"Given the very tiny size of the Ardi skull, the similarity of its cranial base to a human's is astonishing," says Kimbel.
The cranial base is a valuable resource for studying phylogenetic, or natural evolutionary relationships, because its anatomical complexity and association with the brain, posture, and chewing system have provided numerous opportunities for adaptive evolution over time. The human cranial base, accordingly, differs profoundly from that of apes and other primates.
In humans, the structures marking the articulation of the spine with the skull are more forwardly located than in apes, the base is shorter from front to back, and the openings on each side for passage of blood vessels and nerves are more widely separated.
These shape differences affect the way the bones are arranged on the skull base such that it is fairly easy to tell apart even isolated fragments of ape and human basicrania.
Ardi's cranial base shows the distinguishing features that separate humans and Australopithecus from the apes. Kimbel's earlier research (with collaborator Rak) had shown that these human peculiarities were present in the earliest known Australopithecus skulls by 3.4 million years ago.
The new work expands the catalogue of anatomical similarities linking humans, Australopithecus, and Ardipithecus on the tree of life and shows that the human cranial base pattern is at least a million years older than Lucy's species, A. afarensis.
Paleoanthropologists generally fall into one of two camps on the cause of evolutionary changes in the human cranial base. Was it the adoption of upright pos
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Arizona State University