Whence Australopithecus africanus? Comparing the Skulls of South African and East African Australopithecus

Abstract

There is longstanding debate on the position of Australopithecus africanus in hominin phylogeny, possibly due to the phenetic gap that exists between A. africanus and A. afarensis. The fact that A. africanus is phenetically similar to Paranthropus and Homo allows it to endure as similar to each in cladistics analyses without resolving its exact phylogenetic position. This fact permits the development of a variety of both evolutionary and biogeographic models to explain the conundrum of A. africanus. This taxon likely arose from a population derived from somewhere in the middle of the A. anamensis to A. afarensis lineage that was subsequently isolated in South Africa. There are a variety of anatomical features that support this view. The variation through time in A. africanus is somewhat unexpected in that the Makapansgat sample is more Paranthropus-like than those from Sterkfontein, which, as a whole, are more Homo-like. Many of the relationships among these taxa are dependent on the biogeographical models that can be constructed from the limited data that we possess, but there are two basic ways to think about this: (1) eastern Africa was the center of evolution and ensuing distribution, with movement always to the south and with southern populations becoming extinct, or (2) a “piston” type model, which encompasses character displacement among sympatric species as they occur throughout back and forth movement across the continent through time. Each of these fits into evolutionary models in different ways, the former fits better with cladistic analyses and the latter is more stratophenetic. In the end, despite the fact that there are many fossil specimens of A. africanus, its phylogenetic position remains unresolved.