bridge between the more primitive Pliocene hominid represented by Ardipithecus and the anamensis group and the more derived Plio/Pleistocene hominins represented by Paranthropus, Australopithecus, Kenyanthropus, and Homo.
There are three courses that could be adopted in interpreting these results in terms of systematics. One is to simply accept generic paraphyly as an "occupational hazard" in paleoanthropology; this would have the benefit of stabilizing the taxonomy (and so the all-important nomenclature), at least for a while, but at a cost of flouting the principles of phylogenetic systematics (Wiley, 1981; also see Harrison, 1993, for a detailed discussion of this issue).
The second course is much more radical: Refer all to a single genus, for which the prior available name would, of course, be Homo. This would have the superficial drawback that finer interrelationships among species would no longer be defined by generic names. But, as we have seen, this is not possible anyway, given the apparently irreducibly paraphyletic nature of some widely recognized genera. However, it would stabilize generic nomenclature, and it would in addition mean that a reshuffle of species between genera would not occur each time anyone discovers apparently new evidence for their affinities. Groves (2001) has supported giving higher categories some objectivity by allocating a time depth to each: In the case of a genus, this would mean some 4 to 6 million years. Under such a scheme, it is clear that all hominin genera, even Ardipithecus (and perhaps Pan), would be synonymized with Homo, and the tribe Hominini would become unnecessary. CPG favors this proposal (see Table 5.2).
The third course would be to erect further genera, one for each major clade. This approach has been argued and adopted by Cameron (1997a, 1998, 2001, in press) in his analysis of the Miocene hominids. Most of the hominin genera would thereby be monotypic. This is also in agreement with Collard and Wood (1999), Wood and Collard (1999), and Strait and Grine (2001), who argue that genera should ideally correspond to both the grade and clade concepts. This means that a species of the same genus must be both monophyletic and adaptively distinct from species that belong to other genera. This approach has the advantage that a certain stability is introduced to hominid classification, and that the principles of phylogenetic systematics would not be flouted. This is the scheme adopted here. DWC favors this proposal (see Table 5.3).
Cameron's taxonomic scheme supports the generic distinction of the Hadar and Laetoli Pliocene hominins from Australopithecus. This partly follows Day et al. (1980), Harrison (1993), and Strait et al. (1997), who referred them to the genus and species Praeanthropus africanus, though for reasons outlined in Groves (1999), Strait (2001), and Strait and Grine (2001), the species should now be called Pr. afarensis. The paraphyletic nature of the australopithecines has been recognized by others, including Chamberlain and Wood (1987), Groves (1989), Wood (1991), Skelton and McHenry (1992), Lieberman et al. (1996), Strait et al. (1997), Strait (2001), and Strait and Grine (2001). With Strait and Grine (2001), a generic distinction would be warranted for the hominins currently allocated to A. anamensis and A. garhi, which cannot be considered species of Australopithecus. As such, Australopithecus africanus would represent the only species within this genus. Nor does this study support the scheme suggested by Asfaw et al. (1999), that members of the garhi group represent a likely immediate ancestor to early Homo.
Chapter 5 Pliocene and Early Pleistocene Hominids Colin's Mio-/Pliocene Hominid Taxonomy
Kenyapithecinae Ponginae Dryopithecinae Homininae
Gorilla gorilla Gorilla beringei
Pan troglodytes Pan paniscus
Homo kadabba Homo ramidus (australopithecine group) Homo anamensis Homo bahrelghazali Homo afarensis Homo garhi Homo africanus (paranthropine group) Homo walkeri Homo boisei Homo robustus (kenyapithecine group) Homo platyops Homo rudolfensis (habiline group)
Homo habilis (erectine group)
DWC further considers that only species within Paranthropus, Australopithecus, Kenyanthropus, and Homo can be considered true hominins. Ardipithecus and members of the anamensis group are simply plesiomorphic Pliocene hominids, whose evolutionary history likely
TABLE 5.3 ► Dave's Mio/Pliocene Hominid Taxonomy
P. troglodytes P. paniscus
O. tugenensis Sahelanthropus
Garhi deme Australopithecus
A. africanus Paranthropus
P. walkeri P. boisei P. robustus Kenyanthropus
K. platyops K. rudolfensis
Lothagam hominid Ardipithecus ramidus Praeanthropus afarensis Anamensis hominids Bahrelghazali hominids
H. habilis H. ergaster ended in extinction, with no direct contribution to the evolution of the later hominins. Whether Sahelanthropus, Praeanthropus, or members of the garhi group can be considered hominins in this scheme remains unresolved, though Sahelanthropus does appear from the available evidence to be closer phylogenetically to the later hominins than to Pan or Ardipithecus. Given the early dates for Sahelanthropus, this result must be considered of some significance to later human evolution.
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