Skull of Paranthropus robustus
Skull of Paranthropus robustus.

Image credit: Kornelius Kupczik / Max Planck Weizmann Center for Integrative Archaeology and Anthropology

Getting to the roots of our ancient cousins’ diet

The splay of tooth roots reveals how South African hominins Australopithecus africanus and Paranthropus robustus chewed their food, researchers have found.

Ever since the discovery of the fossil remains of Australopithecus africanus from Taung nearly a century ago, and subsequent discoveries of Paranthropus robustus, there have been disagreements about the diets of these two South African hominin species. By analysing the splay and orientation of fossil hominin tooth roots, an international team of researchers from the Max Planck Institute for Evolutionary Anthropology in Leipzig, the University of Chile and the University of Oxford now suggests that Paranthropus robustus had a unique way of chewing food not seen in other hominins, which seems to explain the unique suite of characters observed in this species.

Food needs to be broken down in the mouth before it can be swallowed and digested further. How this is done depends on many factors, such as the mechanical properties of the foods and the morphology of the masticatory apparatus. Palaeo-anthropologists spend a great deal of their time reconstructing the diets of our ancestors, as diet holds the key to understanding our evolutionary history. For example, a high-quality diet (and meat-eating) likely facilitated the evolution of our large brains, while the lack of a nutrient-rich diet probably underlies the extinction of some other species (for example, P. boisei). The diet of South African hominins has remained particularly controversial, however.

Using non-invasive, high-resolution computed tomography technology and shape analysis, the authors deduced the main direction of loading during mastication from the way the tooth roots are orientated within the jaw. By comparing the virtual reconstructions of almost 30 hominin first molars from South and East Africa they found that Australopithecus africanus had much wider splayed roots than both Paranthropus robustus and the East African Paranthropus boisei. 'This is indicative of increased laterally-directed chewing loads in Australopithecus africanus, while the two Paranthropus species experienced rather vertical loads', said Kornelius Kupczik of the Max Planck Institute for Evolutionary Anthropology.

Paranthropus robustus, unlike any of the other species analysed in this study, exhibits an unusual orientation, ie 'twist', of the tooth roots, which suggests a slight rotational and back-and-forth movement of the mandible during chewing. Other morphological traits of the P. robustus skull support this interpretation. For example, the structure of the enamel also points towards a complex, multidirectional loading, while their unusual microwear pattern can conceivably also be reconciled with a different jaw movement rather than by mastication of novel food sources. Evidently, it is not only what hominins ate and how hard they bit that determines skull morphology, but also the way in which the jaws are being brought together during chewing.

The new study, published in the journal Royal Society Open Science, demonstrates that the orientation of tooth roots within the jaw has much to offer for an understanding of the dietary ecology of our ancestors and extinct cousins. 'Perhaps palaeo-anthropologists have not always been asking the right questions of the fossil record: rather than focusing on what our extinct cousins ate, we should equally pay attention to how they masticated their foods', said Gabriele Macho of the University of Oxford.

Molar root variation in hominins is therefore telling us more than previously thought. 'For me as an anatomist and a dentist, understanding how the jaws of our fossil ancestors worked is very revealing as we can eventually apply such findings to the modern human dentition to better understand pathologies such as malocclusions', added Viviana Toro-Ibacache from the University of Chile, one of the co-authors of the study.