Possibly Pivotal Human Ancestor Debated

Fossils yield new look at species that may have spawned humankind.

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By Bruce Bower, Science News

MINNEAPOLIS—Fossils described last year as representatives of an ancient species critical to human evolution have reentered the scientific spotlight and set off a new round of debate over the finds’ true identity.

Researchers described analyses of new and previously recovered remains of a South African species called Australopithecus sediba on April 16 at a meeting of the American Association of Physical Anthropologists. Evidence is accumulating, they reported, that 2-million-year-old A. sediba formed an evolutionary connection between relatively apelike members of Australopithecus and the Homo genus, which includes living people.

It’s now clear that A. sediba shares more skeletal features with early Homo specimens than any other known Australopithecus species does, said Darryl de Ruiter of Texas A&M University in College Station. “We think A. sediba is a possible candidate ancestor for the genus Homo.”

De Ruiter suspects that an isolated population of the hominid species Australopithecus africanus gradually evolved into A. sediba, resulting in a species characterized by an unusual mix of skeletal traits, some typical of Australopithecus in general and others of early Homo.

That scenario, outlined in symposium presentations by De Ruiter and Lee Berger  of the University of the Witwatersrand in Johannesburg, South Africa, remains controversial despite the new fossil discoveries.

Ian Tattersall of the American Museum of Natural History in New York City endorsed A. sediba as a distinct species, probably closely related to A. africanus. “I wouldn’t classify it as the root of the Homo genus, though,” he commented.

De Ruiter acknowledged the possibility that two partial A. sediba skeletons previously excavated from a collapsed, underground cave (SN: 5/8/10, p. 14), as well as newly retrieved fossils from the cave, might represent a late-surviving form of Australopithecus africanus unrelated to Homo. Previous A. africanus finds date from about 3 million to 2.4 million years ago in South Africa. Fossils of A. africanus show lots of anatomical disparities from one individual to another, so that species might well encompass fossils attributed to A. sediba, remarked John Hawks of the University of Wisconsin–Madison. Much uncertainty surrounds the identity of fossil members of the human evolutionary family between 3 million and 2 million years ago, he said.

Possible Homo fossils date to around 2.3 million years ago in East Africa, suggesting that even if A. sediba truly is a new species, it evolved after Homo did.

New A. sediba fossils from the same South African cave complex, many belonging to the previously discovered partial skeletons, underscore this ancient species’ mosaic anatomy, Berger said. A largely complete female pelvis displays relatively straight, vertically aligned hips and an elongated birth canal, much like early Homo species. Other Australopithecus females possessed a relatively short, wide pelvic opening and flaring hip bones.

New A. sediba foot bones include a chimplike heel and a humanlike ankle, Berger said. Fossils from the shoulders, rib cage and spine, as well as surprisingly long arm bones, typify Australopithecus.

Many newly recovered fossils are largely encased in hardened sediment. Computerized scanning produced 3-D images of complete fossils for analysis.

Digital reconstructions also aided an analysis led by De Ruiter of previously recovered A. sediba skulls and teeth, along with newly unearthed teeth. These remains also contain a blend of Australopithecus and Homo traits, De Ruiter reported.

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