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See Inside June 2010

Fossils Of Our Family

A new human species is identified, but does it belong on the line leading to Homo sapiens?

Fossils of a human species new to science could be the direct ancestor of our genus, Homo. Discovered in Malapa Cave, located some 40 kilometers outside of Johannesburg, South Africa, the finds comprise two partial skeletons that are nearly 1.95 million years old. The researchers have named them Australopithecus sediba.

The pair—an adult female and juvenile male that may have been mother and son—appear to have fallen through a hole in the cave ceiling while possibly attempting to access a pool of water inside. So exceptional is the preservation of the skeletons that the discovery is being likened to the famous Lucy fossil from Ethiopia. But the startling mix of primitive and advanced traits evident in the remains is sparking debate over where the new species belongs on the family tree.

Considering the virtually nonexistent fossil trails of our cousins the chimps, bonobos and gorillas, the human fossil record is extraordinary. There are, however, significant gaps in researchers’ knowledge of how we came to be. One such blind spot is the origin of Homo. Most experts agree that our genus evolved from a species of Australopithecus—either A. afarensis (Lucy’s species) or A. africanus. Connecting the dots between one of these australopithecine species and Homo has been difficult, how­ever, because the oldest known Homo remains are so few and fragmentary.

Scientists have supposed that Homo habilis, which lived between 2.3 million and 1.5 million years ago, signaled the debut of Homo and subsequently gave rise to H. erectus, the first hominin (member of the human lineage) to spread out from Africa across the globe and the one thought to have spawned later human species, including H. sapiens.

Enter A. sediba. In a paper describing the fossil remains in the April 9 Science, Lee Berger of the University of the Witwatersrand in Johannesburg and his colleagues suggest that A. africanus gave rise to A. sediba, which in turn gave rise to Homo. Intriguingly, the team hints that A. sediba might even be more closely related to H. erectus than H. habilis is, thus potentially relegating H. habilis to a side branch of the family tree, rather than a coveted spot on the line leading to us.

Berger and his collaborators based their conclusions on A. sediba’s distinctive amalgam of primitive and advanced traits. Features such as small brain size, slight build and very long arms link the creature to the australopithecines, especially A. africanus. Yet the new species also exhibits a number of characteristics seen only in Homo, including its flatter face, robust pelvis and long, striding legs.

Other paleontologists have praised the discovery but are divided on the issue of how to classify the new hominin. “The proposed link between A. sediba and early Homo is forced and tenuous at best,” asserts William L. Jungers of Stony Brook University, noting that alleged skeletal similarities are not very compelling.

A different take comes from William H. Kimbel, director of the Institute of Human Origins at Arizona State University, who argues on the basis of the advanced features of the face and pelvis that the new fossils “probably belong in the Homo genus.” They do not illuminate its origin, however. Kimbel points out that a site where he works in Hadar, Ethiopia, has yielded a Homo specimen that predates the A. sediba fossils by hundreds of thousands of years.

Exactly how the skeletons relate to known Homo species is difficult to establish given the paucity of early Homo remains, observes paleoanthropologist Susan C. Antón of New York University. She suspects that A. sediba may be a dead-end branch of Homo, rather than an ancestor of later species such as H. erectus.

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