Strontium-calcium ratios in fossil bone have been used to reconstruct paleodietary adaptations, and to test the hypothesis that early members of our genus had different diets than other early hominids. The results of an earlier study (Sillen et al., Journal of Human Evolution 28, 1995) suggested that contemporaneous hominids may have had different dietary adaptations, but was not consistent with the prevailing hypothesis of scavenging and/or hunting by early Homo.

Previous studies have employed a solubility profiling technique to account for diagenesis. However, a number of additional factors may alter the interpretation of Sr/Ca ratios. The currently available data suggest differences between A. robustus and Homo, but the small number of samples tested makes interpretation based on dietary variation problematic. A number of natural factors resulting in altered Sr/Ca ratios have been discussed recently (Burton et al., Journal of Archaeological Science 26, 1999), and the natural variability in these ratios is considered in great detail in this study that expands the sample size through the analysis of additional Australopithecus and Homo fossils from Swartkrans and Sterkfontein. Specifically, the correlation between hominid mobility and geological substrates, water pH level, plant species, plant part, and sex of the individual are all considered when possible. Our results are evaluated for potential differences in dietary adaptations of these hominid species, and their dietary adaptations relative to other mammals. The implications of this study on the problems and prospects of Sr/Ca ratio analysis are also discussed.