Final answer:
Gracile australopithecines had smaller teeth and jaws with a more pronounced facial projection, while robust australopithecines had large temporalis jaw muscles, sagittal crests, and larger teeth for tough diets. Robust australopithecines are not ancestors of modern humans but represent an extinct branch. These differences indicate adaptations to diet and environment.
Step-by-step explanation:
The robust and gracile australopithecines are classifications within the Australopithecus genus that are differentiated by distinct physical characteristics. Gracile australopithecines, which include species such as Australopithecus anamnesis, A. afarensis, A. africanus, A. garhi, and A. sediba, are characterized by smaller teeth and jaws, less flared cheeks, a more pronounced facial projection (prognathism), and the absence of a sagittal crest. In contrast, robust australopithecines, classified under the genus Paranthropus and including species like Paranthropus robustus, P. boisei, and P. aethiopicus, have more pronounced features such as large temporalis jaw muscles supported by a sagittal crest, flared zygomatic arches, and larger teeth adapted for chewing tough plant materials. The gracile species appeared around 4 million years ago (MYA) and vanished 2 MYA, whereas the robust species persisted for another million years.
Paranthropus aethiopicus is known to possess traits of both gracile and robust australopithecines, depicting an interesting mix of features. Paranthropus species, which includes robust individuals, are not considered ancestors of modern humans but represent a lineage that ultimately went extinct.
These physical adaptations in gracile and robust australopithecines indicate their various dietary preferences and environmental pressures, which contributed to their morphological diversity within the early hominin fossil record.