Early Human Had "Nutcracker" Jaws

Christine Dell'Amore
National Geographic News
February 2, 2009

Long before "wimpy jawed" Homo sapiens took over the planet, our early predecessors had bites strong enough to split open seeds even harder than walnuts, new research says.

But what such nutcracker jaws were used for has been a source of debate among paleoanthropologists for decades.

Now, a new study finds that Australopithecus africanus had a "whole chewing apparatus" for cracking open large, hard foods such as nuts and seeds when its preferred foods were scarce.

What's more, the species' well-muscled heads could withstand high forces on the premolar teeth—those just behind the canines—unlike the heads of other primates. (Explore the modern human body.)

"You can imagine these guys taking these large objects and sticking part of it in their mouth and cracking it," said study lead author David Strait, an anthropologist at the University at Albany in New York.

The hominins, which lived in woodlands in southern Africa about two million years ago, would then take the shell off and eat the softer "good part."

But at least one expert warns that the study takes "bold leaps in interpretation" and will likely ignite even more controversy.

Brunt of the Bite

Using a novel approach, Strait and colleagues applied an engineering technique, among other methods, to an A. africanus skull model, among other methods, to test how well the face responded to forces generated by premolar bites.

Based on the finding that premolars took the brunt of the bite, the team rejected the idea that the species had large teeth, jaws, and chewing muscles just to eat lots of food at one time.

That's because A. africanus wouldn't need a specially adapted mouth if it ate high volumes of similar foods: Any of its teeth could do the job.

Nor did it make sense that the hominin needed to bite with its premolars to eat small, hard foods, since it could simply put the whole item in the back of its mouth and bite down with its molars.

(Related: "Tooth Study Reveals Diets of Early Humans" [August 3, 2005].)

So the most likely reason for the sheer might of the premolars, Strait said, was so they could crack open larger objects that were too big to cram all the way into their mouths.

The evidence, published this week in Proceedings of the National Academy of Science, adds to an expanding body of thought that evolution favors designs that accommodate "fallback foods"—those critical to survival but difficult to access.

No Walk in the Park

The new study is the latest in a litany that have "challenged wisdom" about the fossils of our distant kin, said Matt Sponheimer, an anthropologist at the University of Colorado at Boulder who was not involved in the study.

For example, the idea that the skull structure of A. africanus and similar species could be explained by preparing food with its premolars is "something that a lot of people won't be happy with," Sponheimer said.

The new engineering techniques used in the study will also engender controversy, he said, in particular because they are based on assumptions about A. africanus' biology.

But "these studies are pretty important for the simple reason that paleoanthropology has to constantly move or it gets stale," he said, and this study is sure to generate many testable hypotheses.

"We're talking about trying to reconstruct a dietary behavior two million years removed—no one ever said this would be a walk in the park."




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