Abstract:
Functional morphology of the atlas reflects multiple aspects of an organism’s biology. More specifically,
its shape indicates patterns of head mobility, while the size of its vascular foramina reflects blood flow
to the brain. Anatomy and function of the early hominin atlas, and thus, its evolutionary history, are
poorly documented because of a paucity of fossilized material. Meticulous excavation, cleaning and
high-resolution micro-CT scanning of the StW 573 (‘Little Foot’) skull has revealed the most complete
early hominin atlas yet found, having been cemented by breccia in its displaced and flipped over
position on the cranial base anterolateral to the foramen magnum. Description and landmark-free
morphometric analyses of the StW 573 atlas, along with other less complete hominin atlases from
Sterkfontein (StW 679) and Hadar (AL 333-83), confirm the presence of an arboreal component in
the positional repertoire of Australopithecus. Finally, assessment of the cross-sectional areas of the
transverse foramina of the atlas and the left carotid canal in StW 573 further suggests there may have
been lower metabolic costs for cerebral tissues in this hominin than have been attributed to extant
humans and may support the idea that blood perfusion of these tissues increased over the course of
hominin evolution.
