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.

The Dart-Throwing Motion of the Wrist: Is It Unique to Humans?

“…42 At the radiocarpal joint of chimpanzees (genus Pan) and gorillas (genus Gorilla), a prominent ridge along the dorsal aspect of a deeply concave distal radial articular surface is thought to contact a corresponding concave facet on the scaphoid, resulting in a closepacked configuration and limited extension at the radiocarpal joint. [43][44][45] At the midcarpal joint, the capitate is stout and has a waisted neck as a result of lateral expansion of a broad head that achieves a highly stable close-packed position with the scaphoid and lunate at maximum extension. 41 This close-packing mechanism in the African apes may be aided by the near 100% occurrence of early fusion of the os centrale to the scaphoid in Pan and Gorilla (a characteristic shared with hominins).…”

“…41 This close-packing mechanism in the African apes may be aided by the near 100% occurrence of early fusion of the os centrale to the scaphoid in Pan and Gorilla (a characteristic shared with hominins). 42,[44][45][46][47][48][49][50][51] The os centrale portion of the scaphoid forms a broad, stable articular surface for the trapezium and trapezoid. In wrist extension, the os centrale becomes lodged within an embrasure formed by the capitate head and the trapezoid, 42,44,50 and scaphoid-centrale fusion might provide increased resistance to extension moments at the midcarpal joint.…”

“…42,[44][45][46][47][48][49][50][51] The os centrale portion of the scaphoid forms a broad, stable articular surface for the trapezium and trapezoid. In wrist extension, the os centrale becomes lodged within an embrasure formed by the capitate head and the trapezoid, 42,44,50 and scaphoid-centrale fusion might provide increased resistance to extension moments at the midcarpal joint. 42 The midcarpal joint is prevented from further extension by contact with a distinct ridge on the concave aspect of the capitate head 44 with the proximal scaphoid-centrale facet.…”

“…In wrist extension, the os centrale becomes lodged within an embrasure formed by the capitate head and the trapezoid, 42,44,50 and scaphoid-centrale fusion might provide increased resistance to extension moments at the midcarpal joint. 42 The midcarpal joint is prevented from further extension by contact with a distinct ridge on the concave aspect of the capitate head 44 with the proximal scaphoid-centrale facet. Extension of the midcarpal joint also appears to be limited because of the proximal expansion of the nonarticular surface of the dorsal capitate in Pan and Gorilla, which shortens the articular path of the distal carpal row.…”

“…Several investigators 44,45,[47][48][49]52 have suggested that there are hints of knuckle-walking human ancestors. For example, using linear and angle measurements of the radiocarpal joint, Richmond and Strait 45 suggested a temporal regression of African-ape-like extensionlimiting features of the joint from early species such as Australopithecus anamensis and A. afarensis, whose radii retained a distinct distally projecting dorsal ridge, to A. africanus, whose radial contour and articular facets were more similar to the modern human radius.…”