Human hands and their remarkable dexterity have given us everything from the guitar of Segovia to the art of the Dutch masters but, says David Carrier from the University of Utah, they evolved to be what they are for a more practical reason.  As a weapon. 

Carrier and colleague Michael Morgan publish their hypothesis that human hands evolved their square palms and long thumb to stabilise the fist and produce a compact club for use in combat in The Journal of Experimental Biology.

Carrier says the idea occurred to him during an impassioned discussion with fellow biomechanic Frank Fish about sperm whales. Explaining that he had published a paper suggesting that the whales might use their spermaceti organs as battering rams, Carrier says "Frank didn't buy the argument and at one point he raised his fist and said, "I can hit you in the face with this, but that is not what it evolved for.'" Carrier says the notion came to him - sure, the human hand evolved for dexterity, but he adds, you could manipulate the proportions of a chimp hand in ways that would enhance manual dexterity, but they would not necessarily end up with the proportions that we have.

Maybe there was more to Fish's challenge than met the eye, he surmised. 

According to Carrier and Morgan, modern chimpanzees have long palms and fingers with a short thumb, while the human palm and fingers are much shorter and the thumb longer and stronger. Carrier explains that this squat arrangement allows us to clench our hand into a fist when we fold the thumb across the fingertips however, chimp fingers form an open doughnut shape when curled. Could the tightly packed human fist provide internal support – buttressing – to the digits to protect them from damage during combat? In addition, Carrier wondered whether curling the fingers into a fist could allow punching men to deliver a more powerful blow (increase the peak force of an impact) than slapping with the open hand.

They decided to find out whether hands are more effective when balled into a fist or wielded in a slap.

"Fortunately, Michael had a lot of experience with martial arts and he knew people who were willing to serve as subjects," Carrier recalls. Asking the athletes to thump a punching bag with their hands in a range of shapes (from open-handed slaps to closed fists) using various delivery styles (over arm, sideways and head on), Morgan and Carrier measured the force of each impact. However, they were surprised to see that the punch did not deliver more force per blow.
 
"In terms of the peak forces or the impulse, it did not matter whether the subjects were hitting with a clenched fist or open palm," Carrier says. 

Next the duo tested whether buttressing the hand by curling the fingers and thumb stiffens the structure. They asked the martial arts experts to roll their hands into variations of the fist shape – two with the thumb extended sideways – and then push the first joint of the index finger against a force transducer to measure the rigidity of the knuckle joint in the presence and absence of the buttressing thumb. Impressively, the knuckle joint was four times more rigid when supported by the thumb.

And when the duo measured the amount of force that the athletes could deliver through the fist surface of the index and middle fingers, they found that the presence of the buttressing thumb doubled the delivered force by transmitting it to the wrist through the metacarpals (palm bones) of the thumb and the index finger.

So, they conclude, our short, square hands are perfectly proportioned to stiffen our fists for use as weapons and allow us – well, males predominantly – to deliver powerful punches without incurring injuries.