The species and diversity of australopiths

In this chapter, the historical, systematic, and anatomical evidence for the diversity of the species within the australopith grade is reviewed. Given a strict evolutionary species definition, nominal taxonomic diversity and specieslineage diversity do not necessarily map onto one another in the fossil record. Species lineages entail statements of ancestry and descent that depend on the consistency of phylogenetic and stratophenetic data. The requirements for identifying species lineages in the fossil record are severe and in the early hominin record are rarely met, most often owing to small sample size, underrepresented character data, nonrepresentation of rare or short-lived taxa, poor chronological resolution, gaps in the time-stratigraphic framework, or some combination of these factors. Because hypotheses concerning the “bushiness” of the hominin phylogenetic tree depend on the identification of lineages, not phenetically based “paleospecies,” confidence with respect to this issue is not justified for the majority of the hominin fossil record. There are two cases in which an approach to this question can be attempted. In one, the evidence is consistent with the evolution of Australopithecus anamensis into A. afarensis via anagenesis. The other, the evolution of A. boisei, most likely entailed a speciation event that gave rise to southern African clade (represented by A. robustus) subsequent to the appearance of A. aethiopicus. The late Pliocene time period in which the latter events transpired (ca. 2.8-2.3 Ma) is one of substantial morphological diversity, high nominal taxonomic diversity, and high probability of synchronicity among known fossil samples. With the exception of the close phylogenetic relationship of A. africanus to A. sediba, it is not possible to connect the later Pliocene australopith taxa (A. aethiopicus, A. garhi) to particular descendants due to defects in the database. Nevertheless, this time period probably documents a previously (and subsequently) unmatched degree of lineage proliferation compared to other parts of the human evolutionary record. The challenge to paleoanthropologists is to devote resources to improving this part of the fossil record and then to create testable phylogenetic and adaptive hypotheses to explain it.