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Complete Fourth Metatarsal and Arches in the Foot of Australopithecus afarensis

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Science  11 Feb 2011:
Vol. 331, Issue 6018, pp. 750-753
DOI: 10.1126/science.1201463

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  1. Fig. 1

    AL 333-160 left fourth metatarsal in dorsal, lateral, medial, plantar, and proximal views.

  2. Fig. 2

    (A) Schematic representation of metatarsal proximal (rounded rectangles) and distal (ovals) ends seen in distal view in a human above and a chimpanzee below. In both species, the metatarsal heads are in contact with the substrate. Because of the arch, the proximal ends of the human metarsals are higher and situated in a transverse arch configuration (indicated by the dashed line). This results in axial torsion within the human fourth metatarsal of the head relative to the base, whereas this is not found in apes. [Modified from (23)] (B) Box plot of torsion values for chimpanzees, gorillas, humans (N = 10 individuals each), and AL 333-160, showing the distinct torsion in both hominins that is lacking in the apes. MT, metatarsal. Data are in table S1.

  3. Fig. 3

    (A) Box plots of angular relations of the proximal and distal metatarsal ends to the diaphysis in chimpanzees, gorillas, humans, and AL 333-160. The proximal ends of hominin metatarsals are angled plantarly relative to the diaphysis, reflecting the average 8° of inclination of the metatarsal in normal arched posture, whereas that of the apes is oriented slightly dorsally. The flattened plantar portion of the hominin distal articular surface is inclined distally, also reflecting this posture and the habitual extension at this joint during bipedal locomotion, something also not seen in the apes who have this surface oriented directly plantarly. This distal plantar surface is also more distally oriented relative to the base in both hominins. In every case, AL 333-160 resembles humans only, strongly supporting the presence of arches in the A. afarensis foot. (B) Left fourth metatarsals of human, AL 333-160, chimpanzee, and gorilla in medial view, showing the orientation of bone ends and diaphysis. The blue arrows indicate the domed portion of the head. AL 333-160 resembles humans in having the doming along the dorsal articular margin, whereas the distal articular surface is domed more plantarly in the apes.

  4. Fig. 4

    (A) Proximal ends of left fourth metatarsals in medial view, showing the dorsoplantar contour of the distal end. The box plot shows measured curvature, measured as maximum distance of the proximal joint surface from a line drawn between dorsal and plantar articular margins, expressed as a ratio to dorsoplantar length, following (11). Data are from (11). All hominins have relatively flat surfaces, rather than the convex profile of apes. (B) Proximal view of left fourth metatarsals, showing the dorsoplantarly expanded articular surface in hominins as compared with apes. The box plot of the ratio of dorsoplantar to mediolateral breadth shows the almost square proportion of apes, but the deep shape of the hominins. Data are in table S2. (C) Above, dorsal view of left fourth metatarsals, showing the articular facet for contact with the third metatarsal (vertical line) and the oblique articular facet for contact with the ectocuneiform in the hominins. Below, dorsal view of articulated cuboid, lateral, and medial cuneiforms and lateral metatarsals, showing the articular configuration of the lateral cuneiform with the third and fourth metatarsals. In apes, the cuneiform is directly medial to the cuboid and does not contact the fourth metatarsal. Both hominins have lateral cuneiform contact and an obliquely oriented facet on the fourth metatarsal for the cuneiform.