Scientists: We Evolved Thumbs Not for Tools, But to Make a Fist

We are a vio­lent, vio­lent species folks.

Don’t ever let civ­i­liza­tion fool you oth­er­wise.  Only thing we’ve done is find more effec­tive ways to kill each other.

But a study just pub­lished in the Jour­nal of Exper­i­men­tal Biol­ogy by Michael Mor­gan and David Car­rier of the Uni­ver­sity of Utah has shown that the exact geom­e­try of the hand is prob­a­bly the result of its destruc­tive rather than its con­struc­tive power.

Most nat­ural weapons are obvi­ous: teeth, claws, antlers, horns. But the hand becomes a weapon only when it turns into a fist. Dr Mor­gan and Dr Car­rier there­fore stud­ied its anatomy to try to find out what makes the fist such an effec­tive weapon—one which, like the pre­ci­sion and power grips, the hand of even a chim­panzee is inca­pable of form­ing properly.

Part of the rea­son is obvi­ous. A fist presents the knuck­les first. That means the force of a blow is trans­mit­ted through a much smaller area than would be the case for its alter­na­tive, an open-handed slap. But the two researchers sus­pected that there might be more to it than that, so they dug a lit­tle deeper.

First, they gath­ered some basic mea­sure­ments. They asked ten athletes—a mix­ture of box­ers and mar­tial artists—to strike a punch bag as hard as they could using either a nor­mal fist or an open palm. The ath­letes did so in many ways, includ­ing for­ward strikes, side strikes and over­head attacks, and Dr Mor­gan and Dr Car­rier mon­i­tored how much force was deliv­ered in each case using an accelerom­e­ter attached to the bag.

They also used a series of pis­tons to mea­sure the stiff­ness of dif­fer­ent hand shapes. These included a fully clenched fist, a semi-fist with the fin­gers curled up but the thumb pointed out­wards, and a poorly formed fist in which the fin­gers were folded over the palm (but not fully curled) and the thumb pointed out­wards. (This lat­ter is rem­i­nis­cent of the clos­est that a chim­panzee can come to mak­ing a fist.) As the ath­letes formed these var­i­ous fists and fist-like shapes, the pis­tons mea­sured the rigid­ity of their hands along the knuckle bones.

Though the accelerom­e­ter in the punch bag sug­gested that a side­swipe made with a closed fist deliv­ers 15% more force than an open-handed strike, a frontal attack with either pro­duces about the same force. The fist’s advan­tage thus seems to be mainly in its geom­e­try rather than it mode of deliv­ery. Part of that advan­tage does, indeed, come from the small sur­face area of the knuck­les (which is about a quar­ter that of an open-handed strike). But the killer app, almost lit­er­ally, is the stiff­ness imparted by the way the bones are arranged in a prop­erly formed fist. This allows the force of a punch to be deliv­ered with an effect that can, for the receiver, be bone breaking.

Two things are cru­cial. One is the way the fin­gers curl back on them­selves, which leaves no empty space inside the fist. That is a prod­uct of the pre­cise lengths of the com­po­nent bones of each fin­ger, which is one rea­son a chim­panzee can­not form a proper fist. The other is the but­tress­ing role of the thumb, which adds yet fur­ther stiff­ness. Again, this requires the thumb to be of pre­cisely the right length, and to orig­i­nate in pre­cisely the right place at the side of the palm. In com­bi­na­tion, Dr Mor­gan and Dr Carrier’s machine indicted, these fea­tures make a prop­erly formed fist almost four times as rigid as a chimpanzee-style fist—for when a chimp curls its fin­gers up it leaves a gap through the mid­dle of the fist, fatally weak­en­ing the struc­ture; and the thumb plays no but­tress­ing role.

All this sug­gests that fists are indeed proper evo­lu­tion­ary adap­ta­tions, with their own his­tory of nat­ural selec­tion, rather than being just the coin­ci­den­tal by-products of humanity’s hand­i­ness with a tool. In fact, it is prob­a­bly eas­ier for the grip­ping role of the hand to adjust to the geo­met­ri­cal require­ments of the punch­ing role of the fist than the other way round. Which makes per­fect sense, for it has long been the case that the species is divided between those who pros­per by mak­ing things with their hands, and those who rely on their fists, or the threat of them, to take what the mak­ers have made.

Empha­sis added.

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