In the past, the unusual features of the gibbon hand have been viewed as structures whose function is to keep the thumb out of the way during brachiation. Recent behavioral and anatomical observations, however, indicate that climbing is as important to gibbons in their high, exclusively arboreal niche as brachiation, and that many pecularities of the gibbon hand actually represent structural adaptations to climbing. The functional reevaluation of the gibbon hand makes it clear that complex and often contradictory selection factors are responsible for phenotypic manifestations and that the classification of species into locomotor types, such as brachiator, semibrachiator, and quadruped, misrepresents the species' total locomotor adaptation and obscures the complex of selective factors responsible for structural evolution.