Thursday, September 08, 2011

Au. sediba Now Considered Possible Ancestor to Homo

New research is suggesting that the overall anatomy of Australopithecus sediba makes it the best candidate for being ancestral to Homo.  The article in Science Daily is quite long and delves into the specific traits that Lee Berger suggests are thought to evolving in the direction of HomoThey write:
Lee Berger, the project leader from the University of the Witwatersrand in Johannesburg, South Africa, explains what these new findings mean for modern humans. "The many advanced features found in the brain and body, along with the earlier date, make it possibly the best candidate ancestor for our genus -- the genus Homo -- more so than previous discoveries, such as Homo habilis"
The age of the Au. sediba fossils has been constrained to about 1.977 million years, which predates the earliest appearances of Homo-specific traits in the fossil record. Until now, fossils dated to 1.90 million years ago -- and mostly attributed to Homo habilis and Homo rudolfensis -- have been considered ancestral to Homo erectus, the earliest undisputed human ancestor. But, the older age of these Au. sediba fossils raises the possibility of a separate, older lineage from which Homo erectus may have evolved.
This position seemed odd to me at first because there is already evidence of “early Homo” dating back to 2.3 million years ago. Then it occurred to me that this article makes a tacit assumption that is not stated: Homo habilis and Homo rudolfensis are not really Homo, but are Australopithecus. This is an argument that has been made by Alan Walker and Bernard Wood who suggest that, despite the size differential between H. rudolfensis and the late australopithecines, they are more similar than different.This levels the playing field and then Au. sediba simply becomes the branch of Australopithecus from which Homo sprung.

Taxonomy is a black art and we run the risk of tying too much significance to our taxonomic designations.  If we argue that Au. sediba gave rise to H. ergaster, despite the fact that there are already two species of Homo running around, it conjures up ideas of convergent evolution where an earlier australopithecine gave rise to Homo habilis and Au. sediba gave rise to Homo ergaster.  If, however we demote earliest Homo down to australopithecine status, the difficulties vanish “like the snows of yesteryear” as Isaac Asimov would say.  Look for this to be challenged as well. 

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