Origin of human bipedalism: The knuckle-walking hypothesis revisited

Abstract

Some of the most long-standing questions in paleoanthropology concern how and why human bipedalism evolved. Over the last century, many hypotheses have been offered on the mode of locomotion from which bipedalism originated. Candidate ancestral adaptations include monkey-like arboreal or terrestrial quadrupedalism, gibbon-or orangutan-like (or other forms of) climbing and suspension, and knuckle-walking. This paper reviews the history of these hypotheses, outlines their predictions, and assesses them in light of current phylogenetic, comparative anatomical, and fossil evidence. The functional significance of characteristics of the shoulder and arm, elbow, wrist, and hand shared by African apes and humans, including their fossil relatives, most strongly supports the knuckle-walking hypothesis, which reconstructs the ancestor as being adapted to knuckle-walking and arboreal climbing. Future fossil discoveries, and a clear understanding of anthropoid locomotor anatomy, are required to ultimately test these hypotheses. If knucklewalking was an important component of the behavioral repertoire of the prebipedal human ancestor, then we can reject scenarios on the origin of bipedalism that rely on a strictly arboreal ancestor. Moreover, paleoenvironmental data associated with the earliest hominins, and their close relatives, contradict hypotheses that place the agents of selection for bipedality in open savanna habitats. Existing hypotheses must explain why bipedalism would evolve from an ancestor that was already partly terrestrial. Many food acquisition and carrying hypotheses remain tenable in light of current evidence.

Evidence in hand: recent discoveries and the early evolution of human manual manipulation

“…Further discoveries of earlier bipedal hominins dating back to at least approximately 6 Ma [65] only increased the gap between bipedalism and stone tools [8]. Thus, tool-related behaviours have generally no longer been thought to have a cause-effect relationship with the origin of hominin bipedalism [50,117,118]. However, it may be worthwhile to revisit Darwin's original hypothesis.…”

Cerebellar hypoplasia and quadrupedal locomotion in humans as a recessive trait mapping to chromosome 17p

“…This way of walking is distinct from the knuckle walking of the great apes, who bear their weight on the dorsal surface of their middle phalanges. 13 Knuckle walking has been regarded as the ancestral method of locomotion based on the anatomy of wrist and phalangeal bones. 14 The phenotype described here demonstrates that a single gene defect can result in efficient palmigrade walking in humans in spite of a wrist that is generally thought to be anatomically not adapted to this form of locomotion.…”

3D‐kinematics of vertical climbing in hominoids

“…This "vertical climbing hypothesis" has been widely accepted (but see Gebo, 1996;Richmond and Strait, 2000;Richmond et al, 2001). Biomechanical considerations suggested that, during vertical climbing, the stresses and muscle activities in the hindlimbs are very similar to those during bipedal walking (Fleagle et al, 1981;Prost, 1980).…”

“…At present, more research on hominoid locomotion and a functional linking with morphological features are clearly warranted (Richmond et al, 2001). A comparative approach should include as many primate species and locomotor types as possible, because the extant apes and humans are all highly specialized in terms of locomotion and have diverged from the primitive condition of their last common ancestor.…”