Anatomical Adaptations for Bipedalism: Feet & Toes

The calcaneus is comparatively more robust in a biped than in a quadruped. In addition, the hallux sits parallel to the rest of the toes in a biped, but is more divergent in extant chimpanzees and gorillas.

The calcaneus is comparatively more robust in a biped than in a quadruped. In addition, the hallux sits parallel to the rest of the toes in a biped, but is more divergent in extant chimpanzees and gorillas.

The rounded articular surface of the medial cuneiform in quadrupeds permits a wide range of movement abduction of the hallux. In bipeds, the hallux sits parallel to the rest of the toes allowing for greater push-off during walking.

Humans have the most distinctive feet of all the higher primates. Since only the hindlimbs (or lower limbs) are used for propulsion, the body's entire torso weight (all of the forces generated by running, walking, and jumping) pass through only one foot at a time as the biped moves between the from swing and stance phases of locomotion. This has important consequences for the anatomy of the foot. Various elements of the foot must be robust enough to accommodate these forces, and at the same time, provide efficient toe-initiated push-off for propulsion. For example, compared to other primates the hallux (or first toe) in bipeds is much larger and more robust than the other four toes.

The calcaneus, or heel bone, is the first bone of the foot to make contact with the ground during the stride cycle. Its robust size helps to provide stability and absorb the high forces encountered during the heel strike in bipedalism. The calcaneus, especially the posterior portion (calcaneal tuberosity), is much larger and more robust in humans than in chimpanzees. The calcaneus is also large and robust in fossil hominins. The shape of the calcaneus provides for the attachment of strong ligaments that run from the arch of the foot to the tibia. These ligaments add support, creating a double arch system that helps to absorb the forces as the foot hits the ground.

The metatarsals are long thin bones in the middle of the foot between the tarsals (on the distal side) and the phalanges or toe bones (on the proximal side). The medial cuneiform is the small tarsal bone that articulates with the hallux and the navicular bone. Apes have a rounded metatarsal I articular surface on the medial cuneiform which permits a wide range of abduction. Humans have a flattened metatarsal I articular surface that restricts the hallux to an adducted position (i.e., the toe is pulled toward the body and lies parallel to the other metatarsals. Evidence from the Laetoli Tracks in Tanzania, where footprints from several australopithecines were preserved in volcanic ash sediments, indicate that Australopithecus had a hallux that was not fully adducted, but was not as divergent at seen in other apes.

In bipeds, the first metatarsal is relatively larger than the other metatarsals and is fully adducted, commonly referred to as a non-opposable big toe. In general, human toes are shorter in relative length than in other primates, and comparatively, humans have almost no grasping ability in their toes and feet. However, walking with longer toes and a divergent hallux bipedally would be energetically costly and impede efficient bipedalism, so toe length is an adaptation for habitual bipedalism^9,19.