Abstract
Reconstructing diet is critical to understanding hominin adaptations. Isotopic and functional morphological analyses of early hominins are compatible with consumption of hard foods, such as mechanically-protected seeds, but dental microwear analyses are not. The protective shells surrounding seeds are thought to induce complex enamel surface textures characterized by heavy pitting, but these are absent on the teeth of most early hominins. Here we report nanowear experiments showing that the hardest woody shells – the hardest tissues made by dicotyledonous plants – cause very minor damage to enamel but are themselves heavily abraded (worn) in the process. Thus, hard plant tissues do not regularly create pits on enamel surfaces despite high forces clearly being associated with their oral processing. We conclude that hard plant tissues barely influence microwear textures and the exploitation of seeds from graminoid plants such as grasses and sedges could have formed a critical element in the dietary ecology of hominins.
Highlights
Reconstructing diet is critical to understanding hominin adaptations
If particles of lignified plant tissue are unable to produce the deep or elaborate scars on enamel it seems unlikely that feeding on dietary items, such as mechanically protected seeds, would produce the complex surfaces textures predicted by traditional interpretations of dental microwear
One groove was observed for both E. guineensis and S. gabonensis and three grooves were observed for M. parviflora. These markings were similar in length to the slide displacement, in the same direction, and undoubtedly caused by them (Fig. 2a–c). They were much less pronounced than marks produced by phytoliths in similar sliding experiments[16] and would barely register in dental microwear texture www.nature.com/scientificreports analyses as conventionally performed[15]
Summary
Isotopic and functional morphological analyses of early hominins are compatible with consumption of hard foods, such as mechanically-protected seeds, but dental microwear analyses are not. The low blunt cusps of australopith molars would be more resistant to fracture against hard foods, as exemplified by the woody casings of what are ‘mechanically protected’ plant embryos (Fig. 1)[10] Whether this casing is derived from the seed integuments or the fruit endocarp, we call this a ‘mechanically-protected seed’ here. If particles of lignified plant tissue are unable to produce the deep or elaborate scars on enamel it seems unlikely that feeding on dietary items, such as mechanically protected seeds, would produce the complex surfaces textures predicted by traditional interpretations of dental microwear. It is plausible that the presence or absence of complex surface textures measured in microwear analysis of tooth facets may not directly reflect the consumption of hard foods, but instead echo levels and types of dietary abrasives[17]
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