To anyone engaged in a skill requiring dexterity—surgery, drawing, and of course playing a musical instrument—the use of the thumb is crucial to successful execution. Thumb-finger opposition is one of the primary characteristics which distinguishes primates from other animals, allowing them to manipulate tools; in humans this potential exists at the highest levels, facilitating the development of skills which make extraordinary use of the hand…witness the moto perpetuo. We also have expressions to describe this relationship when it doesn’t work well: “He/ she is all thumbs!”
To enable this thumb- finger opposition, there is a considerable amount of brain space devoted to the fleshy area of the thumb between the base joint of the thumb (located at the juncture of the wrist) and the joint just above it. In Frank R. Wilson’s brilliant book called “The Hand: How Its Use Shapes the Brain, Language and Human Culture”, there is an extensive discussion of the precursors to the human hand.
As a cellist, I call the thumb the ‘hidden member’ of the hand, placed as it is behind the frog in one hand and behind the neck of the cello in the other. A special effort of attention is required to become attuned to the action of both thumbs. When working in the Alexander Technique, I find the thumbs of cellists most often tell a silent story of overuse. Thumbs that grip and hold onto the instrument or bow don’t necessarily release when the playing stops. Over time the pattern of overuse becomes second nature, and players are not even aware until it is pointed out to them that the three joints of their thumb have to be free to bend or straighten while doing their job. Mobility means the ability to move. If the joints of the thumb remain free and open, the hand has the marvellous ability to work at full speed with no impediment.
Try opening and closing the hand rapidly, with your palm facing the ceiling. In closed position the thumb lands somewhere between the first and second finger, unless one has an exceptionally long thumb. Then it may land as far over as the area between the second and third finger. Furthermore, there is a huge variation in how the thumb is actually set in the hand. I have seen thumbs in this same closed fist position where the thumbnail is hardly visible and ones where the thumbnail is fully visible. Yet another reason why one cannot prescribe a set position for the bow hold. The scientist Roger Williams called this sort of anatomical variation ‘biochemical individuality’; if you go round a roomful of 20 or 30 cellists, you will see what he meant. 
There is a direct relationship between gripping with the neck and jaw, and gripping with the thumb. As one begins to become aware of one end of this equation (usually the neck first,) the other end can come into awareness. Thumbs are not meant to grip but to facilitate the use of the fingers. There is an appropriate amount of effort for the thumb in partnership with the fingers and it is worth exploring what this might mean to us cellists, in the context of the use of the whole self.
 Mary Marzke, a physical anthropologist at Arizona State University, has spent a great deal of time looking at the hand and wrist bones of Australopitheens afarensis. Professor Marzke points out that although Lucy does have an opposable thumb, other primates also have this feature. Chimps and monkeys, in fact, are quite good at bringing the thumb to the side of the index finger. What they don’t do well (as Lucy herself could not do) is bring the thumb tip all the way across the hand to the fourth and fifth fingers. Also, neither the apes nor Lucy flex the fingers on the ulnar side of the hand (the side with the little finger) toward the base of the thumb in the movement known as “ulnar opposition.” We humans do this without the slightest sense of marvel whenever we grasp the handle of a hammer, a golf club, or a tennis racket and prepare to take a swing.
The advances in Lucy’s hand are nearly impossible to appreciate without studying her wrist bones–fitting them together, moving them around, and comparing them to the wrist bones of other anthropoids. Having spent some years doing exactly this, Marzke concludes that Lucy’s hand is at least partially “modern.” The most impressive evidence for its modified design resides in the joint surfaces at the base of the thumb, index, and middle fingers, and in the changes in the size and orientation of the joint surfaces of the wrist bones closest to those digits. The thumb is long in comparison with the fingers, the ratio approaching that found in modern humans and in few other primates. Taken together, these changes move the radial (or thumb) side of Lucy’s hand dramatically toward the twentieth century. The apparent functional advantages of the changes are:
* the thumb, index, and middle fingers can form a “three-jaw chuck,” which means the hand can conform to, grasp, and firmly retain irregular solid shapes (such as stones);
* finer control can be exerted over objects held between the thumb and the tips of the index and middle fingers;
* rocks can be held within the hand to pound repeatedly on other hard objects (nuts, for example), or to dig for roots, because the new wrist structure is able to absorb (dissipate) the shock of repeated hard strikes more effectively than in the ape hand.
These changes would have given A. afarensis the capacity to conform the thumb and first two fingers to a very wide range of object sizes and shapes, allowing them to be held and manipulated easily on the thumb side of the hand. Changes in the ligaments suggest that Lucy had the capacity for prolonged periods of percussion using small stones.
 Williams, Roger J. Biochemical Individuality: The Key to Understanding What Shapes your Health, ed. Jeffrey Bland, 1988.