Three‐Dimensional Trunk Kinematics of Humans and Chimpanzees: New Insights on Early Hominin Bipedalism

The human trunk is involved in a variety of locomotor tasks that promote stability and efficiency during locomotion. The trunk and upper body must create a stable base for the head, yet must also move enough to cancel out angular momentum of the swinging lower limbs and to help shift the center of mass (CoM) laterally towards the stance‐side foot. These abilities are likely achieved in part by the derived morphology of the human trunk. Humans possess a tall and narrow trunk compared to the more squat trunk morphology of our closest living relatives (great apes). Because of some similarities between early hominins and chimpanzees in trunk morphology (thoracic shape in Australopithecus ) and lower limb features that may affect trunk motion (potentially wide step widths in Ardipithecus ), the role of the upper body during locomotion in our early ancestors remains unclear. Here, we investigated three‐dimensional trunk motion in chimpanzees and humans in order to determine the effects of overall trunk morphology on upper body stability during bipedal locomotion. The trunk was divided into three anatomical segments (thorax, lumbar region, and pelvis). In the transverse plane, all chimpanzee trunk segments rotated in the same direction at the same time (in phase), whereas in humans, the thorax rotated out of phase with the pelvis. However, relative to the pelvis, both humans and chimpanzees achieved the same amount of thoracic (17.1° and 19.1° respectively, n.s. ) and lumbar rotations (6.1° and 6.5°, n.s. ). In the frontal plane, chimpanzees laterally flexed (listed) all three trunk segments (13–22°) towards the support side limb, whereas in humans only the thorax listed (3–4°) towards the support side. The greater thoracic listing in chimpanzees may be necessary to compensate for their wider step width, absent a valgus knee. Indeed, in chimpanzees, the angular excursion of the thorax (beyond that which is due to list of the pelvis alone) is correlated with increased lateral displacement of the CoM ( P <0.01), which is in turn correlated with increased mediolateral mechanical work ( P <0.001). The results of this research suggest that some important aspects of bipedal locomotion were already present in the trunks of the last common ancestor (LCA) of humans and chimpanzees. Regardless of the precise reconstruction of the trunk, the LCA was likely already capable of using its thorax to compensate for some degree of rotation of the pelvis. However, if early hominins utilized wide step widths (as has been suggested for Ardipithecus ramidus ), there would have likely been more active listing of the thorax in order to shift the CoM over the support side foot. This, in turn, would have likely been accompanied by an increase in mediolateral mechanical work. Support or Funding Information This research was funded by the NSF BSC 0935321 and The Leakey Foundation.