Molar Microwear, Diet and Adaptation in a Purported Hominin Species Lineage from the Pliocene of East Africa

Abstract

Craniodental morphologies of early hominins have been widely perceived as having evolved to effectively process the generally harder food items that would have accompanied the expansion of drier, more open habitats from the Late Miocene into the Early Pliocene. In particular, it has been argued that Australopithecus anamensis may have been the first hominin to exhibit dentognathic adaptations for processing hard-food objects. The morphology of its presumptive descendant, Australopithecus afarensis, is viewed as having been further enhanced to deal with such items. Molar microwear fabrics in extant mammals vary with diet and, more particularly, the physical properties of the items consumed. Previous studies of molar microwear in these early hominin taxa suggest that while both may have been morphologically equipped to process a hard, brittle diet, neither appears to have necessarily preferred such items. We examined molar microwear in this purported species lineage for fossils from eleven temporal horizons spanning roughly 940 kyr (4.12–3.18 Ma). Six broad paleoecological categories were recognized for these horizons, and were ranked on the basis of floral cover and composition. With the sole exception of wear striation breadth, which is marginally correlated with habitat, microwear variables are not significantly associated with temporal or paleoecological rank. Occlusal striae tend to be narrower in individuals from more closed habitats, perhaps attesting to the importance of exogenous grit in the formation of microwear in some environments. Thus, the mechanical properties of masticated foods do not appear to have altered in the purported Au. anamensis - Au. afarensis lineage through time or in response to different paleoecological circumstances. The microwear fabrics and textures of Au. anamensis and Au. afarensis overlap extensively those of the mountain gorilla (Gorilla beringei) and gelada baboon (Theropithecus gelada). Most importantly, they differ notably from species such as the brown capuchin (Cebus apella) and grey-cheeked mangabey (Lophocebus albigena) that consume hard objects. Explanatory scenarios that describe Au. anamensis and Au. afarensis as part of an evolutionary trajectory involving a more heavily masticated diet with an increased reliance on hard, brittle items may need to be reconsidered. However, fallback foods that were consumed during relatively short, albeit critical periods may have exerted sufficient selective pressure to explain the evolution of the comparatively robust trophic apparatus of this lineage.