Final answer:
Australopithecines differ from chimpanzees with their larger molars for chewing tough plant material, increased brain size over time, and bipedal movement indicated by pelvic anatomy changes. Their evolutionary path showcases important adaptations leading to modern humans.
Step-by-step explanation:
Compared to chimpanzees, australopithecines exhibited numerous distinctive traits linked to their anatomy and evolution. One of the most notable differences is in their molars, which were larger compared to those found in chimpanzees. The increase in molar size is likely a result of dietary changes and the need to chew tougher plant materials. Australopithecines, such as Australopithecus afarensis and Australopithecus africanus, had a more robust dental architecture to support this function.
In terms of brain size, australopithecines had smaller brains relative to their body mass when compared to modern humans, but their brain capacity was larger than that of the common chimpanzee. For example, the endocranial capacity of Australopithecus afarensis was around 400 cc, approximately the same as a chimpanzee, whereas Australopithecus africanus had a slightly larger brain at 500 cubic centimeters. Despite the smaller brain size compared to humans, this upward trend in brain volume was significant on the evolutionary path to later hominins.
Bipedalism is another key feature in australopithecines. Differences in pelvic anatomy indicate that they moved bipedally, unlike tree-dwelling chimpanzees. The pelvis of australopithecines was shaped more like modern humans, being shorter and wider, which supports the assumption that they walked on two legs. As a result, this evolutionary adaptation set australopithecines apart from their four-legged primate cousins and represents a critical step towards modern human locomotion.