Men's faces look different from women's because their ancestors evolved to survive being punched during fights, a new study suggests.

University of Utah researchers David Carrier and Michael Morgan make that argument based on a study that looked at:

The skulls of early hominins — human relatives — called australopiths that lived around four million years ago.

The injuries of modern humans who show up in the hospital emergency room after a fight.

A reconstruction of the head of the early human relative Australopithecus afarensis is on display in the Hall of Human Origins in the Smithsonian Museum of Natural History in Washington, D.C. Australophiths lived about four or five million years ago. (Reconstruction by John Gurche; photographed by Tim Evanson) The faces of australopiths, especially the males, evolved to be very different from those of chimpanzees and gorillas – their jaws grew shorter and more robust, their molars got bigger, their cheek bones grew bigger and thicker, and so did the bones protecting their eyes, said Carrier, a biologist.

Anthropologists had previously suggested these changes may have been adaptations to changes in their diet, such as one that contained more nuts with shells that had to be crushed with hefty jaws and molars. However, Carrier and Morgan, a physician specializing in emergency medicine, used emergency room records as a starting point to explore a different possibility.

The hospital records show that when humans fight hand-to-hand, they usually strike and injure each other's faces, Carrier said.

"The bones in face that break most frequently are the same bones that underwent the most increase in robusticity in australopiths," Carrier said in a phone interview with CBC News.

These are the skulls of, top to bottom, a chimpanzee, an australopith (Australopithecus afarensis), another australopith (Paranthropus boisei), a human ancestor called Homo erectus, and a modern-day human. (University of Utah)

He suggested that australopiths evolved thicker, sturdier bones in those places to protect themselves from blows to the face, which were likely becoming more powerful and dangerous at that point in their evolution.

"The same time that these changes are happening in the face of autralopiths … proportions of the hand are changing in such a way that they were able to form a fist," Carrier said.

The researchers published their findings in the journal Biological Reviews.

The fact that the facial bones and teeth that changed the most in autralopiths are the ones that are most different between modern-day men and modern-day women – who get into far fewer fistfights — is further evidence that they changed to take punches, Carrier suggests.

Male chimpanzees and gorillas, he added, also fight far more than females and have sex differences in their facial features corresponding to the way they fight. For example, males have much bigger canines for biting.

The researchers acknowledge that modern-day humans do have smaller, less robust jaws, teeth and facial bones than australopiths. In fact, some of these bones are smaller than they are in chimpanzees and gorillas. The researchers suggest that as humans evolved, their upper body strength was reduced (compared to earlier human relatives) so their punches were no longer as powerful, and their faces evolved accordingly. They also began using weapons that targeted other parts of their bodies, such as the top of the skull – a part that Carrier says is better reinforced in humans than earlier hominins.