A case of valgus ankle in an early Pleistocene hominin

Bipedal locomotion is a defining character of the hominin lineage. A skeletal correlate of bipedality is a perpendicularly oriented tibia relative to the plane of the ankle joint, positioning the foot directly under the centre of mass. Non‐human primates, in contrast, possess a tibial shaft that tilts laterally away from the plane of the ankle joint (valgus ankle), which positions the foot in inversion and is adaptive for arboreal climbing. KNM‐ER 2596 is a small distal tibia from 1.9 mya sediments at Koobi Fora, Kenya. Though it possesses some morphologies functionally linked to bipedality, such as an expanded metaphysis, it also possesses a valgus tilt to the ankle. We test the competing hypotheses that the KNM‐ER 2596 tibia is from a cercopithecoid, a non‐human hominoid, or a pathological hominin. A survey of the orthopaedic literature and a comparative study of modern human and non‐human primate tibiae support the hypothesis that KNM‐ER 2596 is from a hominin. In order to investigate the non‐phylogenetic causes of valgus ankle, we examined human skeletal tibiae with valgus tilt secondary to fracture of the distal fibula. Untreated breaks of the lower third portion of the fibula during childhood can result in a valgus tilt to the distal tibia and occasionally other peculiar morphologies found in the KNM‐ER 2596 tibia such as a superoinferiorly atrophied medial malleolus. The morphology of this tibia is incompatible with hypotheses that it is from a cercopithecoid or a hominoid, and instead, we suggest that KNM‐ER 2596 belonged to a hominin that may have suffered a fracture of the lower left fibula as a juvenile. Copyright © 2010 John Wiley & Sons, Ltd.