Today's Nature has a study comparing the teeth of the hominins Australopithecus africanus and Paranthropus robustus. A new method for studying microwear in three dimensions allows the researchers to categorize the diets of ancient species:
the 'gracile' Australopithecus africanus ate more tough foods than Paranthropus robustus, and that Paranthropus ate more hard, brittle items as part of more varied diet.
But the bigger advance from the study was that the new method also allowed evaluation of overlap in diet. In the linked interview, the authors emphasize substantil overlap, and speculate that most of the time both hominins both preferred easy-to-get foods like fruits. The differences in wear may actually reflect seasonal specialization. So, to quote from the article:
This suggests that early hominin diet differences might relate more to microhabitat, seasonality or fall-back food choice than to oversimplified, dichotomous food preferences.