Three-dimensional reconstructions ofHomo erectus,Homo sapiensand a Neanderthal suggest a recent evolutionary origin for the comparatively shallow modern human thorax. The tall and narrow body shape of anatomically modern humans (Homo sapiens) evolved via changes in the thorax, pelvis and limbs. It is debated, however, whether these modifications first evolved together in AfricanHomo erectus, or whetherH. erectushad a more primitive body shape that was distinct from both the more ape-likeAustralopithecusspecies andH. sapiens. Here we present the first quantitative three-dimensional reconstruction of the thorax of the juvenileH. erectusskeleton, KNM-WT 15000, from Nariokotome, Kenya, along with its estimated adult rib cage, for comparison withH. sapiensand the Kebara 2 Neanderthal. Our three-dimensional reconstruction demonstrates a short, mediolaterally wide and anteroposteriorly deep thorax in KNM-WT 15000 that differs considerably from the much shallower thorax ofH. sapiens, pointing to a recent evolutionary origin of fully modern human body shape. The large respiratory capacity of KNM-WT 15000 is compatible with the relatively stocky, more primitive, body shape ofH. erectus.