Human hands may have developed the ideal shape over time for punching, said a study using male cadaver arms.

By using the arms to punch a dumbbell while in different positions — a clenched fist, a relaxed fist and an open-palmed slap — scientists were able to determine that the clenched fist made it much safer to serve someone a knuckle sandwich without getting hurt.

The findings, published in the Journal of Experimental Biology, bolsters a controversial theory that ties human physiology to a violent past.

Compared with non-hominin primates such as chimpanzees, humans have developed a very different hand structure, with a shorter palm, shorter fingers and a relatively longer thumb. This is thought to be a result of hands evolving for more dexterity, allowing our predecessors to wield and manipulate tools.

“That’s the standard argument, it makes all kinds of sense. There’s every reason to believe that,” said lead author David Carrier, a comparative physiologist at the University of Utah.

But Carrier and colleagues have put forth a different, perhaps complementary idea: that as the human hand was becoming more delicate, it may have evolved into just the right shape to fit into a fist — all the better for punching opponents without accidentally breaking one’s own bones. The data suggest that humans can safely strike with 55 percent more force with a fully closed fist than with an open one. A fist allows for twice the force of an open-palmed slap before bones begin to break.

It’s a controversial idea, and one that Carrier has been building upon for some time: Last year, he and colleagues released a paper showing that the male human face may have evolved to withstand more impacts from being punched — presumably during competitions for mates.

Brigitte Demes of Stony Brook University was critical of the study, saying, “At best, pugilistic encounters as an explanation for the evolution of the human hand remains just another story.”

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