Three-dimensional models of hominoid skulls used in the study – (a) Hylobates lar; (b) Pongo pygmaeus; (c) Pan troglodytes; (d) Gorilla gorilla; (e) Australopithecus africanus; (f ) Paranthropus boisei; (g) Homo sapiens. They have been scaled to the same surface area, and the colors denote areas of stress (blue = minimal stress, pink = high stress). From Wroe et al, 2010.
It is all too easy to think of human evolution in linear terms. From our 21st century vantage point we can look back through Deep Time for the first glimmerings of the traits we see in ourselves, and despite what we have come to know about the undirected, branching pattern of evolution, the origin of our species is often portrayed as a slow rise from the ape in which brains eventually overtook brute strength. One of the most prominent examples of this was modifications made to our jaws. It has been widely assumed that, compared to apes and our extinct hominin relatives, we have relatively weak jaws – why should we need to exert heavy bite forces if our lineage developed tools to process food before it entered our mouths? It was our relatives among the robust australopithecines – namely Paranthropus – which obviously developed the strongest jaws, but a new study just published in the Proceedings of the Royal Society B questions these long-held assumptions.
As outlined in the introduction of the paper by Stephen Wroe, Toni Ferrara, Colin McHenry, Darren Curnoe, and Uphar Chamoli, the hypothesis that our species has a diminished bite force has primarily been based upon the study of other, obviously heavier-jawed hominins. On the surface this would seem to make sense – our jaws are nowhere near as robust as those of those of the multiple species of Paranthropus – yet our teeth seem well-suited to withstanding heavy bite forces. Among living apes, for example, we have the thickest amount of enamel, one of several features we posses which are consistent with the ability of teeth to withstand strong bites. Some have argued that these features are holdovers from when our prehistoric ancestors required stronger bites to process tough foods, but the team behind the new paper decided to create a detailed test which compared the bite mechanics of our species to some of our close hominin and hominid relatives, both living and extinct.
In order to investigate the “weak bite” hypothesis, the researchers used CT-scanned specimens to create three-dimensional models of the skulls of a white-handed gibbon, orangutan, chimpanzee, western gorilla, Australopithecus africanus, Paranthropus boisei, and, of course, Homo sapiens. (A model a crab-eating macaque skull was also made to check the results of this study with one carried out previsouly. Also, it is worth noting that all the skull models of extant species were based on female specimens and the fossil specimens were probably males.) From there the appropriate virtual muscles were reconstructed on each skull, with missing bits of the fossil skulls also filled in. Once the models were in place the scientists could then simulate the stresses placed upon the skulls as if they were clamping down on a hard object at different points in the jaw.
What the team found was that the amount of bite force each species was able to produce was generally proportional to body size, and while Paranthropus boisei had the highest estimated bite force, the results for our species were consistent with what would be expected for an ape of our size. In other words, the bite forces we are capable of exerting were not remarkably low, but were instead comparable to those of hominids with similar body mass. Additionally, these estimates were in accord with the relatively small amount of bite force data gathered from living people in non-Western populations, and these are probably a better measure of what our species is capable of than the lower results obtained through voluntary bite force measurements of Western study subjects.
But what about the stresses exerted on the teeth? Estimating bite forces at different parts of the jaw is just one part of determining what an organism is capable of. Humans might have powerful bites, but it would do a human little good to bite so hard that they broke their teeth or otherwise hurt themselves. To find out the scientists scaled the models to have the same total surface area and ran tests to see how the skulls and jaws of each species handled bite force stress. In general the highest amounts of stress were felt in the jaws rather than the skull – meaning that it was more likely that the lower jaw would become damaged due to high bite forces – but the human model was distinct because stress was distributed differently through the jaw. While Paranthropus and the gorilla appeared better able to cope with bite force stress in general, our jaws appeared to be well-adapted to reducing the stress exerted by quick, strong bites.
As the authors state, their results resolve what otherwise might seem like inconsistencies between what our teeth can handle and our gracile jaw musculature. We might not have the heavy jaws and massive muscles of Paranthropus, but we are more efficient biters, allowing us to exert high bite forces with a different anatomical arrangement. The lineage of hominins of which we are a part did not slowly lose their ability to bite hard as had previously been supposed, but, interestingly, our jaws are not well-suited to sustaining high bite forces for a long time. In other words, our jaws are capable of cracking open something like a nut or hard fruit which requires brief exertion of high bite forces, but they are not well-adapted to doing something like chewing on tough plant food for a long period of time. This raises some interesting questions about the inferred diets of extinct humans, and by utilizing similar modeling techniques paleontologists may be able to determine whether hominins were adapted to deliver short, strong bites, were chewing hard foods which took a longer amount of time to process, or were doing something different.
To see more images from this study and similar research, see the CompBiomechBlog.
Wroe, S., Ferrara, T., McHenry, C., Curnoe, D., & Chamoli, U. (2010). The craniomandibular mechanics of being human Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2010.0509