Biologists say the human hand is a wonder of evolution, providing dexterity that lets our species perform activities as diverse as bricklaying, writing, ice hockey and brain surgery.
But what were the forces that, over thousands of years, sculpted our hand into the shape it is today?
Was it to grab and use primitive tools, as many experts surmise? To pick fruit and berries?
Now a new concept has risen: It was also to create a fist, so that we could fight other humans.
"The role aggression has played in our evolution has not been adequately appreciated," said David Carrier, a University of Utah professor who is promoting the novel theory with colleague Michael Morgan.
"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."
In a study published on Wednesday in the Journal of Experimental Biology, the pair say their theory is backed by models and experiments in biomechanics.
Modern chimpanzees -- our closest primate relatives -- have long palms and fingers with a short thumb, while the human palm and fingers are much shorter and the thumb longer and stronger.
This squat configuration means we can clench our fingers into a fist and fold the thumb across the knuckles.
By doing so, the thumb provides rigidity to the fist and protects the hand from damage during combat, Carrier and Morgan argue.
Chimp fingers, though, form an open doughtnut-style shape when they are curled.
The pair first asked martial-arts athletes to whack a punchbag, which measured the force delivered by slaps or punches. They were surprised to find that the punch did not deliver more force per blow than a slap.
The scientists then carried out a second experiment.
The same volunteers were asked to make variations of the fist shape -- the traditional one, and two in which the thumb was extended sideways -- and then press the first joint of the index finger against a gadget called a force transducer.
Having the thumb curled under the finger meant the fist delivered twice as much force as having the thumb extended sideways, the investigators found.
The reason: the thrust is delivered directly to the target via the palm bones of the thumb and the index finger.
In addition, having a buttressing thumb meant that the knuckle joint is four times more rigid, protecting the delicate handbones and ligaments from injury.
The authors agree that manual dexterity is a big driver in hand evolution, but argue aggression should also be factored in.
Early humans who acquired fist-making skills would have had a clear advantage in the struggle for resources, said Carrier.
"An individual who could strike with a clenched fist could hit harder without injuring themselves, so they were better able to fight for mates and thus more likely to reproduce," he said.
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