Forget toolmaking, think fisticuffs. Did evolution shape our hands not for dexterity but to form fists so we could punch other people? That idea emerges from a new study, although it runs counter to conventional wisdom.
About the same time as we stopped hanging from trees and started walking upright, our hands become short and square, with opposable thumbs. These anatomical changes are thought to have evolved for tool manipulation, but David Carrier at the University of Utah in Salt Lake City has an alternative explanation.
He says there are several possible hand shapes that would have allowed greater dexterity, making it less clear why we ended up with the hands we have. But only one hand shape lets us make a fist with a thumb as buttress.
Among primates' hands, ours is unique for its ability to form a fist with the thumb outside the fingers. The fingers of other primates' hands are too long to curl into their palms, and their thumbs are too short to reach across the fingers. So when apes fight, they are far more likely to wrestle or hold their opponent down while others stomp on him, says Carrier.
To test the importance of fists, Carrier and his colleagues recruited 10 athletes and measured how hard they could hit a punching bag using a normal fist, a fist with the thumb stuck out, and with an open palm.
The athletes could generate more than twice the force with a normal fist as with the thumb-stuck-out fist, because of thumb's buttressing role. There was no difference in the force they could generate with a normal fist and with an open palm, but Carrier says it's possible that a fist concentrates the force into a smaller area and so does more damage.
Cause or effect?
Mary Marzke of Arizona State University in Tempe says the study is interesting, but it far from proves that the ability to make a strong fist was the main driver behind the evolution of our hands' shape. It is more likely that it was a useful side effect of a whole suite of modifications.
She points out that apes strike with the heel of their hand when knocking fruit out of trees. Carrier's study didn't assess the force that the heel of the hand generates, but if it turns out to be as good as a fist, it becomes less clear that our hands evolved so as to be perfect for fist-making, Marzke says.
But if the hypothesis is true, Carrier thinks it could explain another mystery. It has long been unclear why high levels of testosterone cause men's ring fingers to be longer than their index fingers. He says the finger-length ratio makes sense if it generates a better fist. This would make dominant males even better fighters.
Journal reference: Journal of Experimental Biology, doi:10.1242/jeb.075713
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