Systematic Scheme for the Pliocene and Early Pleistocene Hominids

The previous two chapters described the emergence of the late Miocene, Pliocene, and early Pleistocene hominid groups without examining in any detail their likely phylogeny. This chapter will directly address issues of phylogeny and taxonomy by generating a number of phylogenetic analyses of the Miocene and Plio-/ Pleistocene hominids. Overall this chapter will examine: (1) what the systematic significance of Sahelanthropus is in relation to the Miocene and Pliocene hominids; (2) how the inclusion of A. anamensis and A. garhi affects the often-commented-upon para-phyletic status of Australopithecus; (3) what the phylogenetic relationship of Kenyanthropus is to other Pliocene hominids; and (4) how these relationships should be reflected in taxonomy.

As discussed in the previous chapters, the discovery and description of the new Pliocene genus Kenyanthropus (M.G. Leakey et al, 2001) and the late Miocene hominid Sahelanthropus (Brunet et al., 2002) have thrown into question the origin of the genera Australopithecus, Paranthr-opus, and Homo, not to mention the evolutionary significance of the proto-australopithecines, including Praeanthropus, to them. While numerous parsimony analyses on Pliocene and early Pleistocene hominids have been generated (B.A. Wood & Chamberlain, 1986; Chamberlain & Wood, 1987;

Skelton et al., 1986; B.A. Wood, 1991; Skelton & McHenry, 1992, 1998; D.E. Lieberman et al., 1996; Strait et al., 1997), they have tended to focus on the monophyletic status (all share a common ancestor to the exclusion of other taxa) of the species within Paranthropus (P walkeri, P. boisei, and P robustus) as well as the phylogenetic significance of Pr. afarensis and A. africanus. While some "consensus" has been reached with regard to the monophyletic status of the Paranthropus (see B.A. Wood, 1991; Strait et al., 1997; Strait & Grine, 1998, 2001; Cameron, in press b), the paraphyletic status (do not share a common ancestor and thus are not considered to be particularly closely related because they appear to have their own distinct evolutionary lineage) of australopithecine species is still very much a hot issue.

Whether species currently assigned to Australopithecus form a mono-phyletic or paraphyletic group is extremely important, because if they are shown not to share a common ancestor to the exclusion of all other hominids, then they cannot be considered as belonging to the same genus unless all are included in Homo. This obviously has important implications for the phy-logeny of early Pliocene hominids and later hominins. M.G. Leakey et al. (2001), while recognizing the paraphyletic status of Australopithecus, take the conservative, "grade-sensitive" approach and argue that the currently recognized australopithecine taxa represent stem species to this genus, all sharing a suite of key primitive features. Strait et al. (1997), however, suggest that the paraphyletic status of specimens previously allocated to Australopithecus means that A. afarensis should be removed from Australopithecus and placed within the genus Praeanthropus.

The reallocation of afarensis to Praeanthropus goes some way to resolving the paraphyletic nature of the australopithecines, as is frequently commented upon by researchers (see B.A. Wood & Chamberlain, 1986; Groves, 1989a; D.E. Lieberman et al., 1996; Strait et al., 1997; B.A. Wood & Collard, 1999; B.A. Wood & Richmond, 2000; M.G. Leakey et al., 2001; D.E. Lieberman, 2001; Strait & Grine, 2001). The paraphyletic nature of Australopithecus, however, needs to be further reexamined, given the recent recognition of A. garhi (Asfaw et al., 1999), A. anamensis, and the species sometimes categorized as Australopithecus or Homo rudolfensis (which we tentatively place in Kenyanthropus).

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