- 2017年11月10日11:16 来源：互联网
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The evolution of the hand——Making a fist of it
How a dual-use bodily structure came into existence
THE appendage at the end of a human being's arm is a strange organ.
It is the only one that has different names, depending on what it is being used for. Employ it to hold something, and it is called a hand. Employ it to hit someone, and it is called a fist.
That second use, though, is almost unknown in other primates.
Most primate hands are long of palm and finger, short of thumb, and suited for climbing.
Human hands have short palms, short fingers and long thumbs, which are not.
These proportions do, though, make it possible to grip things in two ways that other apes' hands cannot manage well.
One is by using what is known as a precision grip, in which an object is held between the pads of the finger tips and the pad of the thumb.
The other is by means of a power grip, in which all the fingers and the thumb are wrapped around what is being grasped.
These two grips are crucial to Homo sapiens's characteristic tool-crafting skills, and it has thus long been thought that the widespread use of tools by humanity's ancestors was the driving force behind the modern hand's proportions.
No doubt that was important. 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 and the thumb pointed outwards.
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.
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.