Final answer:
Gracile and robust australopithecines are two groups of early hominins that likely descended from a common ancestor. Gracile species had smaller teeth and jaws and more pronounced prognathism. Robust species had larger teeth and jaws with features such as a sagittal crest, adapted for a diet of tough plant materials.
Step-by-step explanation:
The gracile and robust australopithecines are two different classifications of early hominin species. Species such as Australopithecus anamensis, A. afarensis, A. africanus, and A. garhi are considered gracile australopithecines, which have smaller teeth and more pronounced prognathism. On the other hand, the robust australopithecines include Paranthropus robustus, P. boisei, and P. aethiopicus, known for their larger teeth equipped for grinding tough plant materials with characteristics like a sagittal crest and a more flared face. It is thought that gracile species evolved around 4 million years ago (MYA) and disappeared around 2 MYA, whereas robust species existed up to 1 million years after gracile species became extinct. Both groups likely descended from a common ancestor that was a previous form of early hominin. However, it is important to mention that Paranthropus species, including those that belonged to robust australopithecines, are not considered direct ancestors to modern humans.