The placement of the foramen magnum, the large hole on the cranium through which the spinal column passes, is directly related to the orientation of the cranium. Consider that a primate holds their mandible (or chin) parallel to the ground. In a quadruped, the spinal column also runs parallel to the ground so the foramen magnum is more dorsally placed (i.e., toward the back of the cranium). In a bidped, the spinal column runs perpendicular to the mandible and the ground. The foramen magnum is located more inferiorly (more on the bottom of the cranium). Australopiths have a more inferiorly placed foramen magnum8-10.


Comparision of the spinal cord direction
between a biped and a quadruped.

Comparison of the relative positions of the foramen magnum in modern humans and extant chimpanzee.