There is a theory out there that male hands evolved not just for dexterity — climbing trees, grabbing rocks, using tools — but also so we could fistfight each other over women. Just like most other theories, a group of scientists decided to put it to the test by scientists.

In a study published in the Journal of Experimental Biology, researchers at the University of Utah used cadaver arms to punch and slap padded dumbbells with the goal of finding out whether men’s hands really evolved for the purpose of fistfighting. "The idea that aggressive behavior played a role in the evolution of the human hand is controversial," senior author and biology professor David Carrier said in a press release. "Many skeptics suggest that the human fist is simply a coincidence of natural selection for improved manual dexterity. That may be true, but if it is a coincidence, it is unfortunate."

Carrier suggests an alternative: the shape of the hand when it makes a fist tells us something important about our species and how we evolved. "If our anatomy is adapted for fighting," he said, "we need to be aware we always may be haunted by basic emotions and reflexive behaviors that often don't make sense — and are very dangerous — in the modern world."

Compared to primates, humans have shorter palms and fingers, but longer, stronger, and more dexterous thumbs. Long thought to have evolved to make and use tools, Carrier and his colleagues argue that our hands partly evolved for punching as well. They base this on the fact that the faces of our ancestors — the australopiths — evolved to resist punching. Our faces are more delicate now, however, because violence has evolved in a way that no longer depends on brute force, Carrier suggests. The team set out to find evidence of this theory by using nine cadaver arms in a series of experiments.

"We tested the hypothesis that a clenched fist protects the metacarpal [palm or hand] bones from injury [and fracture] by reducing the level of strain during striking," the authors said.

To conduct the experiment, the cadaver arms were placed in an apparatus that allowed them to swing forward and punch a padded, force-detecting dumbbell — the cadaver hands were shaped in either a fully closed fist, a partially closed fist, or an open-handed slap. Strain gauges, which measured stress on the hand bones during the punch or slap, were glued to the back of the hands, on the metacarpal bones. The gauges also measured bone deformation, specifically how they stretched and compressed.

Punch experiment
A closed fist, an open fist, and an open-hand slap showed how our hands evolved for fistfighting. David Carrier, University of Utah

The researchers placed seven times less strain on the hand bones than would be needed to break the metacarpals — the fishing line they used to keep the fingers in place would snap under too much tension. However, Carrier said it was easy to infer how larger forces would affect the hand "because there is a linear relationship between force applied to a bone and how much it bends."

Though one of the arms was too arthritic to stand up to the hundreds of punches and slaps thrown, the researchers discovered that “humans can safely strike with 55 percent more force with a [closed] fist than with an [open] fist, and with two-fold more force with a [closed] fist than with an open-hand slap." The study concluded that the evolution of the human hand, as well as those of our ancestors, may have improved overall dexterity, but they also made it a good weapon in a fight.

Source: Carrier, D, et al. Dead men punching. Journal of Experimental Biology. 2015.