The fist could have shaped the evolution of the hand by giving early man an edge in combat.
"We're ... really trying to understand evolutionary history," Carrier said. "If selection for aggressive behavior was important in the very beginning ... at some level this is part of us."
He conceived the study a decade ago while arguing with a colleague about a paper he'd written positing that sperm whales developed large foreheads to use as battering rams against each other. The colleague disagreed, and to make his point he curled his hand into a fist.
"He said, 'I can hit you in the face with this, but that's not why it evolved,'" Carrier said. "The thought crossed my mind that maybe it did."
After years of false starts, he and Morgan, a U. medical student, developed a set of three experiments to test the theory in a study funded by the National Science Foundation.
First, they looked at how hard a person can hit with a fist, asking 10 test subjects, men ages 22 to 50, to deliver six kinds of hits. To their surprise, they found a fist doesn't deliver any more force than an open palm though it does deliver the same force with one-third the surface area.
Pressure and force tests showed the fist stabilizes and protects the hand by increasing the stiffness of the knuckle joint and allowing force to transfer from the hand to the wrist.
While the fists' increased force-per-surface area means it could more effectively inflict injuries, "what we think is more important is the protective side," Carrier said. An open palm could be more easily bent and twisted, hurting the aggressor.
And while increased dexterity explains why the thumb became longer, Carrier said it doesn't fully explain why the fingers and palms also shortened. Natural selection favoring clenched fists could.
"We're not arguing that other things couldn't have played a role," Carrier said, including another theory that posits the shape of the hands changed as our feet adapted to life on the ground. "We're just arguing that maybe aggression was also involved."
But the results aren't convincing for biologist Brigitte Demes of Stony Book University in New York. While the study was sound, she said, Carrier's conclusion was a leap.
"He shows that a closed fist is better buttressed for fighting," she said. "That doesn't necessarily mean our hands evolved for it. We do a lot of things with our hands that are probably more important than fighting."
Fossil evidence shows the human hand developing and growing more deft around the same time as the first tools came into use.
"There's not much evidence in the fossil record to point to [the fist developing for fighting]," she said, adding that our closest primate relatives maim and kill effectively without fists.
Milford Wolpoff, a paleoanthropologist at the University of Michigan, disagreed. Violence is certainly a large part of human history, he said, and the idea that fighting may have shaped our evolution makes sense.
"I do believe we're learning something about the use of the human hand that nobody really considered," Wolpoff said. "If [Carrier is] outside the bell curve, he's dragging the bell curve toward him."
Twitter: @lwhitehurst Fighting over fists
To read more about the study, visit http://bit.ly/Vc27iw.