4 Characteristics of the Human Hand

We are deeply familiar with our hands, though we may not pay them much conscious attention. We take their remarkable powers for granted. In particular, the relationship between the thumb and the fingers is critical to their function. John Napier writes: “One cannot emphasize enough the importance of finger-thumb opposition for human emergence from a relatively undistinguished primate background. Through natural selection, it promoted the adoption of the upright posture and bipedal walking, tool-using and tool-making that, in turn, led to enlargement of the brain through a positive feedback mechanism. In this sense it was probably the single most crucial adaptation in our evolutionary history” (Hands, 55). In the human hand, by contrast with the hands of other primates, the thumb is much elongated and the fingers correspondingly shortened, so that the thumb can oppose all four fingers. The thumb is also much more mobile and muscled in humans. Being able to hold and squeeze various objects between the tip of the thumb and the tip of the index finger is unique to humans. The thumb of a chimpanzee is a mere runt compared to the large powerful thumb of a human. With this hypertrophy of the thumb comes the potential for all sorts of grips unavailable to other primates. Yet this human hand evolved by slow modification from the primate hand—it is not a radically novel structure by any means.¹ Selective pressures must have encouraged its growth, power, and mobility. The big meaty thumb is the lever of our biological ascent, as Napier remarks.

The hand had its distant precursor in the lobe fish, whose fins have a pentadactylic structure.² This is a clear instance of ancestral preservation. But, as every anatomist delights to note, the human hand is an extraordinary complex of bones, muscles, and nerves, as well as pads and tiny ridges. It is clearly highly adapted, down to the last intricate detail. This complexity gives rise to great versatility, and the hand is capable of a large variety of grips. Napier distinguished the “power grip” from the “precision grip”: the former is used to grip a hammer or tennis racket and involves the fingers, thumb, and palm; the latter takes place between the pads of the thumb and fingers, as when holding a pen or a strand of hair. The precision grip distinguishes human hands more than the power grip. Much human tool use involves the precision grip, which gives the hand a more delicate range of manipulations. Presumably this type of grip evolved later than the more brutish power grip. Writing is made possible by the precision grip, as is rolling small objects between the fingers, not to mention surgery. Of course, it is possible to alternate power grip and precision grip and also to combine elements of both in a single act of manual prehension. The anatomical complexity and functional range of the human hand, exquisitely combining power and precision, give it enormous versatility and control. No artificial hand is anywhere near matching what every human hand can achieve without its owner even thinking about it.³

Not surprisingly, a large quantity of brain tissue is given over to the hand: the organ clearly needs a powerful computer to supervise its operations. In a cortical homunculus diagram, the thumb by itself is typically depicted as about the size of an entire leg: that is, the same amount of cortex is devoted to the thumb as to the leg. But that is entirely predictable given the wide range of activities in which the thumb is engaged. Some of this cortical machinery is dedicated to motor function and some to sensory function. Not only is the hand a motor champion; it is also highly innervated for sensation and perception. And the two are intimately connected, as sensation in the hand guides the hand’s movements. But that isn’t all: in addition to its motor and sensory prowess, the hand is highly educable—so it must possess a powerful memory. The hand can be trained to remember astonishing sequences of movements—as with playing the piano or other instruments.⁴ Much of human civilization depends upon the educability of the hand, which is to say on hand memory. This capacity must have evolved too, probably to aid in sophisticated tool use. No other primate exhibits this degree of hand know-how, as well as hand plasticity. The hand learns complicated lessons from experience. It is really not possible to isolate the hand as a functional organ from the brain that controls it—we may as well reckon the brain as part of the hand (the hand system: compare the visual system). The brain is the engine that drives the wheels.

The abilities of the hand are not all acquired, however. There must surely be a strong innate component. Hand competence is essential to human survival and hence is best not left to chance. The human infant develops the basic grips and skills without instruction, so that an innate program must be controlling the proceedings. It is rather like language: innate universals specialized into specific forms. You are not born with the ability to play the piano or tennis—these require specific training—but the basic component hand skills have an innate foundation. There is, we may assume, an innate prehension program—a universal “grammar” of grips and manipulations. A human no more learns how to squeeze an object between thumb and forefinger than a fish learns how to swim (which is itself a kind of gripping of the water). The same must be true of other primates and their hands (gibbons must have a “brachiation gene”), but human hands have a far richer innate program, to go with their more elaborate structure and function. In fact, the human hand’s capacity seems to exceed the capacity of the ape hand to about the same degree that human language exceeds ape language (we shall see that these are not unconnected facts). The human hand bears a strong physical resemblance to the ape or monkey hand—not surprisingly, given the evolutionary lineage—but if you look beneath the surface whole continents distinguish them. The human hand is in another league, in terms of motor capacity, perceptual sensitivity, educability, and cortical dedication.⁵

