Australopithecus, was a bidepal primate directly ancestral to modern humans. It has been described as the missing link. They lived in South East Africa, 3.9-3.0 Million Years Ago, in an ideal (for hominid evolution) patchwork of open savannah grasslands and forests. They were about 3-4 ft tall as females and less than a foot taller as males. They are famed as the first truly bipedal primates - though the history of hominids has now been pushed back to 7 Million Years Ago, in the very primtive ape Sahelanthropus tchadensis.
Australopithecus would have been a generalist, able to adapt to changing seasonal conditions and the hardships involved with them, switching diets and supplementing them where-ever possible. They would have primarly feed on soft leaves, berries, nuts, fruits, seeds, plant tubers (rhinozomes) and other vegetation. Crucially, though, they occassionally supplemented their often harshly limited (in drought conditions at any rate) herbivorous diet, with meat. This carnivorous element of their diet, was sourced from carrion, and this gave them in turn a source of high protein. In turn again, this allowed the brains of these early human ancestors to increase in capacity of tens of thousands of years - ultimately leading to the line of hominids in the Homo genus that would eventually give rise to the species Homo Sapiens; us that is.
They would have lived in social groups for mutual support and protection, with males possibly contesting for breeding rights and dominance over a hareem of females and control over rival males. It is possible that females had a large part in deciding the males that could be 'in charge'. They lived in a time of many dangerous mammals that also lived alongside them in the Late Pliocene of Africa; Giant Deinotheres such as Deinotherium, relatives of the elephants only three times the size and less intelligent (so more naturally agressive) bull-dozed their way through the landscape, and an array of mammalian carnivores prowled in the grasses - such as the sabre-toothed cat Dinofelis, as well as the more recently evolved Lions and Leopards. At one site, Schwartzkroft Cave, a grizzly discovery was made. In one cave, hundreds of australopithicine (and others like gazelles) bones and particularly skulls, were found. In all, two distinctive 'bullet-hole punch marks', were found in the cranium area of the skulls. They were always evenly spaced and in level with each other. It seemed to be a site of human cannibalism to some - though in truth, when sabre-teeth were tested to fit into these horrific wounds, it became all too obvious what was actually going on here. All this was, was a sabre-tooth cats piled up meal scraps, built up over a sustained period of time that the killer inhabited the cave. it would have killed its prey, and dragged it back into either a tree or as it seems in this case a handy cave to avoid competition from other predators for eating it. Australopithecines, then, were in a sense, cat food. Nevertheless, they outlasted the predation and led to all future descendants - without their success, we wouldn't even be here at all. It is even possible that they could occasionally 'stand up' to predators - much in the same way as baboons, driving them off as a united front of superior numbers; screeching and holloring.