Australopithecus sediba Hand Demonstrates Mosaic Evolution of Locomotor and Manipulative Abilities

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Science  09 Sep 2011:
Vol. 333, Issue 6048, pp. 1411-1417
DOI: 10.1126/science.1202625

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  1. Fig. 1

    Au. sediba MH2 right hand. Palmar (left) and dorsal (right) views of all MH2 right hand bones found in association with the right upper limb. Features of the MH2 hand traditionally considered primitive or australopith-like are labeled in gray (palmar view), and features considered derived are labeled in white (dorsal view) [(2, 3, 5), but see (14)]. The thumb is pictured in opposition, overlapping with the second metacarpal.

  2. Fig. 2

    Pollical distal phalanx of Au. sediba MH2 hand. (A) Right distal pollical phalanx of MH2 (UW 88-124) shown (from left to right) in medial, dorsal, lateral, and palmar views, compared to (B) StW 294 in medial and palmar views, (C) cast of FLK NN-A of the OH 7 hand in lateral and palmar views, and (D) SKX 5016 in medial and palmar views. Although portions of the MH2 phalanx are missing, the apical tuft at the distal end of the bone is expanded (white outline estimated by mirror-imaging preserved fossil morphology). The presence of an ungual fossa (gray highlighted area in palmar view of MH2) and a well-developed distal ridge (white arrows) to the proximal palmar fossa imply a well-developed FPL (17). The overall shape of the bone is most similar to StW 294 (table S3), which has been attributed to both Au. africanus (13) and early Homo (3), and unlike the flat, broad morphology of OH 7 (6, 16) or SKX 5016, which is attributed to Au. robustus (15). The distal ridge is less pronounced in SKX 5016, and all of the morphological features associated with a well-developed FPL are absent in OH 7 (14). All fossils are to scale.

  3. Fig. 3

    First metacarpal (Mc1) of Au. sediba MH2 hand. (A) MH2 right Mc1 (UW 88-119) in (from left to right) dorsal, mediopalmar, lateral, and medial views compared to palmar (left) and medial (right) views of (B) SK 84 and (C) SKX 5020, both of which are possibly Au. robustus or Homo (15, 20, 28). (D) Au. africanus StW 418 and (E) a female modern human. The projecting sesamoid beak (white asterisks) is found on the palmar surface of the Mc1 head in MH2 and SK 84 (2, 20). Like the other fossil hominin Mc1s depicted here, MH2 displays a human-like, distally extended muscle marking for the insertion of the dorsal interosseous tendon on the medial shaft (white brackets), but the muscle scarring is less prominent. The insertion area for the opponens pollicis tendon on the lateral shaft (gray brackets) is poorly developed in MH2 as compared to the other hominins. SK 84 and StW 418 are from the left side and have been mirror-imaged for easier comparisons. All bones are to scale.

  4. Fig. 4

    Relative size of third metacarpals in Au. sediba. Third metacarpals of (A) a female modern human in palmar view; (B) Au. sediba adult female MH2 (UW 88-116) and (C) the juvenile male MH1 (UW 88-112) (11) in palmar (left) and lateral (right) views; palmar views of (D) Au. africanus StW 68, (E) Au. africanus StW 64, and (F) proximal portion of SKX 3646 of an unidentified taxon from Swartkrans. Although the juvenile MH1 metacarpal is missing its epiphyseal head, the estimated age of the skeleton [12 to 13 human years (11)] suggests that its development is close to completion (metacarpal fusion in chimpanzees occurs at 9 to 10 years of age and in humans at 14 to 16 years of age; table S10). Compared to MH2, the MH1 Mc3 is only slightly shorter in length despite missing the epiphyseal head, and the proximal base and shaft are substantially more robust (table S10). The MH1 metacarpal robusticity is comparable to StW 68 and SKX 3646, whereas MH2 is similar to StW 64. All bones are to scale.

  5. Fig. 5

    Au. sediba MH2 wrist bones. Top, the Au. sediba MH2 articulated right wrist in (A) palmar (with the triquetrum’s pisiform facet outline in white), (B) dorsal, (C) lateral, and (D) proximal views. Middle, (E) the MH2 right scaphoid (UW 88-158) in (from left to right) medial, proximolateral, distal, and palmar views compared with (F) a cast of FLK NN-P of the OH 7 hand attributed to H. habilis (6, 7) and (G) a female modern human. MH2 represents the earliest appearance of a derived scaphoid in the hominin fossil record, with a trapezium-trapezoid facet (outlined in white) that extends onto the tubercle as in humans and a slightly reduced capitate facet. MH2 is distinctly unlike the African ape-like scaphoid of OH 7, which has trapezium facet restricted the scaphoid body and an expansive capitate facet. The OH 7 scaphoid tubercle is not preserved (missing portion is highlighted with a white bracket). Bottom, (H) the MH2 left capitate (UW 88-105, which is more complete than the right) in (from left to right) distal, lateral, proximal, medial, palmar, and dorsal views compared to distal (left) and dorsal (right) views of (I) P. troglodytes, (J) Au. africanus TM 1526 (with right capitate mirror-imaged), and (K) a female modern human. MH2 displays a primitive, more laterally facing Mc2 facet (white arrows) than the distally oriented facet of modern humans. However, Au. sediba is derived in having a palmar (in addition to the primitive dorsal) trapezoid facet (outlined in black on the lateral view; palmar is at the top), minimal “waisting” of the capitate body (black dotted line in dorsal views) like that of modern humans, and a relatively large scaphoid articulation as compared to that of the lunate (lunate facet outlined in proximal view). All bones are to scale.

  6. Fig. 6

    Relative length of the thumb in the Au. sediba MH2 hand. Shown is a box-and-whisker plot of the relative length of the thumb calculated as a ratio of the total length of the Mc1 and first proximal phalanx to the total length of the Mc3 and third proximal and intermediate phalanges within the same individual (bones highlighted in dark gray in outline of MH2 hand) in all taxa apart from Au. afarensis, for which that ratio is derived from multiple individuals from different sites (30, 38). MH2 has a relatively longer thumb than that of other hominins and falls outside the range of variation in modern humans (highlighted by shaded box). See table S14 for sample and methods.

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