PLIO-PLEISTOCENE ENVIRONMENTAL CHANGES AND HOMINID DIETARY NICHES IN SOUTHERN AFRICA

If the "details of the paleoenvironmental framework" for hominid evolution are poorly known for East Africa, the problem is far more acute in southern Africa where stratigraphic complexity, taphonomic uncertainties and poor chronological control of the hominid karst-infill sites conspire to thwart construction of long environment sequences. Nevertheless, taken together, the various components of the sites of Makapansgat, Sterkfontein, and Swartkrans can provide reasonable windows into environments of the subcontinent during late Pliocene and earlier Pleistocene, a period in which large-scale global climate shifts are evident in the marine records. Paleoenvironmental reconstructions based on faunal indicators have suggested stepwise shifts from relatively closed, mesic, to more open, arid landscapes in South Africa at this time. The C3/C4 index, based on the proportions of animals eating C3 and C4 diets obtained from carbon isotope values of the fossil fauna, provides a slightly different perspective. The principle is that in relatively open areas such as grasslands, little C3 vegetation is available for C3 consumers, while in woodier habitats there will be less food for grazers. The carbon isotope results confirm that fairly dense, woody environments pertained ~3Ma, but shifts thereafter are small, with little evidence for a large shift to open environments in the period 3-2 Ma as suggested previously. Rather, the most marked change to open environments occurs at, or shortly after, 1.7 Ma. In spite of environmental changes across this entire period, hominid diets show a consistent, modest contribution of C4 foods to their diets. This observation implies that hominids interacted with grassy environments independently of environmental shifts and that interaction with grassland foods is deeply rooted, setting both Australopithecines and Homo apart from chimpanzees in their dietary ecology.