Australopithecus afarensis and the Evolution of the Human Cranial Base

Differences in ape and human cranial base form are usually attributed to a combination of an enlarged brain, retracted face and upright locomotion in humans. Relative to the apes, the human foramen magnum is anteriorly inclined and, with the occipital condyles, is forwardly located on a broad, short and flexed base; the petrous bones are coronally rotated; the glenoid region is topographically complex; the nuchal lines low; and the nuchal plane horizontal. The proximate causes of these innovations are accessible through the fossil record, the only data source on the temporal pattern of trait evolution in the human lineage. Australopithecus afarensis (3.7‐3.0 Ma) is the earliest hominin species in which the cranial base can be assessed comprehensively. It shows that in small‐brained, bipedal hominins the foramen magnum and occipital condyles are anterior, as in humans, but without the foramen's forward inclination. In some skulls this is associated with a short, broad base and an inferiorly rotated occipital – novelties shared with later australopiths and humans. In others, including a recently recovered female, a more primitive morphology is present: while the foramen and condyles reside anteriorly on a short base, the nuchal lines are high, the nuchal plane is steep, and the base is relatively narrow. We explore implications for sexual dimorphism and functional integration in the hominin cranial base.