It is important not to limit the hand to the anatomical structure on the distal side of the wrist. The wrist itself must be regarded as functionally part of the hand, and the forearm, with its tendons and muscles reaching to the fingers, is also a part of the hand’s operation. Indeed, the entire arm and shoulder are typically involved—the hand does not act in isolation. We can even regard large segments of the human body as geared to the actions of the hands, from the feet up. The body is in many ways the hand’s platform—its base of operations. So the body of man has adapted wholesale to the manual lifestyle, enabling the hands to go about their vital business more efficiently. When the hands came into their own, far exceeding their traditional occupations, some time after our descent from the trees, the body had to go along for the ride; the better the body served the hands, the more it would be selected for. Even the eyes must have felt this selective pressure, because so much of the work of the hands needs the eyes to keep it on track. You can only seize an object accurately if your eyes can first locate it. The human body as a totality is designed for supporting manual prehension, in a kind of anatomical holism. Hand and body are fully integrated. There is an extensive hand system. It is almost as if large segments of the body, from feet to trunk to shoulders to wrist, are components of the hand. It is convenient anatomically to separate the hand from the rest, but functionally the hand extends much further. Just consider the act of throwing: the missile is gripped between the fingers but the entire body goes into the act of throwing—you throw with your feet as well as with your hand. The body, we might say, is the handmaiden of the hand—its infrastructure.⁶

Clearly, the hand of our ancestors was subjected to strong selective pressure after our fall from the branches, or else it would not differ so significantly today from the hands of other primates. It was evidently able to change its structure and function as a result of environmental pressure. We can surmise that this is because the hand was so critical to survival, so that the pressure to modify was particularly intense. What an early human did with his or her hands made the difference between life and death, reproduction and its lack, so that the hand could not afford to remain limply unchanged. The hand became the focus of natural selection during those precarious millennia. Other human organs might not have been under such intense pressure—the eyes and ears of humans do not seem markedly superior to those of other apes—but the hands had to adapt more rapidly and dramatically. The lengthening and strengthening of the thumb must have been especially urgent. Fortunately, the hand was able to absorb the selective pressure and emerge a much more impressive organ than before. We went from hand mediocrity to hand genius in fairly short order, by the standards of evolutionary time. No doubt, we now have the best pair of hands on the entire planet.⁷

That, in crude outline, is the science of the hand; but what of the philosophy? How, to begin, should we define the hand? This question does not prove easy to answer. It obviously won’t do to define the hand as “an organ with four fingers and a thumb,” because there is no conceptual necessity to having that many digits—there are logically possible hands that have a different number of fingers and an extra thumb, or no thumb. Also, it would be nice to have a definition of “finger” and “thumb.” We might try going functional: a hand is an organ used for gripping. That looks like a necessary condition, but it is clearly not sufficient: you can grip things in your mouth or between your knees. Then let us say that a hand is an organ that grips by means of fingers, which mouths and knees don’t. But what is a finger but a part of a hand? To avoid circularity we need to define “finger.” Now things get tricky: is the notion finger primitive? We can’t say a finger is that which points, because not all fingers do and some non-fingers do (you can point with your elbow). Nor can we say a finger is part of a gripping mechanism, because lips and teeth are also part of a gripping mechanism. Perhaps this will serve: a finger is a flexible jointed protrusion that enables gripping. We thus rule out lips and knees as fingers. Are the octopus’s tentacles fingers then, according to this definition? We might want to add that the protrusions need to be affixed to a palm, defined as a flat slab or some such, and that they need to be jointed. Or we might allow that tentacles are fingers. Are toes also fingers—of the foot? What if you are a foot acrobat using your toes to perform the functions of fingers? Are not these flexible toes “the fingers of the feet”? What we have captured in our attempted definition is a certain morphology linked to a function, and that is the heart of the notion of a hand, even if some cases are borderline or debatable. Martian extremities with a hundred prehensile protrusions would count as hands by this definition. Are the wings of a bat also hands? What if they are used for gripping? There can surely be webbed hands and feathery hands and slimy hands (the fins of some fish appear quite handlike). Similar problems surround the definition of other organs of the body—the mouth, the ears, the leg—because ingenious counterexamples can always be generated and intuitions may waver. But we need not be very concerned about these definitional problems here: our concern is primarily with the human hand, as it now exists, not with all logically possible hands (though this is a perfectly interesting conceptual question). Let us content ourselves with saying that the hand is a kind of morphological-cum-functional entity: a prehensive array of protrusions, roughly. But with that remark, I will leave the conceptual analysis of the hand to one side in this book. We certainly have a solid enough conception of what a hand is in order to proceed with our inquiries. And we are concerned with the role of the hand in human evolution and human life, not with all conceivable hands for all conceivable handed beings. So let us focus on the actual properties of the human hand. Our question, then, is what gave rise to them and what they in turn gave rise to. What forces shaped the human hand, and what did this hand enable?