The Miracle of the Human Hand

We take it for granted, but the human hand is a marvel. How did it become that way? Without it, we wouldn’t be human …

Look at your hand. Hold it up. Flex it. Bend it. Make it act like a puppet. It’s a remarkable piece of engineering. Never before have five digits, 14 joints and 27 bones come together in such an interesting and practical way. If you turn it, eight cube-like bones connected by a matrix of tendons in your wrist and forearm enable you to rotate your hand 180 degrees. As a result, it can do things that animals in the natural world, even if they had the inclination, could never possibly carry off -- swing a baseball bat, for instance, pour a glass of milk, play a Duke Ellington piano solo, paint a portrait.

The fingers of our hands have no muscles. They are operated by remote control, like marionettes. A web of tendons, anchored in the palm, mid forearm, and as far north as the shoulder, are the strings that make your digits dance. The whole arrangement provides your hands with an unusually wide range of motion. But the anatomical feature that renders your hands especially special is your first digit, the hand’s version of a big toe. Your thumb.

One of the great beauties of a thumb is its position. Whereas our feet forsook the thumb-like appearance of their first digits and evolution straightened them into our big toes, our hands did not. They drifted in the opposite direction, building on their former status as feet specialized for climbing and grasping.

Even today our hands look remarkably like a gorilla’s foot with our thumbs sitting down below the other four digits, positioned apart, as if it were reluctant to join the rest of the group. You might think this means thumbs evolved prepared to play a backseat role, but looks can be deceiving.

Compared with the thumbs of other primates, ours have acrobatic ranges of motion. Chimp thumbs, for example, can’t rotate in great swirling arcs like ours, and that limits their ability to be the thing that all thumbs secretly long to be … opposable. I say “limits” because, contrary to popular belief, the thumbs of chimps and monkeys are opposable. They just aren’t in the same peculiar way that ours are. What’s different is that we can effortlessly swing our thumbs across the palms of our hands to meet our small and ring fingers, the fourth and fifth digits. Nothing like this exists anywhere else in nature. It’s called the ulnar opposition, and it is this seemingly simple ability that gives our hands the power to grasp and grip, turn and twist, manipulate and touch in ways unknown to other creatures. Because of this ability we can pick up and use a hammer or ax, or turn a stick into a lethal club cupping it in a position that extends the power of our arm, and, with it, the force of the blow it delivers. It’s one thing to flail a stick horizontally for show, like a chimpanzee, another to grip it along the axis of your forearm and bring it down from on high with bone crushing force. In this ability lie the potential of both tools for building and weapons for destroying.

Ulnar opposition also makes all of the difference between simply manhandling a tree branch on a chimp’s way through the forest, and precisely clasping miniscule objects with sweet and exact precision. When picking up something as tiny as a grain of rice, a chimpanzee might have to squeeze it.

between its thumb and the flat of its index finger like we hold a key or credit card. But for chimps this kind of a grip is a struggle because they don’t have the musculature and nerve  structure we do. We can pick it up using the very tips of our thumb and caress it in the closed circle of our finger as if we were making the sign for “okay, perfect,” which, in a sense it is.

These abilities exist because we have developed specialized muscles linked to our thumbs. One, a flexor called the pollicis longus runs from the thumb’s knuckle all the way to the shoulder. Along with three other muscles, it lets us push and mash things as well as open our hands and spread our thumbs away from our palm, movements that come in handy when operating a joy stick, typing on a keyboard or thumbing in the numbers on a cell phone. But it is also very useful for gripping and manipulating sticks and stones, natural artifacts that we probably used to fashion the first human made tools, like spears and small knives.

There would have been an evolutionary advantage in developing hands like these. As we gave up knuckle walking and spent more time upright, our hands would have been freed to hold more, carry more, throw more and eventually manipulate more.

It’s not simply the speed and flexibility of our thumbs, fingers and hands that make them special. It’s also their extraordinary sensitivity. Crammed within every square inch of our digits are 9000 hypersensitive egg-shaped, buds called Meissner’s corpuscles, which lay just below the epidermis, our outermost layer of skin. Inside each bud lie coiled nerves that sense and snatch up the signals initiated by whatever we touch to send it to the brain for processing.  These same nerves are scattered among other particularly sensitive parts of our bodies – our tongues, the soles of our feet, our nipples, penis, clitoris, every erogenous zone. They’re optimized for gathering the finest, most granular pieces of sensual information, and they are why our hands are, as Sir Charles Bell, put it, “so powerful, so free and yet so delicate.”

Without this combination of dexterity and sensitivity, Michelangelo would never have been able to sculpt the godlike face of his Moses, nor Leonardo paint the Last Supper.  Horowitz could not even clunk out the most juvenile version of the Emperor Concerto and Shakespeare would have been incapable of grasping a quill to pen a single word of the thousands he invented for the English language.

But the physical power and dexterity of our hands isn’t all that makes them important. Their biological evolution has also literally changed our minds because each evolved in the other’s presence. They have danced with one another. It not only took human hands for Michelangelo to chisel and shape Moses or paint the hand of God in the Sistine Chapel, but it took the evolution of hands to shape a mind that could even conceive of such a work of art.

(This is an excerpt from Chip’s book Thumbs, Toes and Tears – And Other Traits That Makes Us Human. Visit here to buy or learn more.)