"Turns out when humans fight, the primary target is the face," Carrier said. "It's what people strike at. The vast majority of the injuries that occur in fractures [from interpersonal violence] are localized in the face."
The coauthors chiefly focused on the anatomy of skulls from a type of hominin that dates back 2 million to 4 million years ago in eastern Africa.
"The teeth were very big," Carrier said. "The mandible and the bones of the upper jaw become more stout, more robust. They're thicker; they're bigger."
The jaw muscles would have absorbed some of the energy of a punch and reduced the risk of fracturing or dislocating the upper and lower jaws, their study said.
"The activation of the jaw and neck muscles stiffens the connection between the head and body, decreases acceleration of the brain upon impact and therefore reduces the risk of concussion," their study said.
Morgan said it makes sense to develop structural traits to protect the most valuable human asset the brain. And the skeletal structure of the face was too large to develop for diet alone, they contend.
"One of the things we address in our paper ... is that if you think the jaw and the structure of the jaw and the teeth were built for that diet, they're massively overbuilt," Morgan said.
If the bones formed to increase strength when consuming food, "bone strain produced by chewing would be relatively uniform through the facial skeleton," the researchers said.
Instead, they found bone strain in the nasal region, cheek bones and eye sockets was low during chewing.
Also, data uncovered in recent years suggests the hominins didn't have an exclusively nut and seed diet, they note. Studies analyzing microwear patterns on the teeth of early hominins and other evidence suggest they more often ate fruits and grasses.
The two theories are not mutually exclusive, Carrier and Morgan said. "In nature oftentimes we see coevolution of numerous traits that can serve multiple purposes," Morgan said.
Past findings that males are more violent than females, and more often attack other males, also support the theory, the study said.
The areas of the skull that differ most between the sexes are also the areas that most frequently fracture during fighting, the researchers found. And the mechanics of chewing can't explain the stronger neck and jaw muscles in males and other gender differences, they add.
Skulls became less robust as the most common threat the fist also decreased in size, Morgan said. The researchers speculate that upper body strength decreased as weapons were invented and developed.
Their 2013 study claiming the human hand evolved to form a fist for more effective fighting drew skepticism from some scientists.
Showing that "a closed fist is better buttressed for fighting" doesn't prove that hands evolved for it, biologist Brigitte Demes, of Stony Book University in New York, said after the earlier study was released.
Fossil evidence shows the human hand developing and growing more deft around the same time as the first tools came into use, she said.
Carrier is now studying the foot posture of great apes, continuing to explore the hypothesis that violence played a greater role in human evolution than previously believed. The emphasis on aggression is "very uncomfortable and off-putting to people," Morgan said.
But the researchers argue humans need to understand their violent nature.
"We're going to be better able to prevent violence in the future if we have a better capacity to react to fear and anger and hatred," Carrier said. "That's the reason why this work is important. It's addressing this debate about our history, our deep history."
Morgan added: "We hope that with our research we can hold the mirror up to our faces and say, 'Yeah, this is who we were. How do we change our nature?' "