Abstract
The reduction of masticatory load intensity resulting from dietary changes in human evolution has been proposed as an important factor that alters craniofacial shape in past and current populations. However, its impact on craniofacial variation and on the perceived differences among populations is unclear. The maxillomandibular relationship, which alters masticatory force direction, is a factor often neglected but it can contribute to variation in craniofacial morphology, particularly among modern/urban populations where the prevalence of dental malocclusions is greater than in prehistoric populations. This study investigates the influence of masticatory load intensity and maxillomandibular relationship as a proxy for force direction on the human craniofacial skeleton. By using 3D imaging and geometric morphometrics, we analyzed craniofacial shape variation among 186 individuals from pre-Hispanic and modern Chilean and Argentinean populations that differ in diet consistency (a proxy for masticatory load intensity) and maxillomandibular relationship. We predicted that masticatory load would have a subtle effect on the upper craniofacial bones and that this would be more marked in the maxilla. Our results showed no clear influence of masticatory load on craniofacial shape, particularly in modern/urban populations. Allometry, on the contrary, shows a stronger effect. The degree of integration between the upper craniofacial bones and the load-bearing maxilla depends on masticatory load intensity, decreasing from high to low but showing a conservative pattern of covariation among the groups. The degree of variation in the shape of the maxilla is greater than the upper craniofacial bones. These results suggest that masticatory load has a limited effect in determining differences in craniofacial morphology among populations. This effect is slightly greater for the maxillary region of the face. We propose that the reduction of functional constraints is key to greater shape variation found in modern/urban populations.
Highlights
The assessment of human skull shape has been key for researchers reconstructing prehistoric population history [1,2,3,4,5]
The aim of this study is to examine the intensity of masticatory load, and how the altered direction of forces occurring during altered maxillomandibular relationships, plays a role in shape variation of the human skull
While there is overlap among all groups on both axes, the Class III (CIII) individuals tend be slightly clustered towards the negative end of PC1, while Class II (CII) is distributed across PCs 1 and 2, giving it a slightly more distinctive pattern of variation than the other groups
Summary
The assessment of human skull shape has been key for researchers reconstructing prehistoric population history [1,2,3,4,5]. Pathologies of the hard tissues and other plastic changes that can affect bone due to physiological factors may alter skull shape [10, 11] One such example is the presence of dental malocclusion, which tends to be more frequent in modern/urban populations who eat a heavily processed diet. Some researchers report that a high prevalence of different dental malocclusions in modern populations is an important aspect of variation in the modern human skull [see 16, 17, 18, 19 for examples]. The high prevalence of dental anomalies in modern groups, those affecting the relationship between the maxilla and the mandible, makes them an interesting, yet largely unexplored model to investigate the extent to which different aspects of masticatory loads, such as force magnitude and maxillomandibular relationship (as a proxy for force direction) cause variation among populations
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