Five million years of fist fights has left its mark on the human face, scientists believe.
Evidence suggests it evolved to minimise damage from bruising altercations after our ancient ancestors learned how to throw a punch.
Researchers studied the bone structure of australopiths, ape-like bipeds living four to five million years ago that pre-dated the modern human primate family Homo.
They found that australopith faces and jaws were strongest in just those areas most likely to receive a blow from a fist.
It is a legacy that continues to this day, helping to explain why men's faces are more robust than women's, say the scientists.
Just as they are today, men were the less gentle of the sexes at the dawn of human evolution, and more likely to get into the prehistoric equivalent of a bar fight.
US lead researcher Dr David Carrier, from the University of Utah, said: "The australopiths were characterised by a suite of traits that may have improved fighting ability, including hand proportions that allow formation of a fist; effectively turning the delicate musculoskeletal system of the hand into a club effective for striking.
"If indeed the evolution of our hand proportions were associated with selection for fighting behaviour you might expect the primary target, the face, to have undergone evolution to better protect it from injury when punched."
The study, published in the journal Biological Reviews, builds on previous work indicating that violence played a greater role in human evolution than many experts would like to admit.
In recent years, biologist Dr Carrier has investigated the short legs of great apes, the bipedal posture of humans, and the hand proportions of "hominins", or early human species. He argues that these traits evolved, to a large extent, around the need to fight.
"When modern humans fight hand-to-hand, the face is usually the primary target," said Dr Carrier, commenting on the latest research.
"What we found was that the bones that suffer the highest rates of fracture in fights are the same parts of the skull that exhibited the greatest increase in robusticity during the evolution of basal hominins.
"These bones are also the parts of the skull that show the greatest difference between males and females in both australopiths and humans. In other words, male and female faces are different because the parts of the skull that break in fights are bigger in males."
The debate over the dark side of human nature can be traced back to the French philosopher Jean-Jacques Rousseau, who argued that before civilisation humans were "noble savages".
This idea, that civilisation corrupted the human race and made us violent, remains strong in social science, said Dr Carrier. However, the facts did not fit the theory.
Dr Carrier added: "The hypothesis that our early ancestors were aggressive could be falsified if we found that the anatomical characters that distinguish us from other primates did not improve fighting ability.
"What our research has been showing is that many of the anatomical characters of great apes and our ancestors, the early hominins (such as bipedal posture, the proportions of our hands and the shape of our faces) do, in fact, improve fighting performance."
Co-author Dr Michael Morgan, a University of Utah physician, said: "I think our science is sound and fills some long-standing gaps in the existing theories of why the musculoskeletal structures of our faces developed the way they did.
"Our research is about peace. We seek to explore, understand, and confront humankind's violent and aggressive tendencies.
"Peace begins with ourselves and is ultimately achieved through disciplined self-analysis and an understanding of where we've come from as a species. Through our research, we hope to look ourselves in the mirror and begin the difficult work of changing ourselves for the better."