Anatomical Adaptations for Bipedalism: Lumbar Vertebrae

Natural placement of the center of gravity in modern humans and extant chimpanzees.

Natural placement of the center of gravity in modern humans and extant chimpanzees.

The lumbar curvature (shown in the red box) allows the hips and trunk to swivel forward while walking.

Maintaining balance is one of the most critical issues that must be solved in order to walk on two legs. A biped must balance on one leg while lifting the other foot off the ground and swinging it forward during the walking cycle. In most quadrupedal hominins, the center of gravity is located around the center of the torso. In a modern human, the center of gravity is approximately at the center of the pelvis. As the legs alternate swinging forward during the walking cycle, the center of gravity shifts from one side of the pelvis to the other, making a pattern similar to the figure "8". The lumbar curvature helps to bring the center of gravity closer to the midline and above the feet.

The number and size of the lumbar vertebrae in humans is different than in apes. Humans usually have five comparatively larger lumbar vertebrae. Most large apes typically have four lumbar vertebrae that are relatively smaller than human lumbar vertebrae. The greater number and size of the vertebrae forms a more flexible lower back that permits the hips and trunk to swivel forward when walking. Because the ape lower back is less flexible, the hips must shift a greater distance forward with each step when an ape walks bipedally.

The Australopithecus sacrum is broad, similar to modern human. Australopithecus lumbar vertebral bodies were broad for effective weight transmission from the upper body to the pelvis. Australopithecines had five or six lumbar vertebrae that articulated to form a distinctive lumbar curvature, similar to the morphology of modern humans^8,9,15.