Tooth wear and stable isotopes: what were these fossils eating?

The New York Times has picked up a story on an article which came out in last week’s edition of Science on hominin diets. The review paper, co-authored by Peter Ungar (Univ. of Arkansas) and Matt Sponheimer (Univ. of Colorado) represents the coming together of a huge amount of research on enamel microwear patterns in fossil and extant taxa (Ungar and colleagues) and patterns of stable isotope composition in teeth (Sponheimer and colleagues). Both lines of evidence provide a window into the diet of an organism. By looking at the pits and scratches on the surface of dental enamel it is possible to get some sense of the characteristic physical changes different kinds of food can cause to teeth. The stable isotope data provides evidence of the chemical changes brought about by what we eat.

The big headlines from this article are really about the “Robust” hominins, alternatively placed in the genus Australopithecus or Paranthropus, and primarily represented by the species Au. (P.) boisei in East Africa and Au. (P.) robustus in South Africa. The diet of these taxa have been a major question ever since their initial discovery. I have a photo from my last research trip to Kenya of an old Kenyan shilling, slightly larger than a quarter, resting on the first molar of a boisei specimen and being very nearly equal in size. In contrast, the surface of your first molar is probably about the size of the end of your pinky. And these huge cheek teeth come on a skull whose cranial capacity is about half the size of yours. Quite striking.

You might think identifying the diet of such a striking feature would be easy, but studies which have attempted to reconstruct the diet in these lineages using a variety of evidence have often produced confusing or contradictory results. This study represents, at least to me, a more comfortable convergence on the diet of the robusts. And what is interesting is that the East African boisei specimens look surprisingly different than the South African robustus specimens. In terms of microwear, the boisei specimens show a high degree of similarity, but show a pattern of wear similar to earlier hominins like Au. afarensis. Their robustus sample shows a huge variance in wear patterns, and a pattern consistent with organisms that are “hard object fallback feeders.” In isotopic composition, boisei shows a much higher C4 pattern, indicative of a diet of items like sedges or grasses, than any other hominin they examine. Robustus, in contrast, look like other hominins with a more intermediate C3/C4 signal.

Despite the clearer picture these results help create, the authors rightly point to the work that is still to come:

Microwear and stable carbon isotope analyses are needed for all relevant species, and these results must be integrated with data on masticatory biomechanics, plant distributions and nutritional/mechanical properties, and primate ecology and digestive physiology. An important role for microwear and isotope analyses within contemporary paleodietary research is to focus on underlying processes rather than outcomes, as well as to recognize evolutionary novelties, such as grazing giraffes (48) and grass- or sedge-eating apes (37). When these behavioral proxies are linked to morphological and paleoenvironmental data sets through time, yoking habitat and dietary change to morphological response, our understanding of the patterns and processes of hominin evolution will be greatly augmented

There are other interesting tidbits in this paper. I like the observation, as it is closer to my own area of interest, that both early Homo samples, H. habilis and H. erectus, show an expanded and overlapping range in microwear pattern.

Peter Ungar’s Lab

Matt Sponheimer

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1. Peter S. Ungar, Matt Sponheimer. The Diets of Early Hominins. Science, 2011, 334 (190), DOI: 10.1126/science.1207701