Originally, there were two strains of Australopithecus: one herbivore, the other carnivore.
The herbivore developed a stronger jaw but remained with a small brain, since plants aren't a challenge to find.
The carnivore developed a bigger brain, because they were mostly scavengers and finding good carrion wasn't an easy task. It eventually evolved into the Homo Erectus, and finally ended up having us as its descendants.