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Something changed over the course of human evolution that left a lot of us with weak chins and buck teeth.

So says Janet Monge, the keeper and curator of the physical anthropology section at the University of Pennsylvania Museum of Archaeology and Anthropology.

There are lots of reasons to study the bones of ancient people.

Some scholars are interested in how our distant cousins walked, or the shape of their heads, or how certain muscles fit together.

"A person like Janet Monge doesn't give a damn, I really kind of want to know when they become humans," Monge said.

In particular, Monge wants to know how children become social, how they learn to share or cooperate, for example. A psychologist might observe a group of six year olds while they play. Monge is a paleoanthropologist, so she looks for evidence in skeletons and fossils.

She's studied skulls across the ages and along the way, she and colleagues in the field noticed something: "Nobody in the past had dental problems, like we are talking nobody," she said.

(A fun fact: most mammals in the wild have straight teeth too.)

Monge isn't talking about cavities; she wants to demonstrate how straight teeth used to be. For the proof, she sends an intern to the Penn Museum mummy room to find a specimen from the Hasanlu dig site in Iran.

The skull is from a person who lived maybe 5,000 years ago. And yet it has a Hollywood smile. Straight, white, symmetrical. Two little rectangles at the front. Long eye-teeth at the sides. And wisdom teeth—sitting pretty in the back.

"It's like the upper jaw, the maxilla, and lower jaw the mandible are actually kind of perfectly in unity with each other and the interesting thing is that was everybody in human history," Monge said.

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Janet Monge, Keeper of Physical Anthropology at the Penn Museum, shows the near-perfect alignment of teeth in an ancient skull. (Emma Lee/WHYY)

And that's the crux of a burgeoning debate. Americans spend millions of dollars on braces, retainers—tooth extractions--to shape our smiles. But we don't have a great answer to this fundamental question: Why do we have crooked teeth?

Crooked teeth are an epidemic today. Do malocclusions come from genes and heredity (nature)? Or more from nurture: our environment and habits after we are born?

To share her expert guess, Monge opened up her lab—a well-loved, comfortable jumble of artifacts and papers. On a cart she laid out a collection of about 20 different skulls. The bones are creamy white, brown and yellow. One is from ancient Peru. One is from Philadelphia in the 1800s.

The museum's team of bone and tooth experts use super-powered imaging scanners and an electron microscope that can show a sliver of hard tissue at 40,000 times its actual size.

Monge pinpoints the emergence of crooked smiles to perhaps 150, 200 years ago.

"It happened fast," she said. "Something significant happened, and it's almost global."

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The fragile skull of a six-month fetus is one of the many skulls in the Penn Museum collection. (Emma Lee/WHYY)

What happened? The Industrial Revolution happened. That's the period when a culture shifts from agrarian, rural life to manufacturing jobs in the city.

So here's the hypothesis: As women went off to factories, they couldn't take their babies with them. After that, many mothers nursed for a much shorter time. Eons before that moms carried their babies with them everywhere and they commonly breastfed until a child was age three or four.

By the mid-1800s, there were many more bottle fed babies, Monge said.

The way an infant sucks from a bottle is very different from how she feeds at the breast. Breastfed infants have to work hard to get milk. And that muscle training shapes the growth and development of the face. As those babies eat, with every pull they create a wide—more projected—jawbone, Monge said.

"It's even proper tongue position, which is high up in the palate which actually pushes on the beginning front teeth," she said.

On the other hand, an offset jaw—like a rickety house foundation—starts the trajectory of adult teeth that can come in crooked later.

"It's basically like, we've artificially created the weak face," Monge said.

Mouths are more narrow today, Monge says, but we have the same number of teeth as always. And so the idea goes: that causes crowding, an overbite, snaggle teeth or wisdom teeth that have no room to grow in.

That's one hypothesis on the environmental origins of crooked teeth.

Another possibility is that modern people developed weaker-looking jaws because most everything we eat today is soft. To get our calories, we hardly have to use our jaw muscles at all.

In the distant past, humans had to gnaw on sticks, grasses and other foods that really challenged their teeth. Without powerful, shapely chompers to process food, they didn't eat. Only our ancestors with efficient teeth and well-aligned jaws survived.

"They didn't have to brush their teeth, everything they did everyday brushed their teeth," Monge said.

So what does it matter if you have crooked teeth? A beautiful smile is nice, but dentists say there are also health reasons to covet straight teeth.

Misaligned teeth are harder to keep clean. Protruding lower teeth rub-against and wear-down teeth on the top. Crooked teeth can make it difficult to chew.

And that sends lots of us to the orthodontist.