Evolutionary development of the human thumb.

Abstract

The bones and joints of the human thumb are a mosaic of primitive and unique features, reflecting stages in the evolution of the hand from a support element on the ground to a grasping structure in the trees and eventually to an organ dedicated entirely to manipulation. The trapeziometacarpal saddle joint configuration and associated musculature are shared with most nonhuman primate species, whereas the broad distal phalanx with its specialized palmar pad is unique to humans. Most of the distinctive features of the modern human thumb can be explained by the requirements for a firm grip and tolerance of large stresses associated with the use and manufacture of stone tools, which contributed for several million years to the survival of human ancestors after they returned to the ground. Fossil remains indicate that early members of the human family, Hominidae, had short thumbs relative to the length of the fingers, which were not subject to the large stresses associated with modern human manipulative behavior. Later hominids had very flat trapeziometacarpal joints and large distal phalanges, indicating a capacity for opposition of the thumb to all four fingertips and for tolerance of large stresses. Pathologies involving thumb joints contribute to the understanding of the sequence of changes in thumb morphology in the fossil record.