Were human hands built for punching? A new study of cadaver arms suggests this may be the case.
By using the arms to hit a dumbbell three different ways, scientists determined that a clenched fist makes it much safer to serve someone a knuckle sandwich.
Compared with primates such as chimpanzees, humans have developed a very different hand structure that features a shorter palm, shorter fingers and a relatively longer thumb. The conventional wisdom holds that our hands evolved for more dexterity, allowing our ancestors to wield and manipulate tools.
"There's every reason to believe that," said David Carrier, a comparative physiologist at the University of Utah.
However, Carrier and his colleagues have put forth another, perhaps complementary, idea. They posit that as the human hand was becoming more delicate, it may have evolved into just the right shape to fit into a fist — all the better for punching opponents without accidentally breaking one's bones.
It's a controversial theory, and one that Carrier has been building upon for some time: His earlier work argues that the male human face may have developed its robust jaw, cheeks and brow to withstand more punches, presumably during competitions for mates.
The scientist has long looked at connections between physical evolution and a propensity for violence in humans and other animals. These ideas have been met with a certain amount of skepticism, which the scientists have at times (cheekily) noted in their research. For instance, a study examining the way a dozen experienced fighters threw punches included a surprising expression of gratitude to one of their critics.
"We thank Professor Frank Fish for suggesting the null hypothesis with a wave of his fist and the exclamation, 'I can hit you in the face with this, but it did not evolve for that!'" they wrote.
Carrier and Fish, a biomechanist at West Chester University in Pennsylvania, had been arguing at a conference many years earlier about whether the sperm whale's bulging head had developed for ramming other male whales to compete for access to females. Fish believed the answer was no, and he balled up his hand into a fist to emphasize his point. (No blows were thrown, of course.)
The moment proved inspirational.
Frank was waving his fist at my face," Carrier said. "He was suggesting that the human hand did not evolve for fighting, and I thought, 'Wait, maybe it did!'"
For the new study, Carrier and his team obtained nine male arms from body donor programs. They attached fishing line to the tendons connected to muscles in the forearm, allowing them to control the wrist, thumb and fingers.
They also attached strain gauges to the delicate bones in the palm known as the metacarpals, which would be at a high risk of being damaged during a fight.
The researchers mounted each arm to a platform that swung like a pendulum, bringing the fist into contact with a padded dumbbell that had an accelerometer attached. That made it possible to measure the force the fist experienced upon contact.
One arm turned out to be too arthritic, but the scientists used the other eight to whale on the padded weight in three positions — a clenched fist, an unclenched fist (without the protection of a thumb and fully curled fingers) and an open-handed slap.
As expected, they found that the clenched fist, buttressed with the fingers tightly curled into the palm and the thumb providing reinforcement across the knuckles, reduced deformation in the metacarpals, thus lowering the risk of breakage.
"Our results suggest that humans can safely strike with 55% more force with a fully buttressed fist than with an unbuttressed fist," Carrier and his colleagues wrote. The tight fist also allowed for twice as much force as an open-handed slap, they added.
As our hands grew more dexterous and delicate, the fist shape allowed men to use it as a weapon, presumably to compete for access to potential mates. That behavior is seen in many other primates, Carrier said.
But Fish, who holds a black belt in tae kwon do, pointed out that many body parts — feet, knees and pointy elbows — can easily be deployed in a fight. That doesn't mean they evolved to be effective weapons, just that they are handy for such purposes.
"I think he's moved the topic along," Fish said of the new study, which he wasn't involved in. "I don't think it's the final word because I don't think he has totally invalidated other hypotheses."
David Puts, a biological anthropologist at Pennsylvania State University who was not involved in the research, agreed.
"I don't think that, by itself, it's convincing, but the authors are building an increasingly convincing case," Puts said, referring to their previous work on male faces and fists.
One way to dig deeper, Fish said, would be to study fossils from our predecessors, including species in the genus Australopithecus, and see whether there have been changes through time that developed a better-buttressed fist.
One potential weak spot is the fact that the researchers tested only male arms, not female ones.
"I wonder about sex differences in the hand anatomy," said Richard Wrangham, a biological anthropologist at Harvard University who was not involved in the paper. "One would clearly predict major differences."
Carrier's line of research has its detractors. Carrier said they are probably concerned that some might (incorrectly) conclude that his findings could be used to sanction violent behavior.
"I think some of that's understandable. There's a fear that if there is evidence that we are anatomically specialized for aggressive behavior, that might in some way justify violence, might justify aggression, might justify bad behavior," Carrier said. "And the way I respond to that is by saying understanding is not justification."