The African Plio-Pleistocene hominins known as australopiths evolved a distinctive craniofacial morphology that traditionally has been viewed as a dietary adaptation for feeding on either small, hard objects or on large volumes of food. A historically influential interpretation of this morphology hypothesizes that loads applied to the premolars during feeding had a profound influence on the evolution of australopith craniofacial form. Here, we test this hypothesis using finite element analysis in conjunction with comparative, imaging, and experimental methods. We find that the facial skeleton of the Australopithecus type species, A. africanus, is well suited to withstand premolar loads. However, we suggest that the mastication of either small objects or large volumes of food is unlikely to fully explain the evolution of facial form in this species. Rather, key aspects of australopith craniofacial morphology are more likely to be related to the ingestion and initial preparation of large, mechanically protected food objects like large nuts and seeds. These foods may have broadened the diet of these hominins, possibly by being critical resources that australopiths relied on during periods when their preferred dietary items were in short supply. Our analysis reconciles apparent discrepancies between dietary reconstructions based on biomechanics, tooth morphology, and dental microwear.