Bipedalism in Homo ergaster: An Experimental Study of the Effects of Tibial Proportions on Locomotor Biomechanics

Abstract

A full understanding of the evolution of bipedalism requires consideration of the effects of body form on locomotor biomechanics. This is particularly important given the great range of body proportions found in fossil hominids. For example, although Homo ergaster had an essentially modern body form, it differed from most modern humans in having very long limbs, and particularly long distal limb elements, for its size. This study experimentally investigated the effects of crural index (tibia length relative to femur length) on the kinematics and kinetics of normal human walking. We hypothesized that a relatively longer tibia would lead to higher bending moments along the leg. We found, however, that modern humans with higher crural indices employ a series of postural adjustments that keep bending moments at comparable levels. An examination of the relevant fossil material suggests that early hominids with long tibiae, particularly Homo ergaster, likewise did not experience particularly high bending moments along the leg during locomotion. This implies that early hominids may have modified their locomotion behaviorally to moderate the bending stresses incurred during walking. Although an essentially modern human body form had evolved by the early Pleistocene, the form of walking that characterizes most modern humans may not have evolved until significantly later.