Evolution and function of the hominin forefoot.

The primate foot functions as a grasping organ. As such, its bones, soft tissues, and joints evolved to maximize power and stability in a variety of grasping configurations. Humans are the obvious exception to this primate pattern, with feet that evolved to support the unique biomechanical demands of bipedal locomotion. Of key functional importance to bipedalism is the morphology of the joints at the forefoot, known as the metatarsophalangeal joints (MTPJs), but a comprehensive analysis of hominin MTPJ morphology is currently lacking. Here we present the results of a multivariate shape and Bayesian phylogenetic comparative analyses of metatarsals (MTs) from a broad selection of anthropoid primates (including fossil apes and stem catarrhines) and most of the early hominin pedal fossil record, including the oldest hominin for which good pedal remains exist, Ardipithecus ramidus Results corroborate the importance of specific bony morphologies such as dorsal MT head expansion and "doming" to the evolution of terrestrial bipedalism in hominins. Further, our evolutionary models reveal that the MT1 of Ar. ramidus shifts away from the reconstructed optimum of our last common ancestor with apes, but not necessarily in the direction of modern humans. However, the lateral rays of Ar. ramidus are transformed in a more human-like direction, suggesting that they were the digits first recruited by hominins into the primary role of terrestrial propulsion. This pattern of evolutionary change is seen consistently throughout the evolution of the foot, highlighting the mosaic nature of pedal evolution and the emergence of a derived, modern hallux relatively late in human evolution.

Proximal metatarsal articular surface shape and the evolution of a rigid lateral foot in hominins.

This study quantifies the proximal articular surface shape of metatarsal (MT) 4 and MT 5 using three-dimensional morphometrics. Humans and apes are compared to test whether they have significantly different shapes that are skeletal correlates to comparative lateral foot function. In addition, shod and unshod humans are compared to test for significant differences in surface shape. The MT 4 fossils OH 8, Stw 628, and AL 333-160, and the MT 5 fossils AL 333-13, AL 333-78, OH 8, and Stw 114/115 are compared with humans and apes to assess whether they bear greater similarities to humans, which would imply a relatively stable lateral foot, or to apes, which would imply a flexible foot with a midfoot break. Apes have a convex curved MT 4 surface, and humans have a flat surface. The MT 4 fossils show greater similarity to unshod humans, suggesting a stable lateral foot. Unshod humans have a relatively flatter MT 4 surface compared with shod humans. There is much overlap in MT 5 shape between humans and apes, with more similarity between humans and Gorilla. The fossil MT 5 surfaces are generally flat, most similar to humans and Gorilla. Because of the high degree of shape overlap between humans and apes, one must use caution in interpreting lateral foot function from the proximal MT 5 surface alone.

A complete second metatarsal (StW 89) from Sterkfontein Member 4, South Africa.

The functional anatomy of the hominin foot has played a crucial role in studies of locomotor evolution in human ancestors and extinct relatives. However, foot fossils are rare, often isolated, and fragmentary. Here, we describe a complete hominin second metatarsal (StW 89) from the 2.0-2.6 million year old deposits of Member 4, Sterkfontein Cave, South Africa. Like many other fossil foot bones, it displays a mosaic of derived human-like features and primitive ape-like features. StW 89 possesses a domed metatarsal head with a prominent sulcus, indicating dorsiflexion at the metatarsophalangeal joint during bipedal walking. However, while the range of motion at the metatarsophalangeal joint is human-like in dorsiflexion, it is ape-like in plantarflexion. Furthermore, StW 89 possesses internal torsion of the head, an anatomy decidedly unlike that found in humans today. Unlike other hominin second metatarsals, StW 89 has a dorsoplantarly gracile base, perhaps suggesting more midfoot laxity. In these latter two anatomies, the StW 89 second metatarsal is quite similar to the recently described second metatarsal of the partial foot from Burtele, Ethiopia. We interpret this combination of anatomies as evidence for a low medial longitudinal arch in a foot engaged in both bipedal locomotion, but also some degree of pedal, and perhaps even hallucal, grasping. Additional fossil evidence will be required to determine if differences between this bone and other second metatarsals from Sterkfontein reflect normal variation in an evolving lineage, or taxonomic diversity.

Brief Communication: Shape analysis of the MT 1 proximal articular surface in fossil hominins and shod and unshod Homo.

As a follow-up study to Proctor et al. (Am J Phys Anthropol 135 (2008) 216-224), this study quantifies the first metatarsal proximal articular surface using three-dimensional morphometrics to test for differences in articular surface shape between habitually shod and habitually unshod humans. In addition, differences in shape between Homo, Pan, Gorilla, and Hylobates are compared to the fossil hominin specimens A. L. 333-54, Stw 562, Stw 573 ("Little Foot"), OH 8, SKX 5017, and SK 1813. No difference in surface shape was found between habitually shod and habitually unshod humans. There is a clear quantitative division in articular surface shape between humans and apes that is more pronounced than a previous study by Proctor et al. (Am J Phys Anthropol 135 (2008) 216-224), due to additional landmarks present in this study. The specimen OH 8 is indistinguishable from modern Homo. The fossils A. L. 333-54, Stw 562, and Stw 573 are intermediate in shape between humans and apes. The specimens SKX 5017 and SK 1813 have a more apelike articular surface. When combined with other characteristics, this trait suggests that Paranthropus used a degree of abduction during locomotion that was much less than that in extant apes, but greater than that in Australopithecus, allowing for some small degree of grasping ability.

Quantitative three-dimensional shape analysis of the proximal hallucial metatarsal articular surface in Homo, Pan, Gorilla, and Hylobates.

Multidimensional morphometrics is used to compare the proximal articular surface of the first metatarsal between Homo, Pan, Gorilla, Hylobates, and the hominin fossils A.L. 333-54 (A. afarensis), SKX 5017 (P. robustus), and OH 8 (H. habilis). Statistically significant differences in articular surface morphology exist between H. sapiens and the apes, and between ape groups. Ape groups are characterized by greater surface depth, an obliquely curved articular surface through the dorso-lateral and medio-plantar regions, and a wider medio-lateral surface relative to the dorso-plantar height. The OH 8 articular surface is indistinguishable from H. sapiens, while A.L. 333-54 and SKX 5017 more closely resemble the apes. P. robustus and A. afarensis exhibit ape-like oblique curvature of the articular surface.