Assessing complexity in hominid dental evolution: Fractal analysis of great ape and human molars

Abstract

Objectives: Molar crenulation is defined as the accessory pattern of grooves that appears on the occlusal surface of many mammalian molars. Although frequently used in the characterization of species, this trait is often assessed qualitatively, which poses unavoidable subjective biases. The objective of this study is to quantitatively test the variability in the expression of molar crenulation in primates and its association with molar size and diet. Methods: The variability in the expression of molar crenulation in hominids (human, chimpanzee, gorilla, and orangutan) was assessed with fractal analysis using photographs of first, second and third upper and lower molars. After this, representative values for 29 primate species were used to evaluate the correlation between molar complexity, molar size, and diet using a phylogenetic generalized least squares regression. Results: Results show that there are statistically significant differences in fractal dimensions across hominid species in all molars, with orangutan molars presenting higher values of occlusal complexity. Our results indicate that there is no significant association between molar complexity and molar size or diet. Discussion: Our results show higher levels of occlusal complexity in orangutans, thus supporting previously published observations. Our analyses, however, do not indicate a clear association between molar complexity and molar size or diet, pointing to other factors as the major drivers of complexity. To our knowledge, our study is the first one to use fractal analysis to measure occlusal complexity in primates. Our results show that this approach is a rapid and cost-effective way to measure molar complexity.

Basal Primatomorpha colonized Ellesmere Island (Arctic Canada) during the hyperthermal conditions of the early Eocene climatic optimum

“…dawsonae from Ellesmere Island ( Fig 6B ; Table 3 ), which differs significantly from OPCR scores for M 3 in paromomyids from mid-latitudes ( Table 4 ). Enamel crenulation has often been associated with hard-object feeding, but this phenomenon is present in a wide range of mammals showing a variety of dietary specializations [ 66 ].…”

“…feeding, but this phenomenon is present in a wide range of mammals showing a variety of dietary specializations [66].…”

Finite element analysis of crenulated and non‐crenulated hominid molars: A functional hypothesis explaining the adaptive significance of molar crenulation

“…Cooking and an increase in meat consumption may be tentatively pointed as determinant drivers of this process. Alternatively, if molar crenulation is only a byproduct of development with no significant functional effects (Cano‐Fernández & Gómez‐Robles, 2021), then this trait may have been lost in human evolution due to genetic drift.…”

“…The expression of molar crenulations is less conspicuous in modern humans than in orangutans or pitheciines (Cano‐Fernández & Gómez‐Robles, 2021; Kraus & Oka, 1967), although some examples of highly crenulated human molars can be found in African and Asian populations (Grine, 1981; Pilloud et al, 2018). Highly crenulated molars also appear frequently in the hominin fossil record (Berger et al, 2015; Bermúdez de Castro et al, 1999; Frayer, 1973; Haile‐Selassie, 2001; Martinón‐Torres et al, 2012; Pickering et al, 2016; Simpson et al, 2015; Tobias, 1967; Xing et al, 2018).…”

“…Molar crenulations often resemble the grooves and crests of a squeezer and, therefore, it may be proposed that they can be used to extract the juice of fruits. However, it has never been experimentally tested whether juicing fruits has any adaptive advantage over slicing them and a clear correlation between fruit consumption and the expression of molar crenulations has not been found (Berthaume et al, 2020; Cano‐Fernández & Gómez‐Robles, 2021).…”