Human hands evolved to become 'fists of fury'
Human hands evolved not only for the manual dexterity needed to use tools, play a violin or paint a work of art, but so men could make fists and fight, researchers say.
Compared with apes, humans have shorter palms and fingers and longer, stronger, flexible thumbs - features that have been long thought to have evolved so our ancestors had the manual dexterity to make and use tools.
"The role aggression has played in our evolution has not been adequately appreciated," Professor David Carrier, senior author of the study from the University of Utah, said.
"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," he said.
Humans have debated for centuries "about whether we are, by nature, aggressive animals," he adds.
"Our anatomy holds clues to that question. If we can understand what our anatomy has evolved to do, we'll have a clearer picture of who we were in the beginning, and whether aggression is part of who we are," Carrier said.
Carrier agrees that human hands evolved for improved manual dexterity, but adds that "the proportions of our hands also allow us to make a fist," protecting delicate hand bones, muscles and ligaments during hand-to-hand combat.
As our ancestors evolved, "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 says.
Fights also were for food, water, land and shelter to support a family, and "over pride, reputation and for revenge," he adds.
"If a fist posture does provide a performance advantage for punching, the proportions of our hands also may have evolved in response to selection for fighting ability, in addition to selection for dexterity," Carrier said.
So Carrier and study co-author Michael H. Morgan - a University of Utah medical student - conducted their study to identify any performance advantages a human fist may provide during fighting.
The first experiment tested the hypothesis that humans can hit harder with a fist. So, Carrier and Morgan had 10 male students and nonstudents - ages 22 to 50 and all of them with boxing or martial arts experience - hit a punching bag as hard as they could.
Each subject delivered 18 hits, or three of each for six kinds of hits: overhead hammer fists and slaps, side punches and slaps, and forward punches and palm shoves. The bag was instrumented to allow calculation of the force of the punches and slaps.
To the researchers' surprise, the peak force was the same, whether the bag was punched with a fist or slapped with an open hand. However, a fist delivers the same force with one-third of the surface area as the palm and fingers, and 60 percent of the surface area of the palm alone. So the peak stress delivered to the punching bag - the force per area - was 1.7 to three times greater with a fist strike compared with a slap.
The second and third experiments - which each also involved 10 male subjects - tested the hypothesis that a fist provides buttressing to protect the hand during punching.
To do that, the researchers measured the stiffness of the knuckle joint of the first finger, and how force is transferred from the fingers to the thumb. Both measurements were made with normal, buttressed fists or when partial fists were not buttressed.
Humans buttress - strengthen and stabilize - fists in two ways that apes cannot: The pads of the four fingertips touch the pads at the top of the palm closest to the fingers. And the thumb wraps in front of the index and middle fingers, and to some extent the ring finger, and those fingers are locked in place by the palm at the base of the thumb.
To measure stiffness of the second knuckle joint, the study's 10 male subjects slowly pushed a pressure transducer, with clenched fists or with fingers bent but the fist unclenched. Researchers measured the force and also how much the index finger flexed.
Force transfer from fingers to the thumb also was measured, but in this case the subjects got in a one-handed pushup position, with their knuckles pushing down on a block placed on a different force transducer.
The second and third experiments found that buttressing provided by the human fist increased the stiffness of the knuckle joint fourfold (or reduced flexing fourfold), and also doubled the ability of the fingers to transmit punching force, mainly due to the force transferred from the fingers to the thumb when the fist is clenched.
"Because the experiments show the proportions of the human hand provide a performance advantage when striking with a fist, we suggest that the proportions of our hands resulted, in part, from selection to improve fighting performance," Carrier added.
The study has been published in the Journal of Experimental Biology.