Phylogenetic Analysis of Early Hominids [and Comments and Reply]

The proposal of the new australopithecine species Australopithecus afarensis has led to a multiplicity of hypotheses concerning the evolutionary relationships between the known Pliocene and Pleistocene hominid species. We use phylogenetic analysis to gain a new perspective on the subject. Using 69 traits, we construct a series of 12 complexes, each with a defining polarized morphocline. Four mutually exclusive cladograms are derived from these complexes, the most parsimonious of which implies that Homo habilis and A. robustus/boisei are more closely related to each other than either is to A. africanus and that these three species form a distinct evolutionary group relative to the more primitive A. afarensis. We advocate a phylogeny wherein A. afarensis is ancestral to A. africanus, which is in turn ancestral to A. robustus/boisei and H. habilis. We believe that the evolutionary transition from Australopithecus to Homo involved reduction in the size of the chewing teeth and associated traits leading to a unique derived condition in H. habilis that superficially resembles the primitive condition of A. afarensis. Five other current phylogenies are treated as critiques of this one.