A famous “ancestor” may be ousted from the human family

刘颖

This skull may have grown up to look more like an australopithecine than a human-like member of our genus Homo
Brett Eloff/Courtesy Profberger and Wits University

New Orleans— A remarkably complete skeleton that was introduced in 2010 as “the best candidate” for the immediate ancestor of our genus Homo may just be a pretender. Instead of belonging to the human lineage, the new species of Australopithecus sediba is more closely related to other hominins from South Africa that are on a side branch of the human family tree, according to a new analysis of the fossil presented here last week at the annual meeting of the American Association of Physical Anthropologists.

When fossils from several individuals’ skeletons were found in a collapsed cave in Malapa, South Africa in 2008, their discoverer, paleoanthropologist Lee Berger of the University of the Witwatersrand, noted that they helped fill a key gap in the fossil record 2 million to 3 million years ago when some upright-walking australopithecine evolved into the earliest member of our genus, Homo. But the oldest Homo fossils at 2.4 to 2.9 million years are scrappy, and a half dozen more primitive hominins may have been walking around Africa at roughly the right time to be the ancestor. Researchers have hotly debated whether their direct ancestor was the famous 3.2 million-year-old fossil Lucy and her kind, Australopithecus afarensis from Ethiopia, or another australopithecine.

With its fossils dated to 1.98 million years ago, Au. sediba is too young to be directly ancestral to all members of the genus Homo. But Berger and his colleagues proposed in 2010 and, again in 2013 in six papers in Science, that given the many human-like traits in Au. sediba’s face, teeth and body the Malapa fossils were a better candidate than Lucy or other East African fossils to be ancestral to Homo erectus, a direct human ancestor that appeared 1.8 million years ago.

In a talk here, though, Arizona State University paleoanthropologist Bill Kimbel analyzed the most complete skull of Au. sediba and systematically shot down the features claimed to link it to early Homo. Kimbel noted that the skull was that of a juvenile—a “7th grader”—whose face and skull were still developing. His analysis, with paleoanthropologist Yoel Rak of Tel Aviv University in Israel, concluded that the child already showed traits that linked it most closely to the South African australopithecine Au. africanus, a species of australopithecine that lived in South Africa 3 million to 2.3 million years ago. And had it survived to adulthood, its human-like facial traits would have changed to become even more like Au. africanus.

For example, the breadth of the young Au. sediba’s cheekbones appears narrow as in early Homo. But by studying other australopithecine, ape, and Homo fossils to see how features of the cheekbones change as individuals grow and chewing muscles develop, Kimbel and Rak were able to predict how the boy’s face and skull would have looked if he’d grown up to be an adult. The resemblance to Au. africanus is so striking, in fact, that Kimbel thinks Au. sediba is a closely related “sister species” of Au. africanus--and not a long lost human relative. “We don’t believe…that Au. sediba has a unique relationship to the genus Homo,” says Kimbel.

Other researchers who have long been skeptical that Au. sediba was an ancestor of Homo found Kimbel’s talk persuasive: “Spot on,” says paleoanthropologist Bernard Wood of George Washington University in Washington, D.C. Paleoanthropologist Ian Tattersall of the American Musuem of Natural History in New York agrees with Kimbel that Au. sediba is most closely related to Au. africanus and that neither species is ancestral to early Homo.

But paleoanthropologist Darryl de Ruiter of Texas A&M University at College Station, a co-author with Berger on the 2013 paper describing the skull, says he and his former graduate student came to “the opposite conclusion” when they used computational methods to make their own projection of how the skull would have changed as it matured. “I disagree with his impression that the changes that [the skull) would have undergone had it lived to adulthood would be so extensive as to make it appear like Au. africanus,” said du Ruiter, who heard Kimbel’s talk.

The only way to know what an adult Au. sediba’s skull and face really looked like, he says, is to find one: “The ultimate resolution of the question must await the long hoped for recovery of the adult cranium of Au. sediba.”