Wednesday, May 10, 2017

Australopithecus sediba Voted Off the Island

This does not surprise me.  Science Magazine has a short story which suggests strongly that the characteristics in Au. sediba render it unsuitable for a possible ancestor to the genus Homo
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 humanlike 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, paleoanthropologist Bill Kimbel of Arizona State University in Tempe 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. In his analysis, with paleoanthropologist Yoel Rak of Tel Aviv University in Israel, he concluded that the child already showed traits that linked it most closely to the South African australopithecine Au. africanus, a species that lived in South Africa 3 million to 2.3 million years ago. And had it survived to adulthood, its humanlike facial traits would have changed to become even more like those of 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 could 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.
At this point, it is pretty hard to tell where a precursor might be found. We know that there was considerable variability in australopithecines throughout the Plio-Pleistocene but, as of yet, no good candidate has arisen.What seems to be clear, however, is that there is a general trend toward more modern morphology in the pelvis and hands, as exhibited by Au. sediba.  Whether or not these characteristics are present in other specimens of Au. sediba is, however, unknown.  As Kimbel notes, we need an adult one to see for sure. 

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