Size and Position of Frontal Air Sinuses

In common with most primates, humans and apes have pneumatized bones (that is, bones possessing air sinuses) in the facial skeleton and cranial vault. However, the presence of air sinuses in certain bones, and the size and position of those sinuses, is variable across the hominoids. Absence of a true frontal sinus (air sinus in the frontal bone) is an ancestral characteristic among Primates and the presence of a frontal sinus is confined as a derived trait of African apes and humans. The frontal sinuses is one of the more variably expressed sinuses, which lies deep to the brow ridge (or in the case of humans that lack a pronounced brow ridge, the supraorbital area) near the midline of the face. Despite having small-to-nonexistent brow ridges, humans have relatively large frontal sinuses (although this feature is highly variable across populations). The shape and size of the frontal sinus is so variable that x-rays of the frontal sinus can be used to positively identify individuals in forensic cases. Chimpanzees tend to have smaller frontal sinuses, while those of the gorilla are smaller still and orangutans and gibbons lack them altogether.

The frontal sinuses grow out of small air cells in the anterior portion of the ethmoid bone, and communicate with the nasal cavity via small openings (ostia) into the nasal cavity (or indirectly via the ethmoidal air cells). In humans, the frontal sinus shows a considerable amount of variability in the precise location within the frontal bone that the sinus arises from. Although sample sizes are small, the frontal sinus in chimpanzees and gorillas most often arise as a diverticulum from the suprabullar groove, which is the rarest location for the frontal sinus to arise in humans. The close developmental, anatomical and physiological relationship of the frontal sinuses, along with those of the ethmoid and sphenoid bones, is reflected in their classification as “paranasal” sinuses.

The functional role of the frontal sinuses is unclear, and they may simply be architectural hollows in parts of the face that are largely free of masticatory stress (and therefore do not require the reinforcement of solid bone). Difference in frontal sinus size in certain extinct Homo clade members (Neanderthals) has been attributed as an adaptation for living in colder climates. However, recent data suggests that there is an allometric relationship between sinus size and cranial size and therefore the scaling relationship does not actually result in a distinct increase in the size of the Neanderthal frontal sinus from modern humans. Neanderthal frontal sinus size does not distinguish them from modern humans.