It has long been observed that early members of the genus Homo are associated with more open and arid-adapted fauna than antecedent australopith species (e.g., Vrba, 1980; Wesselman, 1985; Reed, 1997; Bobe and Behrensmeyer, 2004). Moreover, the first appearances of earliest Homo (~2.5 Ma; Hill et al., 1992; Schrenk et al., 1993; Kimbel et al., 1996) and Homo ergaster (~1.8Ma; Feibel et al., 1989) are broadly coincident with a series of global climate changes that included the onset of northern hemisphere glaciation and the Walker circulation respectively (Trauth et al., 2005). Together, these observations suggest the possibility of some linkage between the origin and subsequent speciation of Homo and climatic/environmental change; yet, there are several reasons to be cautious about accepting this linkage at face value. For one, large-scale climate processes are not discrete "events," and thus are very difficult to tie to specific speciation and/or extinction events in the fragmentary, and often chronologically ambiguous, terrestrial fossil record. For another, the paleoclimatic data that are often used to posit such linkages are usually global in nature (e.g., Prentice and Denton, 1988; deMenocal, 1995), and thus their relevance for animals in particular basins and around specific sites is unclear. This is particularly problematic with respect to southern Africa, where climate change may be anti-phased

M. Sponheimer (H)

Department of Anthropology, University of Colorado at Boulder, Boulder, CO 80309, USA e-mail: [email protected]

J.A. Lee-Thorp

Division of Archeological, Geographical and Environmental Sciences, University of Bradford, Bradford BD7 1DP, UK e-mail: [email protected]

or at least partially decoupled with respect to the better-known East African record (e.g., Partridge et al., 2004). In addition, paleoanthropologists working in southern Africa have fewer proxies with which to explore paleoclimate and environmental trends. For instance, the absence of pedogenic carbonates (from relevant ages and places), which have been used to investigate long-term environmental change (e.g., Cerling, 1992; Quade and Cerling, 1995; Wynn, 2004) and document vegetational diversity across paleolandscapes (Sikes, 1994), has made it difficult to cross-check both regional and local paleoenvironmental reconstructions in southern Africa. Thus, researchers in southern Africa have relied perforce upon global or regional datasets that may or may not be relevant, or have relied on the taxonomic or morphological character of hominin-associated fauna.

Here, we draw on stable isotope data from fossil herbivores to augment the taxonomic and ecomorphological studies that have investigated the habitats of Australopithecus africanus and later Homo in South Africa (e.g., Cooke, 1978; Vrba, 1980, 1985; Reed, 1997; Spencer, 1997). We explore the degree to which faunal isotopic proxies bear on two principle questions: (1) Did early members of the genus Homo inhabit more open environments than their predecessor (A. africanus) in South Africa, and (2) If so, was the trend to open environments accompanied by aridification, so that Homo lived in drier conditions than A. africanus? In applying and evaluating the isotopic data, we will also touch on several methodological and theoretical considerations.

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