CT study of early humans reveals evolutionary relationships

A CT scan study of the skulls of extinct human species reveals hidden patterns in the internal anatomy of the face and helps settle a long-standing debate over the relationships of our early ancestors. The study, published in the Proceedings of the National Academy of Sciences, shows that beneath the bony exterior of the Australopithecus face - that for decades has confounded researchers' attempts to sort species relationships - details of internal anatomy reveal that the highly specialized "robust" australopith species from southern and eastern Africa very likely shared a common ancestor after all.

According to paleoanthropologists Brian Villmoare, from the George Washington University and University College London, and William Kimbel, of the Institute of Human Origins at Arizona State University, the two "robust" species of Australopithecus, A. robustus from southern Africa and A. boisei from eastern Africa, which flourished between 2.0 and 1.4 million years ago, have unusual adaptations in their teeth and faces for powerful chewing. However, scientists have disagreed for half a century over whether or not the two species were closely related to one another because A. robustus also shares features with another species from southern Africa, the so-called gracile form A. africanus. A. africanus is linked to A. robustus by a distinctive column of bone that runs along the sides of the nasal opening and extends down to the upper jaw. The eastern "robust" species A. boisei lacks the external pillar but otherwise shares a number of facial and dental features with A. robustus.

"This was a classic evolutionary puzzle," says Villmoare. "These two 'robust' species either independently converged on the heavy chewing adaptation, or the South African species both converged on the anterior pillar, but either way, evolutionary convergence was clouding our ability to accurately reconstruct their relationships."

A detailed analysis of CT scans of the faces of five early human species showed that while externally the South African species shared the anterior pillar, internally their pillars are structurally distinct. In A. robustus the pillar is composed of spongy trabecular bone, whereas in A. africanus the pillar is a hollow tube. Although an external pillar is not usually present in A. boisei, this species shares with A. robustus the dense internal structure around the nasal opening. This means that the external pillar - the most important trait linking the two South African species - may not be a genuine sign of common ancestry.

According to Villmoare, determining the patterns of shared anatomy for the anterior pillar may finally resolve the issue of ancestry for A. africanus, A. robustus, and A. boisei. "The detailed similarities in the internal anatomy of the face strongly supports the hypothesis that there was a single evolutionary branch of 'robust australopithecines' rather than separate eastern and southern lineages. The external similarity of the anterior pillar in both South African species may be an example of convergence on a similar dietary niche, but does not seem to indicate shared ancestry."

The article, entitled "CT-based study of internal structure of the anterior pillar in extinct hominins and its implications for the phylogeny of robust Australopithecus" will be published in the September 19 issue of Proceedings of the National Academy of Sciences.