Four million years ago, if you broke your jaw, it was probably a fatal injury. You wouldn’t be able to chew food. You’d just starve to death. ~ American evolutionary biologist David Carrier
Early hominids losing the sharp canine teeth of their primate ancestors took much of the bite out of biting an opponent. There was compensation at hand.
Australopiths evolved numerous traits that endowed greater fighting ability, including a hand that afforded formation of a fist. This turned a delicate musculoskeletal system into a club.
Contemporaneously, the faces of males diverged from those of females. The facial bones that differ most are those that strengthened to protect the face from injury during fist fights.
The function of the fist was reinforced by bipedality. Unlike other primates, apes and hominids walk on their heels. This body posture lends extra punching power, and suggests physical conflict was instrumental in hominin descent.
Later Homo descent traded weaker muscles for comity. Human arm and upper body strength is much less than australopiths. This reduction in damaging ability afforded more gracile bodies, even as facial bone differences still characterize human sexual dimorphism.