If you learn martial arts, one of the first things you get taught is how to make a fist properly. Classic mistakes include sticking the thumb inside the other fingers or curling it around the side. The right way is to wrap the thumb over the index and middle fingers. In this shape, the fingertips are cushioned against the pads of the palm, and the first two fingers are cushioned against the thumb pad and the thumb itself.

By getting martial artists to hit a punchbag, and measuring the forces acting on their fists, David Carrier and Michael Morgan from the University of Utah confirmed that this shape allows the various parts of the hand to buttress each other, turning a flat hand into a stiff, compact club. This channels the force of a punch into the palm, wrist, and forearm, and protects the delicate fingers.

But more controversially, Carrier and Morgan also suggest that this might explain why the proportions of our hands evolved in the first place—for stability during combat, rather than dexterity during tool use.

Compared with the hands of other apes, our palms and fingers are shorter, and our thumbs are longer, stronger, and more mobile. That makes for a stronger fist, and would have allowed “competing males to strike with greater force and power while greatly reducing the risk of injury to the hand,” they write.

It’s an idea that has divided opinion. I contacted four scientists about the study. Two expressed their respect for Carrier’s wider work but were unconvinced by his new idea (although neither wanted to comment on the record). A third—Brigitte Demes, who studies the limbs of primates at Stony Brook State University—said that the actual experiments were sound, but “the interpretation is far-fetched”.

The problem is that, as Carrier and Morgan write, the goal of their study was “to test the hypothesis that the proportions of the human hand make it an effective weapon”. They did that. But the hypothesis that fist-fights shaped the evolution of our hand is different. Going from one to the other is like showing that computers are good at surfing the Web, and suggesting that this is what they were invented to do. It involves speculative just-so stories—stories that may well be correct, but that need evidence.

But John Hawks, an anthropologist from the University of Wisconsin-Madison, found the paper very reasonable. “We do have a problem understanding why human hands have shortened fingers and longer thumbs compared to apes,” he says. “The change in hand proportions came long before the first appearance of stone tool manufacture, and chimpanzees make and use wooden tools without having our hand proportions.”

View Images From Morgan and Carrier, 2012

In which the idea steps into the ring

Carrier first got his idea about fist evolution after working on sperm whales. He suggested that the males use a big waxy organ in their heads as battering rams during fights. He was discussing the idea with colleague Frank Fish, who didn’t buy it. Fish raised his fist and said, “I can hit you in the face with this, but that is not what it evolved for.”

Carrier started wondering about the forces acting upon a punching fist. First, he and Morgan asked boxers and martial artists to lash out at a punching bag with either open palms or closed fists, while recording the strikes with an accelerometer. To their surprise, both techniques delivered the same amount of force.

Next, they asked the volunteers to press their fists against a force-measuring instrument, with their hands rolled into different positions (see above). They showed that the main knuckle joint of the index finger is twice as stiff if the fingers are properly tucked into the palm (B vs. C), and twice as stiff again if the thumb wraps around the fingers (A vs. B). This classic fist prevents the fingers from moving too much during a punch, and the knuckle from overextending. It also transmits twice as much force into the wrist than the less supported shapes, again saving the finger bones from undue stresses.

So, the story goes like this: Our male ancestors fought each other for mating rights. Their hand proportions evolved from those of a typical ape, to those that allowed them to whack each other without breaking their hands. With their non-self-destructing punches, our fore-fore-fore-forefathers got more sex, and gradually their hands attained the proportions of a modern human’s.

The ability to more dextrously wield tools might have also played a role, but Carrier and Morgan argue that there are many ways that an ape-like hand could have changed into one that was good for manipulating tools. For example, the thumb could have grown longer, or the fingers could have shortened without also shortening the palm. Both would have given a precise grip. As it happens, the route that we took is the only one that also provides a strong fist.

View Images Ape hand and foot, by James Swoboda

In which the idea takes some hits but doesn’t fall down.

But Demes spots several problems with this idea. “For a start, the human hand is not particularly well designed for fist-fighting,” she says. “That’s why boxers wear gloves, and why there is even a fracture known as Boxer’s fracture.”

“Our closest relatives, the chimpanzees, also don’t fight with their fists,” says Demes. “They fight primarily with their teeth, using their hands to grab you and bite.” When they do use their hands, they deliver powerful open-handed slaps. You can see both techniques at work in this video. Pity the martial artist that pits their closed fists against the open hands and teeth of a chimp.

But Hawks adds that “it is clear that early hominins de-accentuated fighting with large canine teeth.” Some people think this was due to less aggression between males. “Carrier gives an alterative point of view: that fighting was still important, at least for some early hominins, but took a different form.”

This is a critical point. Our hands can make strong fists, but that would have given our ancestors an advantage only if they fought a lot with their fists. If they used other equally effective striking techniques, it is hard to see how the advantages that Carrier and Morgan describe would have provided enough evolutionary pressure to mould the shape of our hands. As Fish originally said to Carrier, “I can hit you in the face with [a fist], but that is not what it evolved for.” So how did they fight? We don’t know.

“There are more plausible evolutionary scenarios for all of the features that Carrier bases his fighting story on,” says Demes. These include the idea that our hands allowed us to more precisely manipulate objects or that they changed as a side effect. Hands and feet are governed by similar blueprints, and as our feet adapted for two-legged walking, our hands may have been carried along for the evolutionary ride.

And there is one more possibility: There might be nothing to explain about our hands. Carrier and Morgan’s story assumes that our ancestors had hands with the proportions of other apes. But Hawks points out that many monkeys have hands with proportions like ours. “If our common ancestors with other apes were not ape-like in their hand proportions, there may be nothing about human hand proportions that needs explaining,” says Hawks.

There’s a silver lining, though. “I did learn from the paper that next time I get into a fist fight, I’d better make sure that I curl my fingers up properly,” says Demes.

Update: T. Ryan Gregory points out another flaw that riffs off my opening.

As anyone who has studied martial arts knows, this way of making a fist is NOT intuitive and takes practice to become comfortable. The authors of the paper had trained martial artists strike heavy bags, and compared this to open hand strikes. I don’t doubt that the fist position they use is more effective. But it’s not representative of what most people do today when punching unless they have been trained — let alone what early hominin ancestors were doing. And I REALLY doubt this created a strong enough selective pressure to create a trade-off with tool use or other more obvious functions of the human hand.

Reference: Morgan and Carrier. 2012. Protective buttressing of the human fist and the evolution of hominin hands. J Exp Biol. http://dx.doi.org/10.1242/jeb.075713