Men whacked（重击） punching bags for a University of Utah study that suggests human hands evolved not only for the manual dexterity（手灵巧度） needed to use tools, play a violin or paint a work of art, but so men could make fists and fight. 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," says University of Utah biology Professor David Carrier, senior author of the study, scheduled for publication Dec. 19 by 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," Carrier says. "We're the poster children for violence."

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 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（韧带） 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 says.

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 research was funded by the National Science Foundation.