Three-dimensional morphometrics of the proximal metatarsal articular surfaces of Gorilla, Pan, Hylobates, and shod and unshod humans

There is debate about how fossil hominin pedal morphology relates to terrestrial habits. Were early hominins adapted to a bipedal lifestyle with a significant arboreal component, or were they more dedicated to a terrestrial lifestyle? The proximal articular surfaces of the metatarsals (MT) are examined in Gorilla, Pan, Hylobates, and habitually shod and unshod Homo using three-dimensional morphometrics. The results for MT 1 show three trends. OH 8 (Homo habilis) is indistinguishable from humans, specimens SKX 5017 and SK 1813 (Paranthropus robustus) are apelike, and all other fossil 1 metatarsals are intermediate in shape between humans and apes. The MT 2 and MT 3 analyses show that humans have a narrower surface that is expanded in the plantar aspect relative to apes. These features increase joint stability for the human longitudinal arch. The MT 2 fossils for Stw 573d (Little Foot) and OH 8 are humanlike. The MT 2 specimen of SKX 247 (possibly Paranthropus) is apelike, while all other MT 2 fossils are intermediate between humans and apes. In the MT 3 analysis, Stw 387, Stw 496, Stw 388, and OH 8 metatarsals are humanlike in shape, while Stw 435 and Stw 477 are intermediate between humans and apes. The MT 3 surface of Hylobates is markedly convex, suggesting that the midfoot break in gibbons extends to include this joint in addition to the MT 4 and MT 5 tarsometatarsal joints. The results of the MT 4 analysis show a highly convex surface in apes, with Hylobates extending further to the dorsal aspect of this metatarsal, with a greater range of motion at the midfoot break compared to the African apes. The MT 4 specimens of OH 8 and Stw 628 show greater morphological affiliation with humans. The MT 5 analysis shows that Pan and Hylobates have a medio-laterally extended and concave articular surface that is convex in the dorso-plantar plane. The two human groups are narrower and flatter in the medio-lateral plane, with a little dorso-plantar convexity. There is overlap in shape patterns between groups in the MT 5 analysis.