Integration between the bony nasopharynx and the cervical vertebrae in Homo and Pan

Abstract

Hominin evolution is marked by an increase in the size of the brain and concomitant reorganization of the face and basicranium. As a result, modern human crania are drastically different from those of our closest relatives, including in the nasopharyngeal region, which is necessary for vital life functions such as respiration. The maintenance of nasopharyngeal function serves as an important constraint on the development of surrounding structures, and researchers have documented extensive potential negative consequences. However, little is known about the impact of these functional constraints on the cervical vertebrae, which form the posterior border of the nasopharynx. We tested whether the atlas, axis, face, and the cranial boundaries of the nasopharynx form an integrated module using three-dimensional coordinate data from recent Homo (N =80) and Pan (N =44). We used two-block partial least squares (2B-PLS) to obtain RV values, covariance ratios, and z-scores for each of the following potential modules: nasopharynx-face, nasopharynx-atlas, nasopharynx-axis, face-atlas, face-axis, atlas-axis. P-values were obtained using 999 to 1000 random permutations. Analyses were carried out on MorphoJ and in R using the geomorph package. Our results confirm previous findings that the nasopharynx and face form an integrated module across taxa (Effect size =14.0185, CR =1.0296, P =0.001) but this relationship is weaker within Homo and within Pan. In addition, our results suggest that across taxa the atlas and axis are integrated with the face and nasopharynx (CR = 1.0206-1.058) to a greater extent than they are to each other (CR =0.7797, P =0.001), though again within taxa this effect is less clear. Finally, covariation tends to be stronger between the axis and nasopharynx (Effect size =9.4830), and between the axis and face (Effect size =9.0196), than between the atlas and any structure (Effect size =8.4396-8.7139). However, this difference was not statistically significant and again became less clear within taxa. Together, these results suggest that, across hominids, nasopharyngeal function may be constraining upper cervical vertebral morphology, and especially axis morphology, at least to the same extent as the vertebrae affect each other, if not more. Nonetheless, the importance of these different relationships may vary in different taxa. Future research should focus on clarifying relationships within taxa with larger samples, and expanding the analysis to additional taxa.