(A. afarensis, A. aethiopicus, A. africanus, A. boisei, and A. robustus ) and three of Homo (H. habilis, H. erectus, and H. sapiens). Table 1 presents the geological dates and the estimated body, brain, and tooth sizes of these species and apes.
Analysis of the states of 77 craniodental characters in these species of Australopithecus and H. habilis (Skelton and McHenry, 1992) reveals that the cladogram in Figure 1A is the most parsimonious (tree length = 12,796, consistency index = 0.72). The two late ''robust" australopithecines, A. robustus and A. boisei are the most highly derived and form a sister group with early Homo. This branch links with A. africanus to form a clade containing A. africanus, A. robustus, A. boisei, and early Homo. A. aethiopicus branches from this clade next with A. afarensis as a sister species to all later hominids.
Figure 1B displays the phylogenetic tree implied by the most parsimonious cladogram. This phylogeny implies that A. afarensis is the most primitive hominid and that all later hominids shared a common ancestor that was more derived than A. afarensis. This post-afarensis hypothetical ancestor may someday be discovered. Its morphology can be reconstructed by observing the many ways A. aethiopicus resembles later hominids (especially A. africanus) and not A. afarensis. For example, the canine eminences of the face are prominent in the outgroup and in A. afarensis but are reduced or absent in all other species of hominid, which implies that the common ancestor of all post-afarensis species had canine eminences that were also reduced. This hypothetical ancestor would have a strongly developed metaconid on the lower first premolar. It would not, however, resemble A. aethiopicus in traits related to masticatory hypertrophy (heavy chewing), nor would it resemble any other post-afarensis species because they are all too derived in flexion of the base of the skull, orthognathism (flat faced), and encephalization to have been the ancestor of A. aethiopicus. After the divergence of A. aethiopicus, this phylogeny depicts a common ancestor of A. africanus, A. robustus, A. boisei, and Homo that resembled A. africanus in its development of anterior dentition, basicranial flexion, orthognathism, and encephalization. A second hypothetical common ancestor appears in Figure 1B to account for the numerous derived traits shared by A. robustus, A. boisei, and early Homo that are not seen in A. africanus. This ancestor would have the degree of basicranial flexion and orthognathism seen in early Homo and the amount of encephalization seen in A. robustus and boisei. This phylogeny proposes a third hypothetical ancestor that would be at the root of the lineage leading to A. robustus and A. boisei. This ancestor probably resembled A. robustus in traits related to heavy chewing.
Although the most parsimonious cladogram implies this phylogeny, other cladograms are possible but less probable. A cladogram linking A. aethiopicus to A. boisei and robustus as one branch and A. africanus /early Homo as another requires more evolutionary steps (tree length = 13332;