Bare-knuckle fighting has left its mark on the human face, according to scientists who believe it helped to shape how we look today.
A radical new theory about how violence changed our facial appearance over millions of years suggests it evolved to minimise the damage inflicted by a fast-moving fist.
The transition in facial structure from apes to early hominins had previously been explained largely by the need to chew on nuts and other hard foods that needed crushing, which led to a robust jaw, large molar teeth, a prominent brow and strong cheek muscles.
However, scientists have now devised another plausible explanation based on the need for the face to be buttressed against the impact of flying fists, which had become a principal weapon in unarmed combat between competing males.
"We suggest that many of the facial features that characterise early hominins evolved to protect the face from injury during fighting with fists," said Dr David Carrier and Michael Morgan in a study published in the journal Biological Reviews.
The researchers analysed the facial bone structures of a number of hominins, such as an early human ancestor known as Australopithecus, and compared them with apes and modern man.
They found that the parts of the face that changed most were the ones most likely to be damaged in a fist fight.
They also found that these changes in facial anatomy closely coincided with the ability of the early hominins to clench their fists and to use them as swinging clubs in a fight - a key tactical change from the biting and scratching preferred by fighting apes.
"The australopiths were characterised by a suite of traits that may have improved fighting ability," said Carrier of the University of Utah in Salt Lake City.
"These included hand proportions that allow formation of a fist, effectively turning the delicate musculoskeletal system of the hand into a club for striking,"