Wednesday, August 11, 2010
Its time to sink the genus Australopithecus redux
Back in June I posted a comment on the implications of the discovery of "Kadanuumuu (Big Man) and Karabo" the partial skeletons of Australopithecus afarensis at Woranso-Mille in Ethiopia and A. sediba from Malapa in South Africa. The point I was trying to make was that new discoveries at both ends of the Australopithecine lineage were blurring the distinction between the genera Australpithecus and Homo. The Homo-like features of the Woranso-Mille skeleton suggested that the time separating the primitive looking Ardipithecus ramidus from Woranso-Mille's ancestor A. anamensis was too short to accommodate the degree of morphological change that had occurred between them. On the other end of the timescale the recently described A. sediba has been attributed to early Homo by some because of the amount of morphological overlap (particularly dental) between the two.
Now we have a new report in Nature by McPherron et al. that shows that A. afarenis was a potential stone tool user by ~3.4 mya. This further muddies the water regarding what it is that differentiates Australopithecus from Homo. The Woranso-Mille skeleton mitigates the role of body size and proportions in distinguishing the two from one another. A. sediba minimizes the differences in tooth size, form and function between the two. H. floresiensis, if accepted as a valid hominin species, demolishes the brain size Rubicon separating Australopithecus from Homo. And now we have the last bastion of Homo, stone tool use, falling by the wayside. What is left to distinguish the genus Australopithecus from Homo?
As I previously argued the sinking of Australopithecus into Homo, as a subgenus, seems eminently reasonable, if not now mandatory.
The abstract of the Nature article follows:
The oldest direct evidence of stone tool manufacture comes from Gona (Ethiopia) and dates to between 2.6 and 2.5 million years (Myr) ago1. At the nearby Bouri site several cut-marked bones also show stone tool use approximately 2.5 Myr ago2. Here we report stone-tool-inflicted marks on bones found during recent survey work in Dikika, Ethiopia, a research area close to Gona and Bouri. On the basis of low-power microscopic and environmental scanning electron microscope observations, these bones show unambiguous stone-tool cut marks for flesh removal and percussion marks for marrow access. The bones derive from the Sidi Hakoma Member of the Hadar Formation. Established 40Ar–39Ar dates on the tuffs that bracket this member constrain the finds to between 3.42 and 3.24 Myrago, and stratigraphic scaling between these units and other geological evidence indicate that they are older than 3.39 Myr ago. Our discovery extends by approximately 800,000 years the antiquity of stone tools and of stone-tool-assisted consumption of ungulates by hominins; furthermore, this behaviour can now be attributed to Australopithecus afarensis.Either A. afarensis should be revised to H. afarensis or the possibility must be entertained that the Woranso-Mille individual and the maker of the stone tool cut marks at Dikika represent a new previously unknown species of Homo (perhaps H. antiquus Ferguson 1984) that lived contemporaneously with A. afarensis.
Why not doing the complete opposite? Instead of putting Australopithecus into Homo, why not introduce a new Genus which includes all the ambigious Taxa, such as H. habilis and A. sediba. That way you can solve the Problem of either having a too broad or a too strict definition.