If you have ever watched a kid who's missing their front teeth try to eat an apple, you know how important teeth are from a biological standpoint. Well, a new study looks at the outermost layer of teeth, and as NPR's Nate Rott reports, what it found could apply far beyond the kitchen table.
NATE ROTT, BYLINE: One of the scientists you're about to hear describes teeth as the intermediary between the eater and the eaten. And their new study focuses on the part of teeth that we're all familiar with - enamel.
MACKIE O'HARA: Enamel is the hard outer covering of your teeth.
ROTT: Mackie O'Hara is an evolutionary biologist at Ball State University in Indiana.
O'HARA: It's the white part of your teeth that you see every day.
ROTT: And it's the hardest material in the human body for good reason. Unlike bones or the mushier inside of teeth, which are made of living tissue that can regrow or repair itself, enamel cannot.
PUPA GILBERT: And what that means is that it has to last for a whole lifetime.
ROTT: Pupa Gilbert is a professor of physics at the University of Wisconsin-Madison. She co-authored the new study published in the journal Nature with O'Hara and others.
GILBERT: This means that it had to evolve to actually have mechanical characteristics that are especially durable.
ROTT: To prevent fracturing - scientists have known at the macroscale what we can see with the naked eye. Enamel can be thicker in primates that eat hard foods like seeds or nuts.
GILBERT: Then you go down to the microscale, and you see that there are all these bundles of nanocrystals that crisscross one another like in plywood.
ROTT: Giving the enamel even more toughness - but what Gilbert wanted to know was whether enamel is evolving at the nanoscale, even smaller. So she developed a method using electron microscopy and other tools to look.
GILBERT: What you see is that there are very parallel crystals that are parallel like stray hair in a ponytail, if you wish, or fiberoptics in a bundle.
ROTT: But she noticed that some of these crystalline hair strands weren't exactly parallel. They were slightly askew or misoriented in a way that would prevent cracks from spreading. So with O'Hara and others, they compared the degree of misorientation they were seeing between teeth of different primates. At one extreme, O'Hara says they looked at the enamel of a West African monkey called mangabeys, which eat an extremely hard nut.
O'HARA: They're essentially the candy gobstoppers that are, like, the size of a bowling ball and as hard.
ROTT: And they found the most misorientation of any sample they looked at. On the other extreme, they looked at chimpanzees.
O'HARA: Chimpanzees really do not eat hard foods. They show the least misorientation.
ROTT: Suggesting that even at the nanoscale, teeth are evolving to adapt to different diets - O'Hara and Gilbert were curious if they could see that evolution in human teeth too. So they looked at samples of early humans dating back nearly 18 million years ago from three different species of hominins, one that anthropologists know ate tough, sinewy meat at that time, one that didn't, and one that ate a mixture. And Gilbert says they found...
GILBERT: That meat eating correlates beautifully to an increase in misorientation, which was significant.
ROTT: In other words, enamel got tougher to correspond with our changing diet. With more ancient teeth samples, Gilbert says, this method may even help anthropologists determine when humans started cooking food. But more immediately, she says, this finding - that misorientation of nanocrystals can help resistance to fracturing - is worth exploring as a toughening mechanism for materials in other areas.
GILBERT: From the space industry, the sports industry, the car manufacturing, just you name it.
ROTT: All from a close-up look at some pearly whites. Nate Rott, NPR News.
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