Tuesday, June 22, 2010
Its time to sink the genus Australopithecus
With the recent description of the partial skeletons of Ardipithecus ramidus from Aramis and Australopithecus afarensis from Woranso-Mille, both in Ethiopia, a clear distinction can be made between pre-human and fully human adaptations to bipedality. When we include Au. anamensis in the equation, which is assumed by most to be a slightly older and more "primitive" version of Au. afarensis, the separation between Ardipithecus and Australopithecus is reduced to 200 ky, a mere blip on the paleoanthropological radar screen. The morphological distance is, however, far greater in both cranial capacity (less than 400 cc in Ardipithecus vs. more than 400 cc in Australopithecus) and overall locomotor pattern. To go from Ardipithecus to Australopithecus in such a short span of time seems hard to imagine (see previous post). On the other end of the temporal spectrum the recent description of Australopithecus sediba, dated to 1.9 mya from the Republic of South Africa, highlights the difficulty in separating late occurring australpiths from early members of the genus Homo. It thus seems, on the one hand, that adaptationally Australopithecus is much more similar to Homo than it is to Ardipithecus, while on the other hand, it is becoming more and more difficult to separate late occurring Australopithecus from early Homo. The rationale for maintaining the genus Australopithecus appears to be evaporating. It makes more sense to sink Australopithecus into the genus Homo while retaining it as a subgenus to distinguish it from later occurring, increasingly more human-like fossil hominins. For the sake of convenience, and other considerations, I think the genus Homo can be divided into four subgenera H. (Australopithecus), H. (Paranthropus), H. (Pithecanthropus), and H. (Homo). For example, “Lucy” would belong to H. (Australopithecus) afarensis, South African robusts to H. (Paranthropus) robustus, “Java Man” to H.(Pithecanthropus) erectus and “Heidelberg Man” to H. (Homo) heidelbergensis. Each one of these subgenera represents a clearly definable morphotype identifiable by a set of synapomorphies indicating a shared common ancestry as a "species clade" composed of distinct "paleo-demes" as discussed by Howell (1999). Colloquially we can refer to these constructs as australopiths, paranthropes, pithecanthropes, and archaics (i.e. pre-modern humans) vis-à-vis modern humans.
To quote the venerable resource Wikipedia, “in zoology, a subgeneric name can be used independently or included in a species name, in parentheses, placed between the generic and specific name … However, it is not mandatory, or even customary, when giving the name of a species, to include the subgeneric name.” Thus “Lucy” would commonly be referred to as H. afarensis, etc., etc. This is in partial conformity to the geneticist Morris Goodman’s suggestion that the African Great Apes be sunk into the genus Homo because of their overall genetic similarity to H. sapiens and J. T. Robinson’s suggestion that Au. africanus be revised to H. africanus. Goodman’s view, while biologically correct, seems to be a bit too radical for most people’s taste, while Robinson’s suggestion now appears to have been prescient. In the light of these precedents the sinking of Australopithecus into Homo, as a subgenus, seems eminently reasonable.