Nature News Story on Au. sediba

Nature News has a story on the Australopithecus sediba skeleton described by Lee Berger and colleagues.  Ewen Callaway writes:

At around 420 cubic centimetres, A. sediba 's puny brain compares to those of other Australopithecus specimens and chimpanzees. But a high-resolution synchrotron scan of the brain's impression on the skull shows enlarged frontal areas that are normally associated with humans and linked to higher cognitive abilities, such as planning. A. sediba's pelvis also looks wider than those of other australopiths, raising doubts about the idea that the human pelvic shape evolved to accommodate large-brained babies. "Whatever is driving a relatively human-like shape of the pelvis, it is not a big brain," says Berger. The orientations of its leg and ankle bones suggest that A. sediba walked upright, and its nearly complete ankle resembles that of a human. But its long arms, and some features of its feet and shin bones, are similar to those of a chimpanzee. Taken together, these features suggest that A. sediba was adapted for both bipedalism and tree-dwelling.

This is a wonderful mosaic. As I wrote yesterday, though, it is not clear where it fits. There are three possibilities: A. africanus gives rise to A. sediba and A. habilis and A. rudolfensis.  Here, the two species of early Homo have been demoted and all three have transitional characteristics that represent a general trend toward modernity (after Walker and Wood).

The second scheme has A. africanus giving rise to A. sediba, which then goes extinct, and H. habilis and H. rudolfensis (in some fashion), one of which then gives rise to H. ergaster.  This posits that the traits present in both early Homo and A. sediba represent a general trend toward modernity. 

 

The third scheme has  A. garhi giving rise to early Homo with A. africanus giving rise to A. sediba, which then goes extinct.  The advantage of this is that A. garhi has modern-like limb proportions and is found in northeast Africa, not far from early Homo, while A. africanus and A. sediba are both found in south Africa.

 

Which one of these is right?  Are any of them right?  Who knows.  What we do know is that these critters have relationships to each other in some way, shape or form.