributable to agriculture and specifically sugar consumption. We've always had the plaque bacteria, but until we started consuming massive (relatively speaking) amounts of sugar in everything we eat, they didn't produce enough acidic waste products to be a huge problem.

I'll second this. As evidence, I offer the number of documentaries that I can't count on both hands that I've seen about early civilizations that point out that they had no or minimal tooth decay since they did not eat refined sugar.

It's like the ADA has a mole on the staff of the Discovery Channel!
posted by illovich at 9:16 AM on March 21, 2006

Like replacement enamel or something?

I have heard that there are some new enamal-renewal processes in development—something like "painting" (or perhaps laser-blasting or god knows what) an actual layer of enamal onto existing teeth. I can't recall the (vague, possibly pop-sci) source, however.
posted by cortex at 9:22 AM on March 21, 2006

On a related note; is there any sort of technology for replacing all your teeth (besides dentures) which will withstand the plague of sugar-happy bacterias in our mouths? Like replacement enamel or something?

Yeah. Fluoride. It replaces hydroxide ions in your teeth with fluoride, making them more resistant to bacteria.
posted by landtuna at 9:33 AM on March 21, 2006

The japanese have developed a product/process that repairs enamel with the mineral/gemstone apatite
posted by hortense at 9:35 AM on March 21, 2006

Same reason we don't have claws, chimp-strength, poison darts that shoot from our tear ducts, hawk vision, or sonar. It's all about the brain with us. The dumbest person you've ever met is still one of the smartest creatures on the planet. That is our claw, our super-strength. And it's pretty much the only one we got.
We're not the inevitable end of an evolutionary process that had a goal; we're just smart enough to survive (and propagate) without much else in our toolbox.
posted by willpie at 9:45 AM on March 21, 2006

Short answer: they're not 'designed' at all.
posted by willpie at 9:45 AM on March 21, 2006

Neat, I did not realize that flouride was chemically altering my teeth.
posted by beerbajay at 9:46 AM on March 21, 2006

Thus the "quotes." =)
posted by beerbajay at 9:47 AM on March 21, 2006

Understood. I mostly just couldn't resist the opportunity for mild snark, but it's also a serious answer. The question, phrased as it is with the quotation marks, flirts with answering itself. They're not perfect because they're not designed; that's just how they ended up when we reached the point at which we could sustain ourselves.
posted by willpie at 10:17 AM on March 21, 2006

Tooth decay in young humans would be a big problem. But we're talking about tooth decay that happens later on in adult life. By that time, the humans would have already reproduced, and passed the genes for late-adulthood decaying teeth on to their offspring.

Basically, by the time you're old enough for it to be a problem, you've already reproduced, and served your biological use. So the trait doesn't impact selection.

This happens in elephants sometimes. Some of them continue to grow these giant tusks as they get older, to the point where it's difficult for them to lift their heads. But by the time this defect gets expressed, they've already had kids.
posted by Gamblor at 10:29 AM on March 21, 2006

Evolution, as natural selection, only can work for the ages right when reproduction happens and prior. So considering humans have until quite recently had offspring at around 15-17 years old, the favorable genes are genes for people under 18 or so. Most people that age and younger have great teeth. The rot and decay happen after reproduction ages where you are literally aging and falling apart.

When it comes to longevity and quality of life, the mechanics of evolution don't really apply. On the bright side humans don't die immediatly or get eaten by other humans after giving birth.

What cortex said too. Humans who died before hitting 30 or so seemed to have "better designed and working teeth" as post-30 rot never kicked in because everyone was long dead. Wisdom teeth problems and serious rot don't generally happen before 17 or 18.
posted by skallas at 10:30 AM on March 21, 2006

I can't wait for the ozone tooth decay treatment to become mainstream.
posted by the jam at 10:30 AM on March 21, 2006

Evolution, as natural selection, only can work for the ages right when reproduction happens and prior.

Perhaps nitpicky, but this isn't strictly accurate. Obviously, if all human mothers died during the birth of their last child, that'd have a hugely negative selectionary effect because (unlike many species) human mothers continue to be vital to their children's well-being long after birth. And surely in many instances, even grandparents or other farther removed ancestors than parents have a beneficial effect on children's likelihood of evolutionary success.
posted by BaxterG4 at 10:52 AM on March 21, 2006

Keep in mind that tooth decay resulting from a large amount of sugar in the diet only became a problem with the advent of agriculture and the domestication of cereals. So humans have been dealing with those problems for at most around 10,000 years (for some populations). That is not long for selection to have had a huge impact if the problems are relatively minor. For comparison purposes, it looks like it took about 100,000 years for the effects of latitude to lead to the modern differences in skin melanin and color.

