Final answer:
Gracile Australopithecines, such as A. africanus and A. afarensis, lived around 2 to 3 million years ago and displayed trends like reduced dentition and prognathic jaws. The robust australopithecines, like P. robustus and P. boisei, had adaptations for chewing tough plant materials and survived until about 1 million years ago. Notable specimens like Lucy are crucial for understanding early human evolution.
Step-by-step explanation:
The gracile Australopithecines that appeared later, around 3 million years ago (2mya), are significant in the study of human evolution. Among them, species like Australopithecus africanus and Australopithecus afarensis are well studied. These species had a more slender build and were bipedal, displaying evolutionary trends such as reduced dentition and jaw size compared to earlier hominids and apes. Notably, A. africanus lived between two and three million years ago and had slightly larger brains than A. afarensis, with A. africanus brain size being around 500 cubic centimeters. Australopithecus afarensis, with well-known specimens like Lucy (dated to about 3.75-2.8 MYA), also showed a trend toward smaller teeth and prognathic jaws.
The robust australopithecines, classified under the genus Paranthropus, included species like Paranthropus robustus, P. boisei, and P. aethiopicus, which survived until about 1 million years ago. The robust forms differed from the gracile species by having larger teeth and jaws, a sagittal crest, and more flared cheeks, which were adaptations for a tough, plant-based diet.