Stressed Skull Reveals Early Human Diet

Researchers are using a computer model to generate images such as this one that shows compressive stress in the cranium of Australopthecus africanus, an extinct early human that lived around 2.5 million years ago.

The stresses in the model result from a simulation of biting, with substantial forces resulting from the use of the premolar teeth.

In the image, bright colors correspond to high stresses and indicate that a bony pillar running alongside the opening of the nasal cavity acts as a strut that structurally reinforces the face against premolar loads.

Mark Spencer, an assistant professor at the Arizona State University (ASU) Institute of Human Origins, and Caitlin Schrein, a doctoral student at ASU's School of Human Evolution and Social Change, are part of an international team of researchers who studied the feeding biomechanics and dietary ecology of Australopithecus africanus.

Using state-of-the-art computer modeling and simulation technology, the same kind that engineers use to simulate how a car reacts to forces in a front-end collision, the researchers built a virtual model of an A. africanus skull and were able to see just how the jaw operated and what forces it could produce.

Researchers first took a Computerized Axial Tomography (CAT) scan of an A. africanus skull discovered in 1947, the most complete specimen the team had available. The skull had struts on the side of the nose, but no teeth. Next, they meshed the data from the CT scan with data from another specimen with teeth to make a virtual model of the bone and tooth structure.

"Then we looked at chimpanzees, who share common features with Australopithecus, and took measurements of how their muscles work and added that to the model," said Spencer, who was lead investigator on the project, funded by the National Science Foundation and the European Union.

The researchers validated the model by comparing it to a similar model built for macaques, a type of monkey.  

The result is a rainbow-colored virtual skull that, in simulated bite scenarios, illustrates forces absorbed by the A. africanus cranial structure and shows how the unusual facial features of the animal were ideally suited to support the heavy loads of cracking hard nuts.  Human skulls, on the other hand, have small faces and teeth and are not well-equipped to bite down forcefully on hard objects.  Results from the study reinforce the idea that facial specializations in early human species were adapted to a specialized diet.

"The enlargement of the premolars, the heavy tooth enamel, and the evidence now that they were loading forcefully on the premolars suggest the size of the objects were larger than the previously hypothesized small seeds and nuts," said Spencer.  During periods of climate change when food was scarce, early human species could rely on hard nuts and seeds.

"Our research shows that early, pre-stone-tool human ancestors solved problems with their jaws that modern humans would have solved with tools," added Spencer.

Results from the study were published in the Feb. 17 issue of the journal Proceedings of the National Academy of Sciences (PNAS).

- Christina Whitcomb, National Science Foundation

• Top 10 Missing Links
• Quiz: The Artifact Wars
• 7 Perfect Survival Foods