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. 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.

The currently available data suggest differences between A. robustus (Sr/Ca = 0.26 ± 0.04, n=8) and Homo (0.43, n=1), but the small number of samples tested cannot control for potential variability especially among Homo. Furthermore, Sr/Ca ratios may vary among potential food resources and by geographic location. Although the range in Sr/Ca ratios observed in carnivores (0.17-0.22, n=14) and herbivores (0.3-1.0, n=15) from this region appear not to overlap, it is also possible that Homo could have obtained an herbivorous signature through the selective consumption of underground plant storage organs or other plant parts with high Sr/Ca ratios.

The sample-size limitation of the earlier study is addressed through the analysis of an additional fifteen Australopithecus and Homo fossils from Swartkrans and Sterkfontein, and the natural variability in Sr/Ca ratios is  considered in greater detail. A solubility profile procedure involving 20 dilute acid rinses per fossil specimen is used to monitor diagenesis that takes place after burial, and multiple elemental analyses are performed to ensure reproducibility of our results. Our results are evaluated for potential differences in dietary adaptations of these hominid species, and their dietary adaptations relative to other mammals. Finally, the implications of this study for understanding early hominid evolution are discussed.

Support for this research was provided by Sigma Xi and the Transvaal Museum.