Biometric evidence that sexual selection has shaped the hominin face

Abstract

We consider sex differences in human facial morphology in the context of developmental change. We show that at puberty, the height of the upper face, between the lip and the brow, develops differently in males and females, and that these differences are not explicable in terms of sex differences in body size. We find the same dimorphism in the faces of human ancestors. We propose that the relative shortening in men and lengthening in women of the anterior upper face at puberty is the mechanistic consequence of extreme maxillary rotation during ontogeny. A link between this developmental model and sexual dimorphism is made for the first time, and provides a new set of morphological criteria to sex human crania. This finding has important implications for the role of sexual selection in the evolution of anthropoid faces and for theories of human facial attractiveness.

Figure 1. Male and female H. sapiens ontogenetic trajectories plotted with fossil hominin crania.

The relationship between bizygomatic width (BZW) and upper facial height (FHT) shows a departure from ontogenetic scaling. Major axis slopes and 95% confidence intervals: male 0.7847 (0.74–0.83), female 0.6988 (0.65–0. 75). The FHT value for specimen KNM-ER 406 is a conservative estimate as the subnasal region is slightly damaged . However, a small increase in FHT would align this cranium even more closely to the male ontogenetic trajectory.

Figure 2. Skeletal craniofacial variables relate to facial appearance.

We show that adult males have relatively shorter upper faces for their breadth compared to females (Table 1,2, Figure 1 and Figures S2 and S3). Lines superimposed on the pictures illustrate this facial dimorphism: vertical lines are positioned against the left and right zygion, and horizontal lines are positioned against the nasion and prosthion of the male face. In comparison to the female face, the male face is wider (represented by the distance between left and right point zygion) and the upper facial height (represented by the distance between point nasion and point prosthion) is approximately the same. The photographs are presented as taken, with identical camera-to-subject distance, and without rescaling, in order to represent the actual size of the faces.

Figure 3. A comparison of male and female skeletal traits versus dental category (age class) for Pan troglodytes and Gorilla gorilla.

For the chimpanzee, sexual size dimorphism is shown to be statistically significant in BZW in age classes 6 and 7 (A) and no significant sexual size dimorphism is evident in FHT for any age class (B). For the gorilla, sexual size dimorphism is evident in BZW (C) and FHT (D) for age classes 4, 5, and 7; BZW and FHT are sexually dimorphic in size as adults (age class 7). Age classes 1–7 plotted on the x axis; each class with females (F) plotted first and then males (M). Trait size on y axis (cm). The box plots indicate the median in white and the quantiles in colour. The dotted lines indicate the data range with outliers shown as isolated bars (for sample analysed see electronic Appendix in [17]).