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On the Phylogenetic Analysis of Early Hominids

by WILLIAM H. KIMBEL, TIM D. WHITE, and DONALD C. JOHANSON Institute of Human Origins, 2453 Ridge Rd., Berkeley, Calif. 94709 (Kimbel and Johanson)/ Department of Anthropology, University of California, Berkeley, Calif. 94720, U.S.A. (White). 3 III 86 We are pleased to receive a belated invitation to comment on Skelton, McHenry, and Drawhorn's (CA 2 7:1-43) "Phylogenetic Analysis of Early Hominids." Although the authors introduce their contribution with the claim that their phylogenetic analysis develops a "new perspective" (p. 2 1) on early hominid evolutionary relationships, we believe this is something of an exaggeration, since we previously discussed their preferred phylogeny in detail, ultimately rejecting it for reasons given in those publications (Johanson and White 1979, White, Johanson, and Kimbel 1981; Kimbel, White, and Johanson 1984). In our 1979 paper (Johanson and White 1979) we concluded (1) that the samples of early hominid remains from Hadar and Laetoli are best viewed as conspecific, (2) that these samples (taken individually or combined) are more primitive in aspects of craniodental anatomy than known samples of other hominid taxa, therefore necessitating a new species name, Australopithecus afarensis, for them (Johanson, White, and Coppens 1978), (3) that A. afarensis was broadly ancestral to all later hominids, and (4) that A. africanus (as known in the Transvaal) was an exclusive ancestor of the "robust" species A. robustus and A. boisei, owing to a unique derived, functionally integrated set of craniodental characters hared by these taxa. Our point was that this set of derived features distinguishes the "robust" clade from the Homo clade in the late Pliocene-early Pleistocene. All of these features are related functionally to heavy mastication and are foreshadowed in the skulls and teeth of middle-late Pliocene A. africanus. Skelton et al. agree with our first three conclusions but favor the view that A. africanus was the last common ancestor of the "robust" and Homo clades. We do not intend here to challenge the validity of the phylogenetic hypothesis preferred by Skelton et al. We continue to maintain that this hypothesis (which has also been championed by Wolpoff [1983] and Yaroch and Vitzthum [1984], among others) is a viable, and testable, alternative to our own (in fact, one of us [Kimbel 1986] now feels that this alternative hypothesis fits the available evidence at least as well). Instead, we wish to comment on several aspects of the authors' methods, upon which the credibility of their conclusions depends. Among the original commentators on the Skelton et al. paper, Chamberlain and Wood (pp. 36-37) forcefully point out some of the pitfalls in the authors' use of the cladistic method. While we do not agree with Chamberlain and Wood's criticism that employing the maximum-parsimony principle implies that parallelism and convergence are rare (see, e.g., Sober 1983, Gosliner and Ghiselin 1984), Skelton et al. have given only lip service to these phenomena as regards hominid evolution. It is true, as they point out in their reply, that "simple assertion of the possibility of convergence does not demonstrate its reality"