EVOLUTIONARY RELATIONSHIPS AMONG EARLY HOMINIDS

Although cladistic analysis provides one of the most useful approaches to discovering the phyletic relatiosships among the species of Australopithecus and early Homo, methodological problems continue to beset any attempt to apply it in this context. Two of the most pressing problems are redundancy of traits due to similarity of underlying function and overrepresentation of some anatomical or functional systems in trait lists. In an attempt to mitigate these problems, we collapse our list of 77 traits into sets of traits using two methods. The first method segregates traits into seven groups by anatomical region. The second method segregates traits by function into five complexes which correspond fairly well with recognized trends in the evolution of early hominids (adaptation for heavy chewing, reduction of the anterior dentition, basicranial flexion, increased orthognathism and encephalization). The most parsimonious cladogram describing the relationships among the early hominid species obtained using the original 77 traits or the summary scores from the functional complexes, places Australopithecus afarensis as a sister group to all other hominids. Australopithecus aethiopicus occupies the next branch, leaving A. africanus, A. robustus, A. boisei and Homo as a monophyletic group. The cladogram next separates A. africanus from the remaining hominids and finally divides Homo from A. robustus and A. boisei. Summary scores for anatomical regions produced three equally parsimonious cladograms, one of which was identical to that described above. This result implies that there was a large amount of parallelism in hominid evolution, and that adaptations for heavy chewing evolved separately in the lineage leading to A. aethiopicus and in the lineage leading to A. robustus and A. boisei. Another implication is that Homo descended from an A. africanus-like ancestor and diverged from A. robustus and A. boisei relatively late in hominid evolution by reducing the extent of its adaptation to heavy chewing. Most current phylogenies are not compatible with the cladogram obtained in this study, but are instead compatible with a cladogram obtained when traits related to heavy chewing are used exclusively. The "heavy chewing" cladogram reverses the positions of A. aethiopicus and Homo. Perhaps the reason why most current cladograms resemble the "heavy chewing" cladogram is the over-representation of traits related to heavy chewing in most trait lists. © 1992.