foramen magnum

Ardipithecus ramidus comes from the period around 4.4 million years ago, and has so far been found at several field localities in Ethiopia. It lived shortly after the time that genetic evidence suggests humans share a common ancestor with chipmanzees and bonobos. Many scientists believe that Ardipithecus is on the human lineage, a hominin. Others disagree, suggesting it may be related to gorillas, chimpanzees, or an extinct lineage of apes.

The most complete specimen of Ardipithecus is a skeleton from Aramis, in the Middle Awash field region of Ethiopia. The skeleton has grasping feet with opposable big toes, very long fingers and toes, and arms and legs approximately the same length. The anatomy of the skeleton is roughly like a quadruped, with arms and legs resembling monkeys in proportions rather than the living great apes. But the skeleton's pelvis suggests some changes that may reflect an ability to maintain an upright posture. The anatomy has given rise to a debate about what early hominins may have looked like.

Another part of the anatomy that may reflect posture is the base of the cranium. A well-preserved temporal bone of Ardipithecus allows us to examine the length of its cranial base. You'll be comparing this anatomy (illustrated in the picture below) with some casts of fossil hominins, living humans and living great apes.

Ardipithecus cranial base reconstruction. The temporal bone was mirrored using digital techniques, and the two were aligned by positioning the small semicircular canals of the inner ears (shown in the bottom frame).

The petrous portion of the temporal bone points medially and anteriorly (toward the midline and front) on the cranial base. The hole in the base of the skull is called the foramen magnum. The foramen magnum admits the spinal cord to the brain, so its position reflects the posture of the cervical spine. A foramen magnum that is positioned toward the rear of the skull should reflect a more quadrupedal habitual posture. A position toward the front, with a short cranial base separating the foramen magnum from the palate, should reflect a more vertical habitual posture. When this part of the cranial base is long, the petrous portions of the temporal bones angle forward more strongly; a short cranial base corresponds to a more medial angle of these petrous portions.

We may expect the cranial base to reflect posture in this way, but does it? Examine the species at this station. Can you distinguish the bipeds from the quadrupeds by using the cranial base? What about Ardipithecus: Where does it fit relative to these other species?