Early hominins evolved faces that could withstand club-like blows, researchers say, to provide protection against that most human of weapons—the fist.

Without surgeons (or health insurance), early human males needed to develop their own line of defense against what was then the weapon of choice—the fist.

According to two University of Utah researchers, to protect against potentially deadly injuries sustained during fights with other men, early human males beefed up the bones and muscles of their faces and jaws. These prehistoric brawls likely revolved around resources, as well as potential mates.

“The teeth were very big,” study author David Carrier, a biologist, told the Salt Lake Tribune. “The mandible and the bones of the upper jaw become more stout, more robust. They’re thicker; they’re bigger.”

Additionally, the changes in the shape of the face allowed the jaw muscles to absorb more of the energy from a punch on the chin. This reduced the risk of fracturing or dislocating the upper and lower jaws, and also reduced the risk of concussion.

These changes in facial structure didn’t happen overnight. They evolved over many, many generations, especially in australopiths—the bipedal, ape-like early humans that lived four to five million years ago and immediately preceded the human genus Homo. The researchers claim that the ability of those hominins to take it on the chin—or nose or cheek—paralleled the evolution of the fist itself.

Related News: Is Modern Living Ruining the Human Species? »

The new “protective buttressing” theory goes head-to-head with the prevailing hypothesis that the face structure of early hominins evolved in response to the need to chew hard foods, like nuts and seeds.

Carrier, along with physician Michael Morgan, in a paper published last week in Biological Reviews, point out that recent studies—including those that looked at the wear pattern on fossilized teeth—suggest that early hominins lived less on a diet of nuts and seeds, opting instead for more fruits and grasses.

Given that our stony-faced ancestors lived millions of years ago, it may be impossible to rule out diet completely, or any other number of potential reasons for the face to become more robust.

“In nature oftentimes we see co-evolution of numerous traits that can serve multiple purposes,” said Morgan.

Read More: Ancient Human Secrets Revealed by New DNA Analysis »

To beef up their hypothesis, the researchers turned to recent studies of violence among humans—including one from the University of Bristol Dental Hospital.

“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 same bones that are likely to fracture in modern, untrained hand-to-hand combat between men are also the ones that evolved in early hominins to better withstand such club-like assaults.

Carrier and Morgan have been trying to understand why our ancient faces looked the way they did for many years, and they are no strangers to controversy.

In a 2013 study, they claimed that the early human hand evolved to become a fight-worthy fist, drawing 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 told the Salt Lake Tribune.

Heading off criticism, Morgan said, “I think our science is sound and fills some longstanding gaps in the existing theories of why the musculoskeletal structures of our faces developed the way they did.”

But the researchers will continue to investigate the evolution of early humans, especially as it relates to fighting ability. They are currently working on a study involving the foot posture of great apes, looking for signs that violence played a greater role in human evolution—something that might be equally applicable to modern ultimate street fighting.

The researchers emphasize that their study is really about promoting peace by helping us to better understand both our past and our present.

“Through our research,” said Morgan, “we hope to look ourselves in the mirror and begin the difficult work of changing ourselves for the better.”

Did You Know: One in Three U.S. Teens Experience Dating Violence, Girls More Likely to Get Physical »