Opposable thumbs: one of the features that separates us from other apes, a contributor to the tool use that's made us the dominant species on the planet. Makes you feel quite smug, right? Well, not so fast.
A new study indicates that the real evolutionary advantage of being able to cross your thumb over your palm is that it lets you hit people hard without hurting yourself.
University of Utah researchers have examined the stresses on volunteers' hands when whacking punch bags, and say that the ability to form a fist means far more effective fighting.
"The role aggression has played in our evolution has not been adequately appreciated,” says biology professor David Carrier.
“There are people who do not like this idea, but it is clear that compared with other mammals, great apes are a relatively aggressive group, with lots of fighting and violence, and that includes us. We’re the poster children for violence.”
So Carrier and his team asked 10 male students and nonstudents – aged 22 to 50 and all with boxing or martial arts experience – to hit a punching bag as hard as they could.They tried overhead hammer fists and slaps, side punches and slaps, and forward punches and palm shoves.
To the researchers’ surprise, the peak force was the same. However, a fist delivers the same force with one-third of the surface area as the palm and fingers, and 60 percent of the surface area of the palm alone. So the peak stress delivered to the punching bag – the force per area – was 1.7 to three times greater with a fist strike than with a slap.
The team also carried out experiments to test the hypothesis that a fist provides buttressing to protect the hand during punching. They found that the buttressing from making a fist increased the stiffness of the knuckle joint fourfold, and doubled the ability of the fingers to transmit punching force.
"Because the experiments show the proportions of the human hand provide a performance advantage when striking with a fist, we suggest that the proportions of our hands resulted, in part, from selection to improve fighting performance," says Carrier.
If manual dexterity was the only driving force, humans could have evolved longer thumbs without the fingers and palms getting shorter. But, he says, "there is only one way you can have a buttressed, clenched fist: the palms and fingers got shorter at the same time the thumb got longer."
The current theory is that when our ancestors came out of the trees, selection for climbing vanished and selection for manipulation became dominant.
"An alternative possible explanation is that we stood up on two legs and evolved these hand proportions to beat each other," says Carrier.