The vestiary hypothesis proposes that hair reduction in humans, a characteristic of the species, evolved coincident with a developing intellectual capacity that permitted the use of artificial insulation. Functional hairlessness permitted the elaboration of an extraordinarily effective mechanism for heat dissipation, whole-body evaporation, but with the sacrifice of the heat retention function of typical pelage. The latter necessity was met by clothing, which required capability for fabrication and cultural transmission, and may have developed early in hominid evolution.