The researchers noticed that australopithecus -- the genus of bi-pedal hominids who directly preceded the human genus homo some 5 million years ago -- had facial features conducive to absorbing punches to the face.
Scientists had originally assumed that the robust cheeks, jaws, eye and nose features evolutionary adaptation to their hardy diet. But in their new study, the Utah scientists suggest the diet of the the australopiths was rather soft and full of fruit.
"When modern humans fight hand to hand, the face is usually the primary target," Dr. David Carrier said.
Therefore, Carrier and his colleagues concluded -- in their study, published recently in the journal Biological Reviews -- early humans evolved to protect their faces when violence was prevalent.
Modern humans have sleeker, less buttressed facial features, as the threat of violence has waned.