Robust australopithecines and masticatory overkill

Ever since the discovery of the hominds we call Paranthropus robustus in 1938 and Paranthropus boisei in 1959, the dietary habits of these "robust australopithecines" have been controversial. With skulls that seem to have more in common with gorillas than with Homo habilis, another hominid more closely related to us that lived during the same time, it has long been thought that Paranthropus was a dietary specialist. The saggital crest, large and thickly-enameled teeth, and huge jaws of these creatures has long been interpreted as a mark of a diet largely made up of tough, fibrous plants, but a new study published in the open-access journal PLoS disputes the "classic" view. Instead, the authors argue that Paranthropus was a dietary generalist that was able to make use of a broader range of foods than the weaker-jawed hominids more closely allied to our ancestors.

Although no one has the opportunity to observe the behavior of extinct hominids like Paranthropus today, their teeth have provided paleoanthropologists with some important clues as to what they may have been eating. The PLoS study primarily focuses on the patterns of microwear, scratches and pits made on teeth by food as it is being chewed, and the patterns on the teeth of Paranthropus can be compared to patterns on the teeth of living animals so see if they are consistent with a particular kind of diet. The particular creatures used in comparison for this study were the tufted capuchin monkey (Cebus apella), the grey-cheeked mangabey (Lophocebus albigena), the mantled howler monkey (Alouatta palliata), the silvered leaf monkey (Trachypithecus cristata), and the fossil remains of Australopithecus africanus. It should be noted, however, that the fossil teeth mentioned in this study are replicas made from dental impressions taken during the 1990s. Of the original samples of Paranthropus only seven molars replicas preserved any microwear, and I have to wonder if more microwear would be seen on the original teeth.

Further, it is strange that the two living species of gorilla were not used for comparison. Beyond similarities in the skull, both mountain and western gorillas would have provided an excellent baseline to test the Paranthropus teeth against. The western gorillas (Gorilla gorilla) are primarily frugivorous, tougher foods making up a smaller part of the diet (especially when fruit is not available). Mountain gorillas (Gorilla berengei), by contrast, have a diet mostly made up of stems, bark, and roots, fruit being a very small part of what they consume. Within one genus, then, you have two species that have extremely similar morphology but have diets where fruit and tougher foods are consumed in drastically different proportions, and I can't imagine why gorillas were not used in this study.

Such criticism aside, the results of the study revealed that microwear on Paranthropus boisei teeth fell at the bottom of the range of the living primates studied and was distinct from both P. robustus and Australopithecus africanus. The numerous deep pits that are characteristic of a hard-food diet are missing in Paranthropus boisei, and it appears that its diet differed even from the South-African P. robustus even though they look very similar. Indeed, P. robustus microwear patterns appeared to be closer to those of living primates (like the tufted capuchin) that often eat hard food when softer fare like fruit is not available, and P. boisei exhibited even less dental microwear than the South-African robust australopithecine. As the authors of the paper report, it doesn't appear that the individuals of P. boisei that were studied consumed especially tough food "in the days before death" (although this phrase is left frustratingly vague).

The observations made by the authors led them to conclude that P. boisei probably ate softer foods most of the time and rarely ate hard foods, while P. robustus also preferred soft foods but had to "fall back" on tough foods more often. What is not discussed, however, is why robust australopithecines have skulls and dental features that appear to be specialized for processing hard foods. Was there a dietary shift, the group becoming hard-food specialists but then switching over to softer foods? Could some of the robust features have anything to do with sexual dimorphism? With both questions, there is little than can be said. The evolution of the robust australopithecine side branch is still murky, and given the paucity of postcranial evidence and specimens, sexing the remains of these hominids for comparison is presently impossible.

Although the evolution of the robust australopithecine form awaits further fossil discoveries, the available material is starting to reveal a picture of these hominids that differs from the standard story. If studies like this one are correct, Paranthropus was able to consume a wider variety of foods (regardless of whether the lineage evolved as specialists or generalists), their diets perhaps being largely dictated by seasonality of different kinds of food. More research is certainly needed, but the natural history of Paranthropus does not appear to have been as simple as previously supposed. The fact that they could chew up tough foods doesn't mean that they exclusively ate those foods, and a variety of studies is reflecting a more nuanced version of Paranthropus than is often put forward. Unfortunately, since they are on an evolutionary "side line" they are often stereotyped or otherwise brushed aside in the search for our own ancestors, but I am hoping that people like Ungar keep trying to pick away at the lifeways of the robust australopithecines.

Ungar, P.S., Grine, F.E., Teaford, M.F., Petraglia, M. (2008). Dental Microwear and Diet of the Plio-Pleistocene Hominin Paranthropus boisei. PLoS ONE, 3(4), e2044. DOI: 10.1371/journal.pone.0002044