Why do human hands look the way they do? Apes have long palms and fingers, while human palms and fingers are relatively shorter. According to a new theory, human hands evolved not just for the manual dexterity needed to create tools and paint masterpieces, but for another, less elegant purpose: punching.

"The standard argument is that once our ancestors came out of the trees, the selection for climbing was gone, so selection for manipulation became dominant, and that's what changed the shape of our ancestors' hands," University of Utah professor and study author David Carrier said in a statement. "Human-like hand proportions appear in the fossil record at the same time our ancestors started walking upright 4 million to 5 million years ago. An alternative possible explanation is that we stood up on two legs and evolved these hand proportions to beat each other."

Carrier also points out that, although chimpanzees are better at building tools than gorillas, gorillas' hands are closer in proportion to humans'. Humans also clench their hands into fists in a way to show off aggression. In addition, most of the differences between men and women, which generally appear to stave off competition between males, are in the upper body, particularly the hands - consistent with the idea of hands being used as weapons. Perhaps the most damning support for this theory is the fact that humans are the only primates who hit with a clenched fist.

Carrier and medical student Michael Morgan tested their theory with a series of experiments. In one, they had 10 men aged 22 to 50, all of whom had boxing or martial arts experience, deliver hits using a variety of hits to a punching bag: overhead hammer fists and slaps, side punches and slaps, and forward punches and palm shoves. The punching bag was outfitted to calculate the amount of force used to hit it. They found that both hitting and punching delivered the same amount of force, but with one-third of the palm and fingers exposed, so a punch exerts 1.7 greater force per surface area. That indicates that a punch can incur the maximum amount of damage to the intended recipient, with minimum damage to the person delivering the blows. Subsequent experiments found that a clenched fist does indeed protect the hand from maximum damage.

However, Carrier also provides another theory: natural selection for walking and running led to shorter toes and a longer bigger toe, and that the same gene led to shorter fingers and a longer thumb.

The study was published in the Journal of Experimental Biology.