Hand grasping is one of the unique characteristics of primates. This study reviews our current understandings on hand shape and grasping from two aspects: changes of hand structure in the fossil record, and the significance of hand grasping in arboreal life.
Plesiadapiformes, the sister group of modern primates, exhibits reduction of the relative lengths of matacarpals and distal phalanges within the manus and possesses a widely divergent hallux.The transformation of nails from claws on the all fingers and the appearance of opposability in the hallux occurred after the acquisition of these morphologies. Early in Euprimate evolution, strepsirhines are characterized by wider inter-digit-ray angles and proportionately shorter metacarpals, while haplorhines have nearly parallel metacarpals associated with the development of mesaxony. The latter is more similar to the ancestral condition of Euprimates based on comparisons with other mammals .
The unique characteristics of primates were explained in ambiguous terms as arboreal adaptations ("arboreal theory") until the 1960′s.Subsequent hypotheses, "visual predation theory" and "angiosperm theory", have attempted to correlate these characteristics with specific arboreal activities. It was proposed that hand grasping ability evolved in the primate ancestor because it was beneficial for stabilizing the body and for accomplishing predation at terminal branches. This explanation is based on hand grasping behavior that is also employed by some extant non-primate mammals that make use of terminal branches. Comparisons with other arboreal mammals which have grasping hands and with mechanical models have shown that longer proximal and middle phalanges and shorter distal phalanges relative to metacarpals are required for grasping narrow branches. The fact that the phalangeal ratio change occurred prior to other structural changes typically advanced as critical features in primate evolution clarifies the importance of these characteristics in acquisition of hand grasping ability, a feature that evolved in Plesiadpiformes approximately 55 million years ago.