A couple of recent perspectives on Australopithecus and early Homo, from researchers at my school, the University of Utah.
Tropical grasses and sedges commonly use the C4 pathway for photosynthesis, which evolved from the C3 pathway used by most plants, in response to lower atmospheric carbon dioxide levels. We can trace an isotopic signature in the teeth of animals that eat C4 plants (and animals that eat those animals). From about 3.5 Mya we see evidence of hominins eating more C4 foods: either grasses (maybe including underground parts) or animals that ate grasses. Paranthropus boisei seems to be a particular champion with the C4s. Chimpanzees, even when they live in open woodlands, mostly stick to eating fruits and leaves. By contrast, exploiting grassroots and sedges to varying degrees may be an important component of australopithecine adaptation to the savannah.
Not only do australopithecines have enlarged back teeth relative to great apes, but they also have smaller canines. Canines are mostly good for fighting, so it’s nice to think that canine reduction is a sign of our ancestors becoming more peaceful. Could be. But it’s possible they just switched fighting styles. David Carrier argues that australopithecine faces are built to withstand punches, starting from the time that australopithecine hands were capable of forming fists. This might go together with high levels of sexual dimorphism – males, the more violent sex, being bigger than females (but early hominin sexual dimorphism is a contentious subject.)