Abrasive, silica phytoliths and the evolution of thick molar enamel in primates, with implications for the diet of Paranthropus boisei.

Peter Stuart Ungar,Diana Rabenold,Osbjorn M Pearson

doi:10.1371/journal.pone.0028379

Abstract

BackgroundPrimates—including fossil species of apes and hominins—show variation in their degree of molar enamel thickness, a trait long thought to reflect a diet of hard or tough foods. The early hominins demonstrated molar enamel thickness of moderate to extreme degrees, which suggested to most researchers that they ate hard foods obtained on or near the ground, such as nuts, seeds, tubers, and roots. We propose an alternative hypothesis—that the amount of phytoliths in foods correlates with the evolution of thick molar enamel in primates, although this effect is constrained by a species' degree of folivory.Methodology/Principal FindingsFrom a combination of dietary data and evidence for the levels of phytoliths in plant families in the literature, we calculated the percentage of plant foods rich in phytoliths in the diets of twelve extant primates with wide variation in their molar enamel thickness. Additional dietary data from the literature provided the percentage of each primate's diet made up of plants and of leaves. A statistical analysis of these variables showed that the amount of abrasive silica phytoliths in the diets of our sample primates correlated positively with the thickness of their molar enamel, constrained by the amount of leaves in their diet (R2 = 0.875; p<.0006).Conclusions/SignificanceThe need to resist abrasion from phytoliths appears to be a key selective force behind the evolution of thick molar enamel in primates. The extreme molar enamel thickness of the teeth of the East African hominin Paranthropus boisei, long thought to suggest a diet comprising predominantly hard objects, instead appears to indicate a diet with plants high in abrasive silica phytoliths.

Highlights

Few dental traits have elicited more interest in the study of human origins and evolution over the past several decades than that of molar enamel thickness in primates
As the hominin with the thickest molar enamel, the Plio-Pleistocene East African hominin Paranthropus boisei has been considered the ultimate consumer of hard objects [11]
Pair-wise correlations showed that only percentage of leaves eaten (%_leaves) correlated significantly with Relative enamel thickness (RET) (r = 0.586, p = 0.045)

Summary

Introduction

Few dental traits have elicited more interest in the study of human origins and evolution over the past several decades than that of molar enamel thickness in primates. Researchers suggested a correlation between molar enamel thickness, open habitats, and terrestriality [7,8]. Researchers proposed that hominins foraged for hard objects like nuts, seeds, and underground storage organs on, in, or near the ground of the open savanna [9,10]. Primates—including fossil species of apes and hominins—show variation in their degree of molar enamel thickness, a trait long thought to reflect a diet of hard or tough foods. The early hominins demonstrated molar enamel thickness of moderate to extreme degrees, which suggested to most researchers that they ate hard foods obtained on or near the ground, such as nuts, seeds, tubers, and roots. We propose an alternative hypothesis—that the amount of phytoliths in foods correlates with the evolution of thick molar enamel in primates, this effect is constrained by a species’ degree of folivory

Methods

Results

Conclusion