Early Humans Had Nutcracker Jaws
Computer simulations showed the compressive stress of biting in the cranium of Australopithecus africanus. Bright colors indicate high stresses, and show that a bone running alongside the opening of the nasal cavity acts as a strut to reinforce the face against premolar loads.
Credit: Arizona State University and the Hominid Feeding Biomechanics research team.

Our ancient human relatives had jaws like nutcrackers that allowed the hominids to chomp down on hard nuts and seeds and adapt to changes in food sources in their environment, a new computer simulation reveals.

Today's humans have comparatively small faces and teeth, making us ill-equipped to bite down forcefully on hard objects. Apparently that wasn't the case for Australopithecus africanus, which lived 3.3 million to 2.5 million years ago.

An international team of researchers used computed tomography scanning to digitally recreate an A. africanus skull to see how the jaw operated and what forces it could produce. Then, they added measurements of how the facial muscles work in chimpanzees, which share common features with Australopithecus.

The result is a rainbow-colored virtual skull that illustrates forces absorbed by the cranial structure in simulated bite scenarios. The simulations also reveal how the hominid's unusual facial features were ideally suited to support the heavy loads of cracking hard nuts.

"This reinforces the body of research indicating that facial specializations in species of early humans are adaptations due to a specialized diet," said researcher Mark Spencer of Arizona State University.

Along with evidence of enlarged premolars and heavy tooth enamel, the new model evidence that these hominids were loading forcefully on the molars suggests Australopithecus' diet consisted of foods that were larger than the previously hypothesized small seeds and nuts, the researchers say.

"These fall-back foods — hard nuts and seeds — were important survival strategies during a period of changing climates and food scarcity," Spencer said. "Our research shows that early, pre-stone tool human ancestors solved problems with their jaws that modern humans would have solved with tools."

The research is detailed this week in the journal Proceedings of the National Academy of Sciences.