Wednesday, April 18, 2007

Australopithecus With Gorilla-Like Jaw

Gorilla-like anatomy on Australopithecus afarensis mandibles suggests Au. afarensis link to robust australopiths. 2007. Y. Rak, A. Ginzburg, and E. Geffen. PNAS 104: 6568-6572.

Juvenile Australopithecus afarensis specimen, A. L. 333-43b (left), and Au. robustus specimen SK 34. Note the similarity in the size and shape of the coronoid process and the mandibular notch.
Abstract: Mandibular ramus morphology on a recently discovered specimen of Australopithecus afarensis closely matches that of gorillas. This finding was unexpected given that chimpanzees are the closest living relatives of humans.

Because modern humans, chimpanzees, orangutans, and many other primates share a ramal morphology that differs from that of gorillas, the gorilla anatomy must represent a unique condition, and its appearance in fossil hominins must represent an independently derived morphology. This particular morphology appears also in Australopithecus robustus.

The presence of the morphology in both the latter and Au. afarensis and its absence in modern humans cast doubt on the role of Au. afarensis as a modern human ancestor. The ramal anatomy of the earlier Ardipithecus ramidus is virtually that of a chimpanzee, corroborating the proposed phylogenetic scenario.