Berger and his collaborators based their conclusions on A. sediba's distinctive amalgam of primitive and derived traits. Features such as its small brain, small body, and very long arms link the creature to the australopithecines, especially A. africanus, whom A. sediba resembles in details of the face and teeth. Yet the new species also exhibits a number of characteristics seen only in Homo, including its flatter face, robust pelvis and long, striding legs. Similarities to H. erectus in particular, according to the team, are evident in details of the skull and pelvis.
Paleontologists not involved in the new work agree that the two South African skeletons represent a major discovery. But they are very much divided on the issue of where this new hominin belongs on the family tree. "It is truly an incredible find," comments Meave Leakey of the National Museums of Kenya, who recently traveled to South Africa and looked at the A. sediba remains. "I found it was hard to believe what we saw in view of the quantity and quality of the fossils. However, I do not think that they are ancestral to Homo or have anything to do with Homo." Rather, she says, "these fossils reinforce my view that the australopithecines in South Africa underwent a separate radiation that had little to do with East African species that have been called Australopithecus, other than that they share a common ancestor." In 2001 Leakey and her colleagues announced their discovery of a hominin they called Kenyanthropus platyops, and suggested that it could be another possible ancestor of Homo.
William Jungers of Stony Brook University concurs with the discovery team's interpretation of the new skeletons as a new species of Australopithecus that is probably descended from A. africanus. But he disputes the ties to Homo. "The proposed link between A. sediba and early Homo is forced and tenuous at best," he asserts, noting that the alleged postcranial similarities between the two groups are not very compelling.
William Kimbel, director of the Institute of Human Origins at Arizona State University, has a different take. Considering the advanced features of the face and pelvis, the new fossils "probably belong in the Homo genus." They do not illuminate its origin, however. Kimbel points out that a site in Hadar, Ethiopia, where he works, has yielded a Homo specimen that, at around 2.3 million years old, predates the A. sediba fossils by hundreds of thousands of years. For their part, Berger and his co-authors contend that the Malapa fossils represent neither the first nor the last instances of A. sediba, and that they could be a late-surviving population of a species that originated rather earlier. "That's a suggestion that further finds would need to clarify," Kimbel counters.