Final answer:
Australopithecines likely used simple stones and sticks as tools for digging and breaking open hard shells, which are not preserved in the fossil record. These early tool uses are considered important evolutionary steps towards advanced cognition and even the development of language.
Step-by-step explanation:
The habitual tool used among Australopiths, including gracile and robust australopithecines, can be attributed to very basic stone tools that existed prior to the more advanced Oldowan tools associated with Homo habilis. These ancestors of humans had not yet developed the refined tool-making techniques that would come later, but evidence suggests that they likely used simple stones and sticks for tasks such as digging and breaking open hard shells. Given that these tools would have been made from organic materials such as wood, they have not survived in the fossil record; hence, we rely on our understanding of their dietary habits and other indirect evidence to infer the use of such tools. The capability to produce and use these tools has been linked to evolutionary developments in cognition, such as imitation, which may have laid the groundwork for language. Robust australopithecines, in particular, had adaptations like large temporalis jaw muscles indicated by a sagittal crest, implying a diet that included tough plant materials which might have necessitated the use of tools for processing them.