Evolution of the distal radioulnar joint

The evolution of the wrist joint began 400 million years ago with the pectoral fins in primitive fish. The five-rayed extremity with 13 carpal bones and a syndesmotic distal radioulnar joint first appeared in the primitive amphibian, Eryops, some 230 million years ago. These characteristics remained similar through the evolution of the amphibians and of mammals and were retained when primates evolved to hominids. As later-developing, larger-sized hominids began competing for food, brachiation (the ability to swing from tree branch to tree branch) became more important, and a synovial distal radioulnar joint evolved, allowing pronation/supination of the wrist. The development of bipedalism freed the upper extremity from the requirements of locomotion, placing greater emphasis on increasing wrist mobility. The ability to supinate and pronate was an immense advantage to hominids in caring for their young, defending themselves, and gathering food. It was also critical in efficient tool handling, which developed two million years ago. The mobility of the distal radioulnar joint, along with the prehensile thumb and increasing brain function, are hallmarks of the late-evolving hominids.