Functional morphology, stable isotopes, and human evolution: a model of consilience

TL;DR

This paper introduces a process-based model linking molar enamel thickness, dietary choices, and foraging constraints in human evolution, explaining the role of stiff foods like underground storage organs in hominin diets.

Contribution

It develops a novel model integrating functional morphology and isotopic data to explain dietary adaptations and the evolution of megadontia in hominins.

Findings

Thicker enamel reduces mechanical costs of chewing stiff foods.

USOs likely served as fallback resources for early hominins.

Model reconciles morphological evidence with microwear data for Paranthropus.

Abstract

Foraging is constrained by the energy within resources and the mechanics of acquisition and assimilation. Thick molar enamel, a character trait differentiating hominins from African apes, is predicted to mitigate the mechanical costs of chewing obdurate foods. The classic expression of hyperthick enamel together with relatively massive molars, termed megadontia, is most evident in {\it Paranthropus}, a lineage of hominins that lived ca. 2.7 to 1.2 million years ago. Among contemporary primates, thicker molar enamel corresponds with the consumption of stiffer, deformation-resistant foods, possibly because thicker enamel can better resist cracking under high compressive loads. Accordingly, plant underground storage organs (USOs) are thought to be a central food resource for hominins such as {\it Paranthropus} due to their abundance, isotopic composition, and mechanical properties. Here, we present a process-based model to investigate foraging constraints as a function of energetic demands and enamel wear among human ancestors. Our framework allows us to determine the fitness benefits of megadontia, and to explore under what conditions stiff foods such as USOs are predicted to be chosen as fallback, rather than preferred, resources. Our model predictions bring consilience to the noted disparity between functional interpretations of megadontia and microwear evidence, particularly with respect to {\it Paranthropus boisei}.