Also, keep in mind that before agriculture, grit in the diet was the biggest danger for the teeth of relatively old humans/hominids. It caused teeth to be worn away. So selective pressures on teeth would have been very different before agriculture.

Finally, there is evidence that hominids have been helping out their teeth for over a million years. At least one researcher claims to see evidence for toothpick use on 1.8 mya specimens. They were presumably getting meat caught between their teeth. If hominids have been using their manual dexterity and intelligence to help their teeth along as we do with dentistry today, then there would be even less selective pressure to modify teeth morphology.
posted by Tallguy at 10:52 AM on March 21, 2006

Putting it another way, the human skull has changed shape over the evolution of upright hominids. Our massive brain as compared to our forebears certainly had an effect on the shape of our lower half of our heads. Consider that we are still evolving. Were perfectly straight teeth necessary to our survival these days, they would evolve in that direction.
posted by toastchee at 11:06 AM on March 21, 2006

Basically, by the time you're old enough for it to be a problem, you've already reproduced, and served your biological use. So the trait doesn't impact selection.

This never quite made sense to me.

Og has good teeth. Og starts siring children upon the fertile women of his tribe on a regular basis, and dies an old man having fathered 30 children who lived to reproduce.

Grog has bad teeth. He has five kids, two of which live to reproduce, before he dies of a raging oral infection at 25.

So why does the distribution of children in the next generation not reflect Og's greater contribution of good teeth? Why does selection operate only dichotomously (you have kids or don't)?
posted by ROU_Xenophobe at 11:24 AM on March 21, 2006

I'd say resistance to gum disease or just general resistance to infections is not related to dental problems. Rot doesn't equal death. How many typical dental problems lead to death? Two or three out of dozens? Cavities, dental pain, tooth loss, or even a bad case of thrush and gingivitus doesn't stop Og from getting his game on.

Age leads to teeth loss and other problems, but Og could still have lots of kids with bad teeth. Hell, look at the British!
posted by skallas at 11:32 AM on March 21, 2006

ROU, you're presuming that Og's good teeth are a dominant factor in his survival and the survival of his offspring.

If Og has good teeth and weak bones, the Ogs die off. If Grog has bad teeth and strong bones, the Grogs propogate. If Og has good teeth and a low sperm count, the Ogs die off, whilst virile, tooth bedamned Grog gets fertile and multiplicative.

Who ever lives, wins. Whatever baggage they bring with them is secondary. Good teeth, useful though they be, may have just be baggage in human development.
posted by cortex at 11:34 AM on March 21, 2006

If Grog has bad teeth and strong bones, the Grogs propogate.

my old medieval lit professor once stated that the greatest gain in human lifespan was caused as a result of dental hygeine. no idea if it's remotely true, and i'm not sure if it was the rotting out teeth that killed you with infections or what, but it was said!
posted by soma lkzx at 11:47 AM on March 21, 2006

Who ever lives, wins. Whatever baggage they bring with them is secondary.

Exactly. ROU, you could say the same thing about Alzheimer's disease, cancer, or any one of a long list of ailments that primarily affect the elderly. Why haven't these been weeded out via natural selection? Because they don't impact the organism before or during prime reproductive age. They're just (unfortunate) genetic baggage that shows up later on, after the fact.
posted by Gamblor at 12:33 PM on March 21, 2006

Fire. Cooking.

Cooked food generally makes it softer and easier to eat. As a result, there's been less selection pressure on Homo teeth for a couple milion years.
posted by orthogonality at 2:25 PM on March 21, 2006

Man, evolution is so dumb. Fix my unsatisfactory tooth design, nature!
posted by beerbajay at 6:09 PM on March 21, 2006

Teeth are superbly evolved. The fact that we have only two sets: a baby set and an adult set (normally) is the problem.

I believe that very soon stem cell treatments will solve this problem by allowing dentists to induce the growth of new teeth.

I would not be surprised if they could do it allready. But think now... Won't that be a shock to the dentistry industry. Would they allow it?
posted by Arno at 3:36 AM on March 29, 2006