Ecogeographic patterns of modern human craniofacial diversity suggest external nasal structures reflect climatic adaptations for respiratory and thermoregulatory functions. Regarding internal structures, the maxillary sinus supposedly varies as a function of the nasal cavity while not contributing in respiratory function. Owing to conflicting results, this study reinvestigates that claim by evaluating maxillary sinus variation in a larger sample (n=200) spanning 11 ecogeographic regions. The surface-area-to-volume (SA:V) ratio (i.e., relative mucosal area) was collected in addition to sinus volume and linear dimensions. Pearson correlations show nasal cavity breadth and maxillary sinus volume are not significantly correlated, and individuals from cold, versus hot, climates exhibit larger volumes with lower surface-area-to-volume (SA:V) ratios. Individuals from high altitudes display a unique configuration with high SA:V ratios and large maxillary sinus volumes. Analyses of variance largely fail to find significant differences among the 11 samples. However, a canonical variate analysis of nasal and sinus dimensions indicates clear separations between the heat- and cold-adapted populations, as well as among the cold-adapted populations. Specifically, Arctic populations display smaller sinus volumes and lower SA:V ratios. Mantel tests indicate certain sinus dimensions depart from isolation-by-distance models. Results indicate that maxillary sinus form does not simply follow isolation-by-distance models and cannot simply be explained in terms of nasal cavity breadth or craniofacial architecture— suggesting that environmental pressures are directly acting on the sinus. Functional possibilities for the sinus include thermoregulatory functions among Arctic populations and/or nitric oxide production for high-altitude populations. Additional considerations and future lines of research are presented.

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