Gorilla-like anatomy on Australopithecus afarensis mandibles suggests Au. afarensis link to robust australopiths

doi:10.1073/pnas.0606454104

Gorilla-like anatomy on Australopithecus afarensis mandibles suggests Au. afarensis link to robust australopiths

*Department of Anatomy and Anthropology, Sackler Faculty of Medicine, and
^‡Department of Zoology, Faculty of Life Sciences, Tel Aviv University, Tel Aviv 69978, Israel

Edited by David Pilbeam, Harvard University, Cambridge, MA, and approved February 26, 2007 (received for review July 28, 2006)

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.

Footnotes

^†To whom correspondence should be addressed. E-mail: yoelrak{at}post.tau.ac.il
Author contributions: Y.R. designed research; Y.R. and A.G. performed research; Y.R. and E.G. analyzed data; and Y.R. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/cgi/content/full/0606454104/DC1.
↵ § Although the height of the gorilla coronoid can be considerable, it is the constellation of the noted features (the width of the base of the coronoid process, the flatness of its superior contour, and the location of the highest point of the process and the lowest point in the notch relative to the width of the ramus) that determines the shape of the superior ramal contour in our comparative primate sample and differentiates the two morphologies.
↵ ¶ Kimbel, W., Rak, Y., Johnson, D. (2003) Am J Phys Anthropol Suppl 36:129 (abstr.).
↵ ‖ A much less likely scenario is that the Au. afarensis ramal morphology was apomorphic (newly derived) for all hominins and many other primates and that a reversal to the chimpanzee-like condition occurred in the Homo clade.
↵ ** In light of the debate about which modern chimpanzee species is more representative of the common ancestor (“prototype”) of modern humans and chimpanzees (25, 26), we note that the posterior probability for the Ar. ramidus mandibular ramus in our analysis is highest for common chimpanzees (twice as high as for pygmy chimpanzees).