Hard plant tissues do not contribute meaningfully to dental microwear: evolutionary implications

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.

Applying dental microwear texture analysis to the living: Challenges and prospects

“…However, recent research suggests that enamel hardness exceeds that of most foodstuff, and consequently, that food cannot abrade enamel directly (Lucas et al, 2013; Sanson, Kerr, & Gross, 2007; van Casteren et al, 2018). Indeed, a new experimental study seems to confirm that hard plant tissues do not contribute substantially to dental microwear (van Casteren et al, 2020). Instead, exogenous grit, dust, and possibly phytoliths present in the food are responsible for producing microwear (Lucas et al, 2014; van Casteren et al, 2018, 2020).…”

“…Indeed, a new experimental study seems to confirm that hard plant tissues do not contribute substantially to dental microwear (van Casteren et al, 2020). Instead, exogenous grit, dust, and possibly phytoliths present in the food are responsible for producing microwear (Lucas et al, 2014; van Casteren et al, 2018, 2020). In this new approach, the interpretation of microwear features relies on the mechanical properties of food, like shape, rather than on the material properties, like hardness.…”

“…As such, this approach suggests that isodiametric (i.e., roughly spherical) food (e.g., nuts) produce complex pitting in dental surfaces as food rolls between the teeth and through the action of grit present in food, whereas thin food (e.g., leaves) produce aligned scratches or grooves through the action of grit being dragged across dental contact facets as they slide against one another. In this sense, dietary signals in dental microwear are indirect and depend on abrasive particles present in the food, emphasizing the impact that food processing may have on microwear by removing or adding such particles (Lucas et al, 2014, 2013; van Casteren et al, 2018, 2020). However, the debate between material and mechanical properties of food as drivers of microwear is far from settled, as considerable research has shown that items “softer” than enamel can still mark it (Daegling, Hua, & Ungar, 2016; Gügel, Grupe, & Kunzelmann, 2001; Xia et al, 2015).…”

“…So far, proof of concept for the usefulness of DMTA in dietary assessment derives from (a) agreement between results obtained through DMTA and diets reported in the literature, or with other means of dietary assessment (e.g., El Zaatari, 2014; Grine et al, 2012; Schmidt et al, 2019; Schmidt, Beach, McKinley, & Eng, 2015; Scott, Teaford, & Ungar, 2012), and (b) a few in vitro studies (Daegling et al, 2016; Hua, Brandt, Meullenet, Zhou, & Ungar, 2015; Lucas et al, 2013; van Casteren et al, 2020; Xia et al, 2015; Xia et al, 2017). As reported in some of these studies, DTMA was successful in identifying differences in microwear pattern among species with substantially different diets.…”

Tooth chipping patterns inParanthropusdo not support regular hard food mastication

“…Direct evidence from enamel microwear studies of P. boisei suggests little to no hard object feeding (Ungar et al, 2008). In P. robustus , higher enamel surface complexity may indicate hard foods were consumed more frequently (Ungar, 2019; Scott et al, 2005; Peterson et al, 2018), potentially in the context of “fallback foods.” However, the role of hard plant tissues in generating microwear features is currently debated (van Casteren et al, 2020; Teaford et al, 2020). From stable carbon isotopes, the diets of P. robustus and P. boisei appear substantially different (Cerling et al, 2011; Ungar and Sponheimer, 2011).…”