20 December 2012

Human hands evolved for punching, not just dexterity, says prof

by Will Parker

A University of Utah study suggests human hands evolved not only for the manual dexterity needed to use tools or play a violin, but also so we could make fists and punch hard.

"The role aggression has played in our evolution has not been adequately appreciated," says Professor David Carrier, senior author of the study appearing in the Journal of Experimental Biology. "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."

Humans have debated for centuries about whether we are, by nature, aggressive animals. Carrier has explored this theme previously in another study that focused on the combat benefits of standing upright.

Carrier agrees that human hands are evolved for manual dexterity, but he says that's only part of the story. "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.

Carrier suggests that 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.

To investigate, Carrier and co-researcher Michael H. Morgan (pictured) tested the hypothesis that humans can hit harder with a fist. Working with subjects who had boxing or martial arts experience, the researchers measured the force delivered as the subjects hit a punching bag with six kinds of strikes: overhead hammer fists and slaps, side punches and slaps, and forward punches and palm shoves.

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 was 1.7 to 3 times greater with a fist strike compared with a slap. "Because you have higher pressure when hitting with a fist, you are more likely to cause injury to tissue, bones, teeth, eyes and the jaw," Carrier noted.

The second and third experiments tested the hypothesis that a fist provides buttressing to protect the hand during punching by measuring 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 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.

Carrier 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 contended.

Carrier's work challenges the idea that once our ancestors came out of the trees, the selection for climbing was gone, so evolutionary selection for manipulation became dominant, changing the shape of our ancestors' hands. "Human-like hand proportions appear in the fossil record at the same time our ancestors started walking upright 4 million to 5 million years ago. An alternative possible explanation is that we stood up on two legs and evolved these hand proportions to beat each other," Carrier suggests.

Carrier says that if manual dexterity was the only driving force, humans could have evolved manual dexterity with longer thumbs without the fingers and palms getting shorter. But, he adds, "there is only one way you can have a buttressed, clenched fist: the palms and fingers got shorter at the same time the thumb got longer."

Carrier notes that the human hand presents something of a paradox. "It is arguably our most important anatomical weapon, used to threaten, beat and sometimes kill to resolve conflict," he said. "Yet it is also the part of our musculoskeletal system that crafts and uses delicate tools, plays musical instruments, produces art, conveys complex intentions and emotions, and nurtures. More than any other part of our anatomy, the hand represents the identity of Homo sapiens. Ultimately, the evolutionary significance of the human hand may lie in its remarkable ability to serve two seemingly incompatible but intrinsically human functions."

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Source: University of Utah