Plio-Pleistocene climate-driven facial morphology in southern African australopithecines

Intensification of Northern Hemispheric Glaciation (INHG) has been cited as the driving force for climate change in southern Africa, despite the fact that rare and fragmentary continental records for the region allow for only a loose correlation between local faunal events and global climate change. Determining climatic influence from the limited climate history is exacerbated by the difficult chronology for the cave sites. The depositional history of the caves nonetheless reveals a turnover of southern African mammals, and variability, among other forcing factors, have implicated the INHG to explain the evolution of these novel forms. We suggest that evolutionary changes in southern African fauna may have also been driven by a subsequent climate event, the Onset of Walker Circulation (OWC) at ~ 2 Myr. The OWC, with enhanced high frequency climate variability, may have been more dominant than INHG in driving southern African mammalian evolution. For example, Pleistocene Australopithecus robustus, but not Pliocene Australopithecus africanus, exhibits relatively broad palates, postcanine megadonty and deep mandibular corpora that correspond to a dietary niche involving heavy mastication. These adaptations may have been selected for in unstable Pleistocene environments initiated by the OWC. Moreover, the temporal context of Australopithecus robustus, found in the Pleistocene dated caves of Swartkrans, Kromdraai and Drimolen, coincides more closely with the OWC than with the INHG.