The nasopharynx is an important anatomical structure involved in respiration. Its bony boundaries, including the basicranium and upper cervical vertebrae, may be subject to selective pressures and ...constraints related to respiratory function. Here, we investigate phenotypic integration, or covariation, between the face, the basicranial boundaries of the nasopharynx, and the atlas and axis to understand constraints affecting these structures. We collected three‐dimensional coordinate data from a sample of 80 humans and 44 chimpanzees, and used two‐block partial least squares to assess RV (a multivariate generalization of Pearson's r2), rPLS, the covariance ratio, and effect size for integration among structures. We find that integration is significant among some of these structures, and that integration between the basicranial nasopharynx and vertebrae and between the face and vertebrae is likely independent. We also find divergences in the pattern of integration between humans and chimpanzees suggesting greater constraints among the human face and nasopharynx, which we suggest are linked to divergent developmental trajectories in the two taxa. Evolutionary changes in human basicranial anatomy, coupled with human‐like developmental trajectories, may have required that the face grow to compensate any variation in nasopharyngeal structure. However, we were unable to determine whether the nasopharynx or the face is more strongly integrated with the vertebrae, and therefore whether respiration or biomechanical considerations related to positional behavior may be more strongly tied to vertebral evolution. Future work should focus on greater sample sizes, soft tissue structures, and more diverse taxa to further clarify these findings.
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.
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