The capitate of Australopithecus afarensis and A. africanus

Abstract

The capitates of Australopithecus afarensis (AL 288-lw and AL 333-40) and A. africanus (TM 1526) have the identical combination of modern pongid, modern hominid, and unique characteristics. These traits include the combination of a length that is proximodistally shortened (Homo sapiens-like), a facet for the second metacarpal that is distolaterally facing (unique), the reduced styloid process on the third metacarpal (pongidlike), a dorsally placed trapezoid facet (pongidlike), mediolaterally constricted metacarpal III facet (pongidlike), a prominent palmar beak (pongidlike), a single elongated facet for the second metacarpal (H. sapiens-like), a waisted neck (pongidlike), and a reduced amount of "cupping" in the third metacarpal facet (H. sapiens-like). In overall shape the bones are more like H. sapiens than other extant hominids, although they are uniquely different. The two A. afarensis capitates provide no evidence that there are two postcranial morphotypes at Hadar. Available evidence shows that A. afarensis and A. africanus are strikingly similar postcranially. The morphological differences between the capitate of Australopithecus and H. sapiens may relate to the retention of climbing ability and an absence of certain grip capabilities in these early hominids.

Origin of human bipedalism: The knuckle-walking hypothesis revisited

“…As with the hamate and the trapezoid, the capitate's metacarpal articular surface is more complex than in Asian apes and other catarrhines (see below). McHenry (1983) stresses the chimpanzee-like morphology of australopithecine capitates, and this is supported by newer material described in Ward et al (1999a,b). McHenry (1983) interprets australopithecine chimpanzee-like capitate characters to be retained primitive characters, and at least one of these features, a waisted capitate neck, is functionally consistent with knuckle-walking.…”

“…McHenry (1983) stresses the chimpanzee-like morphology of australopithecine capitates, and this is supported by newer material described in Ward et al (1999a,b). McHenry (1983) interprets australopithecine chimpanzee-like capitate characters to be retained primitive characters, and at least one of these features, a waisted capitate neck, is functionally consistent with knuckle-walking. Ward et al (1999a,b) also describe a "keel" on the capitate head that may correspond to the lengthy lunate-scaphoid ridge that Corruccini (1978) considered to be a knuckle-walking adaptation.…”

“…8). The head of the capitate, which forms most of the ball of the ball-and-socket midcarpal joint in African apes and humans, is broad transversely and, in African apes and fossil humans, separated from the distal portion by a deep notch, contributing to a waisted morphology (Bush et al, 1982;McHenry, 1983;Ward et al, 1999a) (Fig. 8).…”

“…He also noted that "cupping" does not occur in the australopithecines he examined (A. afarensis and A. africanus), though it does appear to have been present in other specimens of Australopithecus (Ward et al, 1999a,b). While there are other important differences in the capitate distal articular surfaces between fossil humans and African apes (Bush et al, 1982;Marzke, 1983;McHenry, 1983;Marzke et al, 1992;Ward et al, 1999a), they all share this keeled morphology, which is lost in later humans. The distal articular surface of the hamate is also keeled in African apes and fossil humans, separating to some degree the facets for the fourth and fifth metacarpal bases, which are thus somewhat divergent (Bush et al, 1982;Marzke, 1983;Ward et al, 1999a).…”

Early Pleistocene third metacarpal from Kenya and the evolution of modern human-like hand morphology

“…1A) is another distinctive feature of the modern human/Neandertal hand that is absent in all apes and other nonhuman primates, as well as Pliocene and Early Pleistocene hominins (11,14,16,(21)(22)(23)(24)(25)(26)(27). The dorsal extension of bone seen in a single specimen of Au.…”