brain size: 310 - 530 cubic centimeters
intelligence: used simple tools (such as sticks), probably had some form of language
largest group size: 55
other: Australopithecus only differed from Ardipithecus and earlier ancestors in that it walked upright, or was bipedal, and may have had a slightly larger brain.
Timeline
Ardipithecus | Australopithecus | Homo habilis | Homo erectus | Neandertals | Modern Humans