Humans pack an efficient bite, but at a cost

Published 27 July 2016

Compared with our closest living primate relatives, the human skull is quite striking in its appearance. In addition to our unusually large brains, humans have small jaws, teeth, and faces that are thought to reflect a diet of cooked, sliced, or softer foods. However, the human skull presents a puzzling contradiction: our small and flat faces offer increased efficiency, or leverage, to the muscles involved in biting and chewing, which is something usually found in animals that subsist on foods that are difficult to chew or crack open.

X-ray CT “slices” were used to construct 3D finite element models of human crania. These models were then subjected to a series of mechanical analyses that simulate feeding. The image to the far right depicts a finite element model during a bite on its left first premolar. “Warm” colours indicate regions of high strain, while “cool” colours indicate regions of low strain.

X-ray CT “slices” were used to construct 3D finite element models of human crania. These models were then subjected to a series of mechanical analyses that simulate feeding. The image to the far right depicts a finite element model during a bite on its left first premolar. “Warm” colours indicate regions of high strain, while “cool” colours indicate regions of low strain.

But new work by an international team of researchers shows that the cranium of modern Homo sapiens is poorly-suited to produce large biting forces due to an important limitation involving the jaw joint.

“We found that humans have high biting leverage, which some previous studies have noted, but we also found that humans are at risk of dislocating, and possibly damaging, the jaw joint during powerful molar biting,” said Dr Justin Ledogar, lead author and researcher at the University of New England’s School of Environmental and Rural Sciences in Australia. “Additionally, we found that the human facial skeleton is generally weaker during biting than chimpanzees, which is not surprising given our small faces.”

The study, published in the journal PeerJ, describes biomechanical testing of computer-based models of human crania. The methods used in the new study are similar to those used by engineers to test whether machine parts or other mechanical devices are strong enough to avoid breaking during use.

“This limitation on forceful biting appears to be much more pronounced in modern humans than in chimps or many of our fossil ancestors,” Ledogar said. “And because the need to bite powerfully is very unlikely to have led natural selection to favour a facial configuration that puts the feeding system at risk of being compromised, the flat faces of modern humans are probably unrelated to such behaviours.”

The new findings provide support for the hypothesis that softer and more processed foods led to the characteristically small and gracile faces of modern humans. However, the study does not directly address why the human face is so flat, if unrelated to powerful biting.

“We suspect that limitations on producing high bite forces may have characterised the origins of our genus,”Dr Ledogar said. “Going forward, it will be important to examine the various hypotheses that attempt to explain human facial flatness. There are a few different ideas, but we’re really not sure what was driving this unusual characteristic.”