A new study claims that punching may have helped our ancestors evolve a unique human hand shape, one capable of forming a fist. The experiment, which relied on cadaver arms strung up with fishing line and guitar tuners, is delightfully offbeat. But many researchers seem unconvinced by the findings.

David Carrier's ideas about the human hand are controversial, but that doesn't seem to bother him much. Ironically, his hypothesis — that ancient fisticuffs helped drive the evolution of the human hand — first came to him in the middle of a loud argument.

A decade ago, the University of Utah biologist had published a paper suggesting that sperm whale foreheads evolved as battering rams, meaning that male-on-male sexual competition had driven a quintessential feature of the species. An old friend of his didn't buy it, and soon they were yelling at each other in the hallway.

"At one point, to illustrate the point he was trying to communicate, he held his fist up in front of my face and said 'I can hit you in the face with this, but that’s not why it evolved!' And I thought hey, maybe it did," Carrier told The Post. "I didn’t say that at the time, because he was already upset."

Since then, Carrier has published several papers on the subject — and they've all been controversial. The studies center on the thesis that the human hand, with its relatively delicate fingers, long thumbs, and short palms, may have been driven into its strange shape by fighting between males. No other primate can make a fist, Carrier points out, which is quite a convenient coincidence for us. He's even argued that the human face evolved its shape to better take a hit from a fist. Brian Switek, who blogs about evolution for National Geographic, accused one of Carrier's studies of being "bro science — dudes pummeling each other driving human evolution."

Even Carrier admits that the more traditional hypothesis, which is that our hands evolved to hold tools "makes perfect sense."

"We're masters of dexterity," Carrier said. "But what we’re suggesting is that maybe there’s another component."

In other words, Carrier feels that fisticuffs could have been a secondary but vital driver of hand evolution.

His latest study, published Wednesday in the Journal of Experimental Biology, sets out to test the resilience of the human fist as a weapon. He wanted to test how much different bones in the hand deform under the force of a blow, so he and his co-authors got hold of some cadaver arms — all donated to science, of course — and tested their punching power.

"We were using little transducers you have to glue directly to the bone," Carrier explained, "Which is obviously too invasive to do on a living subject."

The apparatus sounds pretty gruesome: Strong fishing line was tied directly to tendons in each arm, and the limbs were then mounted on a platform. The fishing line connected to guitar tuners, which the researchers could use to "tune" each arm tightly into whatever position they wanted. Then the whole platform would swing, sending the hand into a dumbbell with an accelerometer on it to calculate force.

The data suggests that humans can safely strike with 55 percent more force with a fully closed fist than with an open one (as shown above). A fist allows for twice the force of an open-palmed slap before bones begin to break.

Arizona State University's Mary Marzke was unimpressed with the hypothesis, pointing out that chimps and gorillas do manage "fist-like" grabbing gestures, even if they don't have thumbs long enough to make a true human fist. These animals don't use their half-fists for fighting, but for breaking up food.

"It also is surprising that the authors compared fist-punching with palm-slapping, instead of with striking by the heel of the palm," Marzke told The Post. "The latter is well known to be very effective in fighting. It has the advantage of concentrating force over a relatively small area of the hand, an advantage that the author highlighted for the fist in an earlier study. The palm heel punch does not require hominin thumb and finger proportions."

Brigitte Demes of Stony Brook University was also critical of the study, suggesting that the data didn't really prove anything one way or another — it just fit the narrative Carrier likes.

"The conclusions drawn are not based on evidence," Demes wrote in an e-mail. Just as Carrier's angry colleague suggested a decade ago, the capability to punch — even if punching really is superior to other hand-to-hand combat techniques — is not proof that the hand evolved for it. "At best, pugilistic encounters as an explanation for the evolution of the human hand remains just another story. There are many other behaviors that could be marshaled to explain the morphology of human hand bones," Demes said.

As far as Carrier is concerned, his naysayers are just scared.

"There’s a fear, and I don’t think it’s necessarily valid, but there’s a real fear that evidence suggesting we’re anatomically built for fighting could be used to justify bad behavior," Carrier said when asked about scientists who discount his research. "What I would argue is that if our goal is to reduce violence in the future, we need to understand what this dark side of human nature is all about."

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