Knuckle‐walking and the evolution of hominoid hands

Abstract

Knuckle-walking is a pattern of digitigrade locomotion unique to African apes among Primates. Only chimpanzees and gorillas are specially adapted for supporting weight on the dorsal aspects of middle phalanges of flexed hand digits 11-V. When forced to the ground, most orangutans assume one of a variety of flexed hand postures, but they cannot knuckle-walk. Some orangutans place their hands in palmigrade postures which are impossible to African apes. The knuckle-walking hands and plantigrade feet of African apes are both morphologically and adaptively distinct from those of Pongo, their nearest relative among extant apes. These features are associated with a common adaptive shift to terrestrial locomotion and support placing chimpanzees and gorillas in the same genus Pan. It is further suggested than Pan comprises the subgenera ( a ) Pan, including P. troglodytes and pygmy chimpanzees, and ( b ) Gorilla, including mountain and lowland populations of P. gorilla.African apes probably diverged from ancestral pongids that were specially adapted for distributing their weight in terminal branches of the forest canopy. Early adjustments to terrestrial locomotion may have involved fist-walking which later evolved into knuckle-walking. Orangutans continued to adapt to feeding and locomotion in the forest canopy and their hands and feet became highly specialized for four-digit prehension. Although chimpanzees retained arboreal feeding and nesting habits, they moved from tree to tree by terrestrial routes and became less restricted in habitat.While adapting to a diet of ground plants gorillas increased in size to the point that arboreal nesting is less frequent among them than among chimpanzees and orangutans.Early hominids probably diverged from pongids that had not developed prospective adaptations to knuckle-walking, and therefore did not evolve through a knucklewalking stage. Initial adjustments to terrestrial quadrupedal locomotion and resting stance probably included palmigrade hand posturing. Their thumbs may have been already well developed as an adaptation for grasping during arboreal climbing. A combination of selection pressures for efficient terrestrial locomotor support and for object manipulation further advanced early hominid hands toward modern human configuration.The hands of great apes, like those of other primates, are complex organs which have evolved due to selection for locomotor functions, both in trees and on the ground, and for manipulative functions such as feeding, nesting, arboreal resting postures, grooming, and tactile communication.Previous studies of great ape hands have concentrated mainly on their adaptations to brachiation and on the possibility that great ape hands might represent precursor forms of the human hand. Both of these problems are associated with the Keith-Gregory hypothesis that man evolved from a brachiator. Behavioral and field studies indicate that the hands of great apes, particularly those of chimpanzees and gorillas, are more than mere hooks used for arboreal loc...

Origin of human bipedalism: The knuckle-walking hypothesis revisited

“…Terrestrial functionality is further supported by the observation that some captive orangutans (likely to have practiced much more terrestriality) have more elongated and palmarly directed pisiforms like those of African apes (Sarmiento, 1985). Dainton and Macho (1999) recently documented differences in the later stages of ontogeny of the ulnar carpal region, and interpret these develop- Tuttle (1967), who first identified knuckle-walking specializations in the radiocarpal joints. Note the distal projection (arrows) of the dorsal margin in Gorilla, Pan, and the radii attributed to A. anamensis (KNM-ER 20419) and A. afarensis (AL288-1).…”

“…During knuckle-walking, African apes bear their weight on the backs of their middle phalanges (middle segments of their fingers), which involves strongly flexed proximal interphalangeal joints, and extended metacarpophalangeal joints (Tuttle, 1967). Cineradiographic experiments (Jenkins and Fleagle, 1975) and preliminary data from ongoing kinematic experiments show that the wrist maintains a slightly extended posture throughout the late support phase of knuckle-walking.…”

“…In African apes, the dorsal margin of the distal radius projects distally (Fig. 4), buttressing the scaphoid as it rotates during extension (Tuttle, 1967;Jenkins and Fleagle, 1975;Corruccini, 1978;Richmond and Strait, 2000). When the rotating proximo-dorsal surface of the scaphoid contacts the distal radial joint surface, and when other joints, such as the scaphoid-capitate joint, achieve a "closepacked" position, little to no further extension is possible (Fig.…”

“…As noted by Jenkins and Fleagle (1975, p. 221), "There is little reason to assume that only one morphological feature is principally responsible; rather, this limited excursion is probably reflected in many aspects of wrist morphology." Thus, African ape distal radii have distally projecting dorsal margins, a broad and dorsally oriented scaphoid notch, and relatively coplanar scapho-lunate joint surfaces (Tuttle, 1967;Jenkins and Fleagle, 1975;Corruccini, 1978;Richmond and Strait, 2000).…”

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).…”