Evolution of eating is a mouthful; Study: Researchers have sunk their teeth in, and those of ancestors millions of years ago, to analyze primates' dietary change from leaves to nuts to steaks.


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 for 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 two ways, Ungar says.

"First, diets are very closely linked to aspects of animal ecology - how they moved around, what group size was like, and how they lived in the forest" - insights otherwise hard to get from the fossil record.

Second, it can better inform 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, that there's a discordance between what we evolved to eat" and what most of us actually consume, Ungar says. The 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 Gunung Leuser National Preserve, on the island of Sumatra, in Indonesia.

He spent more than a year there recording what the gibbons, macaques and orangutans ate. He then 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 the knowledge 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 says. Abrasive grit in the food slides along the tooth, too, causing telltale linear scratches.

With electron microscopes and laser mapping devices, Ungar and Teaford can see linear scratches in the teeth of modern leaf-eating primates, and in the Miocene apes.

But by 4.1 million years ago, during the Pliocene, a new genus called Australopithecus appears. "With Australopithecus anamensis," Ungar said, "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," Ungar said.

That kind of blunt chewing equipment is designed to crush and grind. And microwear studies reveal less scratching, and more pitting characteristic of crushing action.

Clearly, these are animals evolving an ability to eat hard and brittle things, such as nuts, seeds and tubers.

"It's subtle," Ungar says. The Australopithecines could still chew up flowers, buds and shoots, and they were adding an ability to process more and more hard foods, such as nuts.

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

The shifts probably came in response to a changing habitat, Ungar says. 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.

One, Australopithecus paranthropus, was a "robust" type that evolved to specialize in the savanna's resources - roots, tubers and seeds. His dental gear included big, flat molars, thick enamel to resist breakage, and a heavy jaw with massive chewing muscles.

"He is a hard-object specialist," Ungar says, "but he has lost the ability to consume soft, or tough foods."

The other is Homo erectus, our own lineal ancestor. He remained more of a generalist, with a flat molar design that allowed him to eat some hard, brittle foods, but ill-adapted for meat.

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

Later, as more modern humans evolved, our tools became even more specialized, opening up more new food sources and new habitats. Our teeth and jaws, meanwhile, became less so - smaller and lighter.

Ungar reasons that tool use took the selective pressure off. There was no more need to maintain big teeth and heavy jaws.

But it's at that same moment, about 2 million years ago, that oddly worn grooves begin to appear between H. erectus' molars.

Which brings us back to toothpicks.

In a paper being prepared for publication, Ungar and Teaford describe a groove worn into the enamel of "OH 60," a 1.8 million-year-old tooth, probably from H. erectus, found at Olduvai Gorge in Tanzania. They contend that the shape, positioning and microwear on this groove are best explained by tooth-picking, perhaps with gritty bone shards or sticks.

"These are called interproximal grooves," Ungar says. First noted a century ago by dentists examining Neanderthal teeth, the peculiar wear pattern always appears near the gum line, on the enamel surfaces where adjacent molars or premolars contact each other.

The pattern has been found, in a variable percentage of individuals, on every continent, but in no living primates. And it is never found in Australopithecus, making the habit unique to our genus - Homo.

And significantly, Ungar and Teaford believe, it first appears at the time when Homo erectus began butchering and eating meat.

Other explanations have been advanced for these grooves. But Ungar and Teaford challenge and reject each one in turn.

"It is compelling," Ungar says, only half in jest, "that we tend to find toothpicks in steakhouses, but not in vegetarian restaurants." (In fact, today's toothpicks are too soft to wear grooves in our teeth. Dentists just don't see them.)

So should the toothpick 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," he says.

Still, he says, "moderation is always a good thing. My grandmother told me that, and it still holds today."

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