Beginning with Homo erectus, inferred changes in diet appear to be associated with dental reduction (Wrangham et al., 1999). With Homo sapiens, further reduction in crown size is associated with simplified trait expression (Bailey and Hublin, 2013). Trait simplification, particularly following the adoption of agriculture, may be an adaptive response to consuming soft, carbohydrate-rich foods that promote cavities and dental disease (Calcagno and Gibson, 1988; Meiklejohn et al., 1992). In contrast to the general trend of dental reduction and trait simplification, increases in the number and size of molar cusps in modern humans have been documented in the presence of developmental stress (Riga et al., 2014) and at least in one case, during the adoption of agriculture (Coppa et al., 2007). The question this dissertation addresses is how these different patterns of morphological change can be understood in terms of our current understanding of dental crown morphogenesis. The Patterning Cascade Model (PCM) has emerged as a developmental framework through which to predict patterns of dental morphological variability in terms of the size and placement of earlier forming cusps and the duration and rate of crown growth (Jernvall, 2000). These developmental events can be tracked by examining characteristics of the completed crown, assuming that distances among principal cusps reflect the original location of cusp-initiating centers (enamel-knots) and that the final size of the crown reflects the duration and rate of crown growth (Jernvall, 2000; Hunter et al., 2010; Skinner and Gunz, 2010; Morrmann et al., 2013; Ortiz et al., 2018). This dissertation is the first to investigate the extent to which the PCM can explain changes in dental morphology associated with both developmental stress and the shift to agriculture.
For this research, three individual studies (presented as data chapters within the dissertation) were undertaken, each with a different sample or method to address questions about changes to dental morphology in the contexts of either developmental physiological stress and/or a dietary shift to agriculture, and whether the observed patterns are consistent with predictions of the PCM.
The presence of physiological stress may affect dental characteristics such as crown size (McKee and Lunz, 1991), principal cusp spacing (Townsend et al., 2003), and molar trait expression (Riga et al., 2014). Focusing on upper dentitions of modern subadults from the rural Mexican town of Tezonteopan, Chapter 2 addresses the whethere there is an association between nutritional deprivation and dental characteristics in a sample of individuals with known-nutritional status. As part of a previous study, about half of the individuals were given dietary supplementation (n=34) and showed normal growth patterns, while the other half were malnourished (n=39) and showed reduced growth patterns (Chavez and Martinez, 1982). Following expectations of the PCM, it was predicted that individuals in the non-supplemented group would exhibit smaller molar teeth and differences in molar principal cusp spacing, both of which would lead to an increase in peripheral (Carabelli) and central (cusp 5) cusp expression. Furthermore, based on previous studies, it was predicted that anterior dentition anterior dentition (incisors and canines) would not follow the same patterns of molar teeth. Contrary to expectations, differences in molar crown size were not statistically significant between groups. Changes to molar cusp spacing in the undernourished group appear to have led to an increase in Carabelli cusp expression, and perhaps cusp 5 a well; however, with respect to cusp 5, specific patterns of principal cusp spacing did not strictly conform to expectations of the PCM. As expected for anterior dentition, the supplemented group showed larger crown sizes and increased trait expressions compared to the undernourished group. Findings from this study suggest that the presence of developmental physiological stress, in this case mild to moderate malnutrition, may be affecting molar teeth and anterior teeth differently. Additionally, molar principal cusp spacing and trait expression appear to be affected by nutritional deprivation to a greater extent than molar crown size. Results indicate that in the presence of nutritional deprivation, expectations of the PCM are substantiated for peripheral cusps but not necessarily for central cusps. Given that changes in principal cusp spacing did not strictly follow expectations of the PCM in the case of cusp 5, it may be that within the context of physiological stress, expectations of this model are more complicated for central cusps than previously suggested.
Trends in human evolution suggest that dental reduction is accompanied by a simplification in trait expression (Bailey and Hublin, 2013). Yet, within the context of developmental physiological stress, increases in molar cusp expression can occur (Riga et al., 2014). The transition from hunting and gathering to agricultural subsistence provides a unique opportunity to assess these different findings, as both dental reduction and increased physiological stress (reflected by developmental defects of enamel known as linear enamel hypoplasias (LEH)) occurred following the shift in lifeways. Molar crown size, principal cusp spacing, accessory cusp expression, and linear enamel hypoplasia (LEH) presence were recorded for hunter-gatherers (n=62) and agriculturalists (n=108) from prehistoric Ohio (Chapter 3). Results show that while dental reduction occurred from hunter-gatherers to agriculturalists, predicted simplification of trait expression, particularly for upper parastyle, lower protostylid and lower cusp 6, did not. Furthermore, when developmental stress was present within the agricultural group, individuals exhibited more variable principal cusp spacing and expressed more accessory cusps. For instance, LEH-affected individuals showed more variable entoconid-hypoconulid distances and increased expression of lower central cusp 6. Thus, contextualizing these results within the PCM, when either dietary shifts or increased developmental stress are present within these populations, patterns of cusp spacing and cusp expression conform to expectations of this model.
Previous studies demonstrate the concordance between the morphology of the outer enamel surface (OES) and the underlying enamel-dentin junction (EDJ) (Skinner et al., 2008; Olejniczak et al., 2008; Skinner et al., 2009a,b). No study has previously assessed similarities or differences between these topographic surfaces within the context of dietary shifts and developmental physiological stress, even though in these contexts, changes in the OES are likely to be reflected at the EDJ. Therefore, a sub-sample of second molars from the Chapter 3 study population of hunter-gatherers (n= 11) and agriculturalists (n= 12) were microcomputed tomography (microCT) scanned and reconstructed as 3D models. Principal cusp spacing was recorded and compared between the EDJ and the OES of this sub-sample to determine level of concordance between A) subsistence groups, and B) LEH presence/absence. Furthermore, because these is a dietary shift from tough foods to softer foods (Schmidt, 1998, 2001; Schmidt et al., 2019) within this sample, topographic measures associated with dietary adaptations (Dialectic normal energy (DNE), curvature; relief index (RFI), relief; orientation patch count (OPCR), complexity) were measured on the EDJ to address whether changes could be found within this transitional period. Predictions were that there would be concordance in cusp spacing between the OES and EDJ for subsistence and LEH comparisons, and that values for RFI and OPCR on the EDJ would decrease in the agricultural groups, indicating simplification of morphology associated with eating softer foods. Following expectations, results for cusp spacing show that there were some similarities between the OES and EDJ for subsistence comparisons and LEH presence. Contrary to expectations, there were no differences in RFI values between groups, and the agricultural group showed higher values of OPCR compared to hunter-gatherers. Therefore, rather than decreasing in EDJ complexity with softer foods, the agricultural group actually shown an increase in topographic complexity. It is possible the increased OPCR values in the agricultural group are associated with an increase in trait expression and not solely indicative of dietary composition.
The results presented in this dissertation provide evidence for changes in dental morphology, specifically crown size, principal cusp spacing, and trait expression, within the contexts of developmental physiological stress and dietary shifts. Furthermore, this is the first study to situate these findings within the explanatory framework of the PCM.