Most previous studies rely on traits of uncertain functional significance or are constrained by sampling concerns. Bone shaft strength is plastic and reflects in vivo mechanical loading. Because interlimb bone strength proportions among living apes correlate with arboreality, they are ideal contributors to this debate. We present the most comprehensive analysis yet of relative limb shaft strengths in Australopithecus and early Homo by adding two individuals [StW 573, ∼3.67 million years ago (Ma); and D3901/D4167/D4507, ∼1.77 to 1.81 Ma].