Biologists have used the arms of dead men to punch and slap padded dumbbells in a macabre experiment supporting a hotly debated theory that our hands evolved not only for manual dexterity, but also so males could fistfight over females.

Scientists at the University of Utah tested the hypothesis that a clenched fist protects the fragile bones inside the hand — and found a clenched fist could strike a blow with double the force of an open-handed slap.

Humans have shorter palms and fingers and longer, stronger and more flexible thumbs than other apes.

Our hands are a unique feature, which have long been thought to have evolved so our ancestors had the manual dexterity to make and use tools.

A clenched or buttressed fist, an open or unbuttressed fist, and an open hand. ( Supplied: Denise Morgan, University of Utah )

But the study, led by biology professor David Carrier and published in the Journal of Experimental Biology, argues that human male hands evolved partly for punching.

The eight male cadaver arms used in the experiment were placed in a pendulum-like apparatus so they could swing toward and punch a padded, force-detecting dumbbell with either a buttressed fist (thumb locked around index and middle fingers, which are curled tightly to the palm) or an unbuttressed fist (thumb outward and not touching the loosely folded fingers) or side-slap the target with an open hand.

After hundreds of punches and slaps, the study suggested, "that humans can safely strike with 55 per cent more force with a fully buttressed fist than with an unbuttressed fist, and with twofold more force with a buttressed fist than with an open hand slap".

Scientists counter claims of 'bro science'

Professor Carrier has previously argued that the evolution of our faces was also partly driven by violent behaviour.

In 2014, he led a study hypothesising that the faces of our ancestors had evolved to resist punching and that human faces became more delicate as our violence became less dependent on brute force.

The theory has been criticised by some, including National Geographic evolution blogger Brian Switek, who called the theory "bro science — dudes pummelling each other driving human evolution".

Professor Carrier acknowledged the idea that aggressive behaviour played a role in the evolution of the human hand was controversial.

"Many sceptics suggest that the human fist is simply a coincidence of natural selection for improved manual dexterity. That may be true, but if it is a coincidence, it is unfortunate," he said.

"As an alternative, we suggest that the hand proportions that allow the formation of a fist may tell us something important about our evolutionary history and who we are as a species.

David Carrier has suggested human hands did not evolve solely for manual dexterity, but also for male-male fistfights over females. ( Supplied: University of Utah Marketing and Communications )

"If our anatomy is adapted for fighting, we need to be aware we always may be haunted by basic emotions and reflexive behaviours that often don't make sense — and are very dangerous — in the modern world."

Critics have also said that if men were adapted for fistfighting, humans would exhibit much larger differences in body mass between men and women.

But Professor Carrier said the male-female difference in lean body mass and upper body muscular strength are relatively large — and that differences in hand shape and size are among the most significant.