Final answer:
The word 'gracile' refers to slender or delicate features, and in paleoanthropology, it describes australopithecines with lighter, less robust skeletal features.
Step-by-step explanation:
The term gracile generally means slender or delicate in form. In the context of human evolution, it refers specifically to certain species of australopithecines that had lighter, less robust skeletal features compared to their counterparts. Gracile australopithecines such as Australopithecus anamensis, A. afarensis, A. africanus, A. garhi, and A. sediba had a more pronounced projection of the jaw (prognathism), less flared cheeks, no sagittal crest, and smaller teeth and jaws.
On the other hand, the robust australopithecines like Paranthropus robustus, P. boisei, and P. aethiopicus had features adapted for chewing tough plant materials, including larger teeth, jaws, and a prominent sagittal crest accommodating larger temporalis jaw muscles. These physical traits offer insight into the different roles and diets these species may have had in prehistoric landscapes.