Proportional limb strengths signal an adaptive shift in arboreality in early human evolution.

Abstract

How much arboreality characterized the <i>Australopithecus</i> locomotor repertoire and how this differed from early <i>Homo</i> are vigorously debated. 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 <i>Australopithecus</i> and early <i>Homo</i> by adding two individuals [StW 573, ∼3.67 million years ago (Ma); and D3901/D4167/D4507, ∼1.77 to 1.81 Ma]. We show that <i>Australopithecus</i> individuals of varying body sizes spanning at least 0.5 Ma exhibit African ape-like interlimb strength proportions, indicating frequent arboreal behavior, and that the <i>Australopithecus</i> lower limb exhibits human-like intralimb strength proportions. In contrast, early <i>Homo</i> individuals are human-like in both strength proportions, reflecting fundamentally different selective pressures favoring terrestrial bipedalism and an unambiguous departure from arboreality by ∼1.8 Ma.