The development and function of the nasopharynx and its role in the evolution of primate respiratory abilities

The nasopharynx is a centrally located region of the upper respiratory tract (URT) integral to several physiological functions. However, few have focused on this area within the context of human evolution. This study investigated osseous morphology, soft tissue histology, development, and evolutionary change of the nasopharynx. Multimodal analyses were performed: Analysis 1: This study tested hypotheses on the morphological relationships of the osseous nasopharyngeal boundaries with the splanchnocranium and basicranium among dry crania representing humans and non-human primates using 3D geometric morphometrics (3D-GM). Results showed that humans, the most orthognathic group, exhibited the widest nasopharynges. Over human development, the nasopharynx grows vertically taller and anteroposteriorly shorter while the path of the cartilaginous Eustachian tube (CET) grows longer and more vertically oriented. Timing of these growth changes coincide with changes in frequency of otitis media. Analysis 2: The nasopharynx was hypothesized to warm and humidify air only via its bony, non-contractile surfaces. Air conditioning capacity was assessed by presence of submucosal blood vessels, mucous cells, and serous cells on histological slides of nasopharyngeal surfaces. Results indicated that all of these microstructures were present on all nasopharyngeal surfaces, rather than being restricted to non-conctractile bony surfaces. Analysis 3: Ambient climate was hypothesized to influence URT growth. Two groups of adult male Macaca mulatta raised in Oregon (cold climate) and California (warm climate) outdoor colonies were used. CT imaging and 3D-GM were performed. Results revealed no shape differences but the Oregon individuals exhibited larger airway size and smaller body mass than the California individuals. Thus being raised in cold climates appears related to development of larger URT proportions relative to body size. Analysis 4: Nasopharyngeal morphology of fossil hominins, including Neanderthals and mid-Pleistocene Homo (MPH) from Europe and Africa, were reconstructed using 3D coordinate data and analyzed via 3D-GM. Neanderthals exhibited greater CET length than MPH and modern humans while the horizontal CET orientation of Neanderthals resembled the human infant condition. Results strongly suggest that Neanderthals possessed CET morphology and physiology distinct from modern humans, likely impacting susceptibility to middle ear disease and supporting species-level distinction