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Chew on This

What is it about humans and the need to pick our teeth? Is it a habit we got from our ancestors because of the meat we eat?


The next time you start probing with a toothpick for that elusive piece of sirloin, and your date threatens to leave the restaurant, try this dodge:

Tooth-picking is one of mankind's oldest and most widespread habits. There is evidence of it in wear patterns on teeth from every continent, and in fossil teeth as far back as Homo erectus--the first of our kind--2 million years ago.

It's quite possibly a legacy of our ancestors' taste for meat--a tough and stringy substance that often gets jammed between molars that evolution designed mostly to crush nuts and fruits, not to slice a steak.

So . . . it's not your fault.

You can say you heard it from Peter Ungar, an associate professor of anthropology at the University of Arkansas, or Mark Teaford, an anatomy professor at the Johns Hopkins School of Medicine.

They have spent years making connections between tooth design, diet and wear patterns in living primates and then applying that knowledge to the fossil remains of our hominid ancestors.

Discovering what our ancestors ate helps us in two ways, Ungar said. It can give us insights about behavior otherwise hard to glean from the fossil record, and it can shed light on the current debate over so-called "Paleolithic" or "cave man" fad diets.


Ray Audette, in "Neander-Thin: A Cave Man's Guide to Nutrition," and S. Boyd Eaton, in "The Paleolithic Prescription," argue that most modern health problems arise from today's Western diet--heavy in sugar, grains, salt and fat.

"All these people sort of make the assumption that we ought to be eating what our ancestors ate," Ungar said. Trouble is, "we just don't know that much yet about what our ancestors were eating."

Ungar began his search for answers a decade ago in the vast rain forest of the island of Sumatra.

He spent more than a year there recording what the gibbons, macaques and orangutans ate. Then he matched their diets to their tooth and jaw design and to microscopic wear patterns on their teeth.

Armed with similar data they and others gathered in Costa Rica, Venezuela and Africa, Ungar and Teaford began to apply what they'd learned to the remains of our earliest ancestors.

Five million years ago, it turns out, at the end of the Miocene epoch, the many ape species had teeth and jaws adapted to a wide variety of diets. But most were dietary generalists. Like chimps today, they had teeth with moderately sharp cusps for cutting and slicing through such things as fruits, flowers, shoots, leaves and insects.

"When you're slicing through soft, tough foods, you want your teeth to shear past one another like scissors," Ungar said. Abrasive grit in the food slides along the tooth, too, causing telltale linear scratches.

But by 4.1 million years ago, during the Pliocene epoch, a new genus known as Australopithecus had appeared.

"You're starting to see bigger back teeth, a thickening of the enamel, thicker jaws and an ability to at least include more hard, brittle foods in the diet," such as nuts, seeds and tubers, Ungar said.

But that ability came at the expense of being able to easily break down tough, pliant foods such as leaves, soft seeds or tough fruits. For the same reason, it also made them poorly adapted to eat meat.

The shifts probably came in response to a changing habitat, Ungar said. Global temperature and rainfall were becoming more variable, so it was to our ancestors' advantage to adapt to a more varied menu.

By 2.5 million years ago, the woodlands of eastern and southern Africa were opening up with the spread of savanna grasslands. And two new hominids appeared, including Homo erectus, our own lineal ancestor.

Homo erectus's molars were flat--great for hard, brittle foods, but not so hot for chewing meat.

However, H. erectus remains are also found with simple tools--stone flakes for cutting--and animal bones bearing evidence of butchering. If these hominids couldn't process meat well with their teeth, said Ungar, they evidently learned to cut it first, then eat.

Later, as humans evolved, the tools became even more specialized, opening up more food sources and new habitats. Probably because of increasing tool use, teeth and jaws became smaller and lighter. Around this time, oddly worn grooves began to appear between H. erectus' molars.


Which brings us back to toothpicks.

In an upcoming paper, Ungar and Teaford describe a groove worn into the enamel of a 1.8 million-year-old tooth, probably from H. erectus, found at Olduvai Gorge in Tanzania. They contend that the best explanation for the groove is the very thing that drives etiquette mavens crazy: tooth-picking, perhaps with gritty bone shards or sticks.

Significantly, Ungar and Teaford said, the grooves first appear right around the time Homo erectus began butchering and eating meat.

"It is compelling," Ungar said, only half in jest, "that we tend to find toothpicks in steakhouses, but not in vegetarian restaurants." (Not to worry, though. Today's toothpicks are too soft to wear grooves in our teeth.)

Should the toothpick trail of evidence lead us to conclude that evolution did not intend us to eat meat?

Ungar doesn't think so: "Part of what makes us human is that we can take a really broad variety of foods, because we evolved to be generalists."

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