Knuckle-walking - Human Evolution

Human Evolution

One theory of the origins of human bipedality is that it evolved from a terrestrial knuckle-walking ancestor. This theory is opposed to the theory that such bipedalism arose from a more generalized arboreal ape ancestor. The terrestrial knuckle-walking theory argues that early hominin wrist and hand bones retain morphological evidence of early knuckle-walking. The argument is not that they were knuckle-walkers themselves but that it is an example of "phylogenetic "lag"". "The retention of knuckle-walking morphology in the earliest hominids indicates that bipedalism evolved from an ancestor already adapted for terrestrial locomotion. ... Pre-bipedal locomotion is probably best characterized as a repertoire consisting of terrestrial knuckle-walking, arboreal climbing and occasional suspensory activities, not unlike that observed in chimpanzees today".

It has however been suggested that knuckle-walking evolved independently and separately in Pan and gorillas and so was not present in the human ancestors. This is supported by the evidence that gorilla and chimpanzee differ in their knuckle-walking related wrist anatomy and in the biomechanics of their knuckle-walking.

Indeed as Tracy L. Kivell and Daniel Schmitt note "Features found in the hominin fossil record that have traditionally been associated with a broad definition of knuckle-walking are more likely reflecting the habitual Pan-like use of extended wrist postures that are particularly advantageous in an arboreal environment. This, in turn, suggests that human bipedality evolved from a more arboreal ancestor occupying a generalized locomotor and ecological niche common to all living apes".

Arguments for the independent evolution of knuckle-walking have not gone without criticism, however. A more recent study of morphological integration in human and great ape wrists suggests that knuckle-walking did not evolve independently in gorillas and chimpanzees, which "places the emergence of hominins and the evolution of bipedalism in the context of a knuckle-walking background."