Functions of the Hand in Primates

Nearly 100 years ago, Wood Jones ( 1920 ) characterized the hand as a specialized sensory organ and the use of the hands in exploration as a defi ning characteristic of primates. Thus, from the beginning, researchers have investigated how primates use their hands to touch and handle objects, as well as during locomotion. Understanding how primates use their hands is fundamental to reconstructing and interpreting the evolution of the order to which humans belong to (Napier 1960 , 1961 , 1980 ). Primates were present in the Eocene, feeding and moving predominantly on thin and fl exible terminal branches (Cartmill 1974 ; Sussman 1991 ; Sussman et al. 2013 ; see Chap. 14 ). They used both hands and feet to locomote in this environment and the hands to feed. Prehension of small objects (such as insects or fruits) by one hand is considered a primitive characteristic of primates (Washburn 1951 ; Napier 1961 , 1980 ) and is integral to feeding in all living primates. However, this characteristic is shared with other orders as primates are not alone in using one appendage in prehension. Indeed, Iwaniuk and Whishaw ( 2000 ) reported that, of 29 tetrapod orders for which the presence or absence of “skilled forelimb movements” (which includes reaching, grasping, and manipulation with one forelimb) was examined, 13 orders were characterized by such movements. Iwaniuk and Whishaw ( 2000 ) showed that skilled movements of the forelimbs in mammals probably share a common origin in early tetrapods, appearing after the divergence of therian mammals (marsupials and placentals) from the monotremes. The prehensile actions of primates must therefore be considered against this ancient backdrop.