The Upper Limb of Australopithecus sediba

Abstract

The evolution of the human upper limb involved a change in function from its use for both locomotion and prehension (as in apes) to a predominantly prehensile and manipulative role. Well-preserved forelimb remains of 1.98-million-year-old Australopithecus sediba from Malapa, South Africa, contribute to our understanding of this evolutionary transition. Whereas other aspects of their postcranial anatomy evince mosaic combinations of primitive (australopith-like) and derived (Homo-like) features, the upper limbs (excluding the hand and wrist) of the Malapa hominins are predominantly primitive and suggest the retention of substantial climbing and suspensory ability. The use of the forelimb primarily for prehension and manipulation appears to arise later, likely with the emergence of Homo erectus.

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

“…However, the length of the proximal and intermediate phalanges (relative to metacarpal length) is similar to that of modern humans, suggesting long fingers were not a functional requirement of its locomotor strategy. Within the context of the remainder of the MH2 skeleton, it may be that the functional requirements of locomotion were fulfilled by the long, ape-like upper limb [84] and mildly curved phalanges [10], while the hand proportions of Au. sediba largely reflect the requirements of enhanced dexterity.…”

Fossil hominin shoulders support an African ape-like last common ancestor of humans and chimpanzees

“…The hominin fossil record is similarly complicated. Scapular remains attributed to Australopithecus afarensis are described as similar to Gorilla (6-8) whereas the more recent Australopithecus sediba (MH2) displays morphometric affinities to both African apes (gorillas, chimpanzees, and bonobos) and Pongo (9). This mix of character states raises the question of whether modern human morphology reflects evolution from a more derived African ape morphology or retention of primitive traits from an earlier ape ancestor.…”

Inner structural organization of the distal humerus in Paranthropus and Homo

“…Accordingly, the lower diaphyses of P. boisei (not considered in our study) and of non-erectus early Homo are "unusually AP flattened (ML broad) among both fossil hominins and extant hominids, though non-erectus Homo humeri are most extreme in this regard and markedly different from the more modern human-like specimens attributed to H. erectus" (Lague, 2015: 27). Fresh evidence from the A. sediba's ulna shows that, in aspects of its size and shape, the outer morphology of its elbow, which shows adaptations to suspensory and climbing behaviours (Rein et al, 2017), falls within the range of variation of eastern and southern australopiths, including some specimens commonly attributed to P. robustus, such as TM 1517 and SK 10294 (Churchill et al, 2013).…”

“…In its original description, Susman stated that "(t)his specimen is a reasonably good match for TM 1517 in both size and shape, and on this basis it most likely belongs to Paranthropus" (Susman, 1989: 462). However, based on its cross-sectional shape and comparisons with the expanded record at Swartkrans, this specimen has since been allocated to Homo (Lague, 2015;Susman et al, 2001; Braga et al, 2016a, b;Broom, 1938b;Broom and Schepers, 1946;Bruxelles et al, 2016;Churchill et al, 2013;Di Vincenzo et al, 2015;Lague, 2014Lague, , 2015Lague and Jungers, 1996;McHenry and Brown, 2008;Ryan and Sukhdeo, 2016;Senut, 1981a;Skinner et al, 2013;Straus, 1948;Susman, 1989;Susman et al, 2001;Thackeray et al, 2001 SK 24600 Swartkrans Chavaillon et al, 1977;Churchill et al, 2013;Di Vincenzo et al, 2015;Puymerail et al, 2014;Ryan and Sukhdeo, 2016;Senut, 1979Senut, , 1981a a In some papers cited in this study, the taxon Paranthropus is subsumed within Australopithecus. b Braga et al, 2016a, b;Bruxelles et al, 2016.…”

“…An interesting case is represented by the long-term debate about the allocation to Paranthropus (or australopith s.l., following some authors) or Homo of isolated remains of the distal humerus remains (e.g., Bacon, 2000;Churchill et al, 2013;Di Vincenzo et al, 2015;Lague, 2014Lague, , 2015Lague and Jungers, 1996;Patterson and Howells, 1967;Senut, 1981a;Susman, 1989;Susman et al, 2001). While potentially well suited for assessing postcranial variation because of their abundance in the record and the biomechanical specificities of the hominin elbow joint, Plio-Pleistocene specimens from both eastern and southern Africa have nonetheless been the matter of frequent controversial attributions (rev.…”