Evolution and function of the hominin forefoot

Abstract

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, 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 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 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.

The evolution of the human foot

“…The stiffening of the lateral midfoot allowed early hominins to push off the outside of the foot while striding bipedally without sacrificing the grasping ability of the medial forefoot. We therefore support previous suggestions about the mosaic nature of hominin foot evolution proposing a “lateral first” timing on the evolution of the foot . The medial foot in these Pliocene hominins bears a striking resemblance to that found in modern apes, still retaining an inverted set to the ankle joint (based on the Ardipithecus talus), and a divergent, grasping hallux.…”

“…We therefore support previous suggestions about the mosaic nature of hominin foot evolution proposing a "lateral first" timing on the evolution of the foot. 15,16,70,71 The medial foot in these Pliocene hominins bears a striking resemblance to that found in modern apes, still retaining an inverted set to the ankle joint (based on the Ardipithecus talus), and a divergent, grasping hallux.…”

“…Despite the limitations of the fossil record, some patterns are emerging in our attempt to understand the initial pedal adaptations to bipedalism that evolved soon after the hominin‐panin divergence. The very first adaptive modifications for bipedalism appear to happen along the lateral column of the foot . It is likely that these early changes to the hominin foot rendered the lateral column stiffer for bipedal propulsion, while the medial foot retained mobility critical for climbing.…”

“…The very first adaptive modifications for bipedalism appear to happen along the lateral column of the foot. 15,16,[70][71][72] It is likely that these early changes to the hominin foot rendered the lateral column stiffer for bipedal propulsion, while the medial foot retained mobility critical for climbing. These generalizations are based on quite limited evidence and are subject to change as new fossils are unearthed.…”

Effect of the upward curvature of toe springs on walking biomechanics in humans

“…It is well established that the ability to dorsiflex the toes relative to the rest of the foot at the metatarsophalangeal (MTP) joints is one of the key evolved features that enable humans to walk and run bipedally effectively and efficiently. In addition to having shorter, straighter phalanges, human metatarsal heads are characterized by more dorsally oriented and mediolaterally broad articular surfaces compared to those of our closest relatives, the African apes 5 . During the propulsive phase of walking, the dorsally oriented metatarsal heads in the human forefoot are thought to increase the range of dorsiflexion motion at the MTP joints by providing more dorsal articular surface area on which the proximal phalangeal base can slide 6 – 10 .…”