The formation of the human paranasal sinuses.

The paranasal sinuses, which formed in mammals as turbinates and air spaces to perform or aid olfaction, eventually came to participate in the formation and maintenance of the entire cranium and to have concomitant functions as well. Evolving through primates to humans, they were influenced by the retraction of the maxillo-facial cranium and the enlargement of the cerebral cranium and thus reached the state unique to humans. The evolutionary phenomena involved in the formation and functions of the paranasal sinuses include adaptation to olfaction, respiration, and cranial formation. These were the phenomena which caused allometric development. Therefore, the paranasal sinuses did not always perform the same functions as they did when they originated, but, influenced by the evolutionary level of each animal and especially by the evolution to apes and humans, the original functions changed to different ones, mostly as instances of preadaptation, according to the evolutionary stage of each paranasal sinus. As the sense of smell ascribed to the mammalian nose began to regress together with the retraction of the jaw (maxilla and mandible), the orbits started to form in primates, and the ethmoturbinate became full of cavities and formed the lateral walls of the nasal cavity. This occurred because the evolution of primates and higher made olfaction non-essential and respiration foremost. When hominization began and the facial cranium distinctly retracted, the maxillary sinus enlarged because of the enlargement of the molars, the change in the dental arch, and the enlargement of the masticatory muscles, and its air-conditioning function became established. The secondary sinuses of the frontal and sphenoid sinuses, which had expanded in mammals, started to regress in response to the new conditions. Especially when the cranium became spherical in the transition from the homo sap. neanderthalensis stage to the homo sap. sap. stage, the transformation of the cerebral cranium forced an increase in the angle between the forehead and the frontal cranial base and a decrease in the angle of the cranial base at the sella turcica. The frontal and sphenoid sinuses then expanded in adjustment to this transformation.