In recent years there have been an increasing number of studies investigating the relationship between diet preference, skull shape and biomechanics of primates. Understanding the relationship between morphological and mechanical variations and extrinsic factors, such as diet and feeding behaviour, provides insights into the evolution of our own species. However, most diet-related morphological studies have focused on the mandibles and dentition, while most biomechanical studies have concentrated on species that either feed on mechanically challenging foods (hard-object feeders) or are mechanically challenged during food acquiring (e.g., tree-gouging exudate feeders). These mechanical studies are usually small scale with only a few species and there is a lack more comprehensive comparisons. Studies on the relationship between cranial shape and biomechanical performances in primates are also limited. In the present study, whether cranial morphology and/or mechanical performance reflect dietary preferences were examined in Anthropoidea, which is a clade of primates that includes all the New World and Old World monkeys. It was hypothesised that cranial shape and its mechanical behaviour reflect diet regardless of phylogeny and cranial size.

Three-dimensional geometric morphometrics was used to investigate the relationship between cranial shape and diet. Finite element analysis was used to assess the mechanical performance in the anthropoid crania during premolar, molar, and incisor loadings. Von Mises strain magnitudes and distributions were used to determine the ability to withstand high bite forces. Mechanical advantages were calculated as the ratio of bite force per muscle force. Results showed that diet had very limited influence on cranial morphology. New World monkeys generally exhibited a stronger correlation between diet and cranial shape than Old World monkeys. This suggested that cranial morphology in primates was not the sole result of dietary selection. Variations of strain magnitudes were also found to be mostly insignificant with diet. However, results showed that mechanical advantages were a better predictor of diet preference, especially for species that require higher bite forces during feeding. Hard food feeders were more mechanically efficient at producing high premolar and molar bites compared to other dietary groups. Exudate feeders were also relatively efficient at producing high bite forces at the incisors. While cranial morphology was found to have limited correlation with diet, there was a strong relationship between cranial shape and mechanical advantage. This result indicated that mechanical advantage can be achieved by different combinations of craniofacial features.

There is controversy around the mechanisms that guided the change in brain shape during the evolution of modern humans. It has long been held that different cortical areas evolved independently from each other to develop their unique functional specializations. However, some recent studies suggest that high integration between different cortical areas could facilitate the emergence of equally extreme, highly specialized brain functions. Here, we analyse the evolution of brain shape in primates using three-dimensional geometric morphometrics of endocasts. We aim to determine, firstly, whether modern humans present unique developmental patterns of covariation between brain cortical areas; and secondly, whether hominins experienced unusually high rates of evolution in brain covariation as compared to other primates. On the basis of analyses including modern humans and other extant great apes at different developmental stages, we first demonstrate that, unlike our closest living relatives, Homo sapiens retain high levels of covariation between cortical areas into adulthood. Among the other great apes, high levels of covariation are only found in immature individuals. Secondly, at the macro-evolutionary level, our analysis of 400 endocasts, representing 148 extant primate species and 6 fossil hominins, shows that strong covariation between different areas of the brain in H. sapiens and Homo neanderthalensis evolved under distinctly higher evolutionary rates than in any other primate, suggesting that natural selection favoured a greatly integrated brain in both species. These results hold when extinct species are excluded and allometric effects are accounted for. Our findings demonstrate that high covariation in the brain may have played a critical role in the evolution of unique cognitive capacities and complex behaviours in both modern humans and Neanderthals.