Researchers have suggested that human hands have evolved for fighting.
Compared with apes, humans have shorter palms and fingers, and stronger, longer, flexible thumbs.
Experts have long thought that these features evolved to make tools.
But now new research from the US suggests that these traits were shaped by violence too.
One of the most notable traits of the hand is that it can form a fist for punching.
Professor David Carrier, from the University of Utah, said on the matter: "The role aggression has played in our evolution has not been adequately appreciated.
"There are people who do not like this idea but it is clear that compared with other mammals, great apes are a relatively aggressive group with lots of fighting and violence, and that includes us.
"We're the poster children for violence."
To test the theory, Carrier conducted experiments with volunteers who had martial arts experience.
They were encouraged to hit a punch bag in a variety of different ways.
The findings, published in The Journal of Experimental Biology, supported the conclusion that tightly clenched fists are much more efficient weapons than open or loosely curled hands.
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