A cool paper came out today [1] in which researchers used finite element analysis to test hypotheses about the biomechanics of chewing in Australopithecus africanus, a South African hominin that lived probably 3-2 million years ago. Finite element analysis is a technique adapted from engineering in which one creates a virtual model of a structure (here, CT data to make a composite cranium of two fossils, Sts 5 and Sts 52), gives the structure mechanical properties (here, the properties of bone), apply various forces of various direction and magnitude, and then can see how the structure is affected by these loads. It’s an interesting technique that has only recently become popular in physical anthropology, probably as the technology becomes better and cheaper, even though Chris Ruff recommended the technique twenty years ago [2].

The authors used this technique to see if the bony facial structure of A. africanus was adapted specially withstand special bite-loads, specifically at the premolars. For example, A. africanus‘s ‘anterior nasal pillars’—bony struts on either side of the nasal aperture—have long been thought to be buttresses against bite forces, but this had yet to be demonstrated. The authors tried different loading patterns—molars only, premolars only, and molars + premolars–to see how strains were distributed across the cranium.

Regardless of the loading regime, the strains through most of the face were the same. However, when just the premolars were loaded, strains increased noticeably on the nasal margins, compared to the other loading patterns. Thus, the results were consistent with the hypothesis that the face of A. africanus, specifically its anterior nasal pillars, are special adaptations for high-strain bite forces concentrated on the premolars. The results also indicate that the A. africanus cranium was adapted to such a high-force diet, rather than a high-volume (i.e. chewing lots of food) diet, because overall strains were the same regardless of whether the only the molars or all the posterior teth were loaded.

This last result leaves me perplexed as to the significance of a major trend in hominin evolution (except with the inception of Homo): postcanine megadontia. On the whole, through hominin evolution the molars and premolars get bigger and bigger (again, not really in Homo)—this is extreme in A. boisei, one of the latest surviving, “robust” australopiths. Presumably, this tendency toward megadonty increased the amount of food that could be ingested at a given time. But, the paper suggests that premolar loading was, if nothing else, an important selective factor in A. africanus, and presumably later hominins–this explains molarization of premolars (again, most noticeably in A. boisei). But it does not explain why molars should also enlarge, unless molar and premolar size are integrated, that is, under the same genetic and developmental control(s). Hey, that’s a cool hypothesis.… Also, I suppose the results don’t refute the possibility that molar size increased in order to accommodate higher volume of food, but rather they simply indicate that the face was not specially adapted to withstand such a diet.

It would also have been interesting to see how the cranium performed under more loading regimes, expecially of the anterior teeth. What really distinguishes A. africanus from its offshoot A. robustus (and especially A. boisei, to whom I don’t think africanus is ancestral) is its retention of large canines and incisors. How do the crania of these taxa differ in their ability to withstand different dental loading patterns? The DNH 7 skull [3], of a very small female from the S. African site of Drimolen, is probably complete enough to be subject to a similar study.

Anyway, it’s a cool paper that really shows off the promise of ‘virtual anthropology.’

Suggested Reading
1 David S. Strait, Gerhard W. Weber, Simon Neubauer et al., Proceedings of the National Academy of Sciences, USA (2009).
2 C. B. Ruff, Folia Primatologica 53 (1-4), 142 (1989).
3 A. W. Keyser, S. Afr. J. Sci. 96 (4), 189 (2000).