The functional role of the paranasal sinuses in man has long been in dispute and as yet no satisfactory explanation has been offered for these 'unwanted' spaces. An answer may be found by study of the comparative evolutionary development of the sinuses in man and other higher primates. Several unique physical characteristics of man not seen elsewhere in the ape family, or indeed in other terrestrial mammals, including some relating to the upper aerodigestive tract, are not satisfactorily explained by the traditionally held theory of evolutionary development of early man directly from the arboreal ape. It is argued that these developmental differences are much more logically explained by a period of aquatic adaptation at a crucial period in the evolution of pre-hominid man. A new theory is proposed which might explain the importance of the sinus air cavities as buoyancy aids for protection of the upper airway tract in such an aquatic environment. Further evidence is offered relating to a pathological condition of the external ear canal which supports this theory that man at some stage in his early development acquired an affinity for an aquatic environment. Explanation of these unique hominid characteristics in terms of an aquatic evolutionary theory may help to resolve some of the enigmatic inconsistencies between man and other higher primates, and may account for man's eventual emergence as the dominant extant species, and perhaps an explanation for the 'missing link'.