Final answer:
Gracile australopithecines had a more pronounced jaw projection, smaller teeth, and less robust facial features, suited for a softer diet; robust australopithecines had larger teeth and a distinctive sagittal crest for chewing hard plant materials. They are separate branches of early hominids, with robust forms surviving longer. Paranthropus aethiopicus shows traits of both classifications.
Step-by-step explanation:
The student's question pertains to the comparison between gracile and robust australopithecines, which are two classifications of early hominid species based on their physical and morphological differences. Gracile australopithecines, such as Australopithecus anamensis, A. afarensis, A. africanus, A. garhi, and A. sediba, emerged around 4 million years ago (MYA) and disappeared by 2 MYA. They had a more pronounced projection of the jaw (prognathism), no sagittal crest, less flared cheeks, and smaller teeth and jaws suited for a softer diet. In contrast, the robust australopithecines, classified under the genus Paranthropus, which include Paranthropus robustus, P. boisei, and P. aethiopicus, had a distinctive sagittal crest, flared zygomatic arch, and large teeth designed to chew tough plant materials. Robust australopithecines existed for another million years beyond the gracile forms. While Paranthropus is not considered ancestral to modern humans, the gracile forms are part of our evolutionary lineage. Paranthropus aethiopicus presents an interesting case with characteristics of both robust and gracile australopithecines.