The Common Hand, Carl Zimmer

National Geographic, May 31, 2012

The hand is where the mind meets the world. We humans use our hands to build fires and sew quilts, to steer airplanes, to write, dig, remove tumors, pull a rabbit out of a hat. The human brain, with its open-ended creativity, may be the thing that makes our species unique. But without hands, all the grand ideas we concoct would come to nothing but a very long to-do list.

The reason we can use our hands for so many things is their extraordinary anatomy. Underneath the skin, hands are an exquisite integration of tissues.

The thumb alone is controlled by nine separate muscles. Some are anchored to bones within the hand, while others snake their way to the arm. The wrist is a floating cluster of bones and ligaments threaded with blood vessels and nerves. The nerves send branches into each fingertip. The hand can generate fine forces or huge ones. A watchmaker can use his hands to set springs in place under a microscope. A pitcher can use the same anatomy to throw a ball at a hundred miles an hour.

The hand is so remarkable that the great Scottish surgeon Sir Charles Bell wrote an entire book in 1833 praising it, The Hand: Its Mechanism and Vital Endowments, as Evincing Design. At the time, the notion that life evolved was beginning to circulate, but Bell thought a close look at the human hand would dispel such silly talk. “It presents the last and best proof of that principle of adaptation, which evinces design in the creation,” he wrote.

There’s just one problem with Bell’s argument: It didn’t explain why other species have hands too. No one would doubt that the five fingers at the end of an orangutan’s arm are anything else. In other cases we have to look closer. A bat’s wings may look like sheets of skin. But underneath, a bat has the same five fingers as an orangutan or a human, as well as a wrist connected to the same cluster of wrist bones connected to the same long bones of the arm.

When Charles Darwin wrote Origin of Species, he singled out this odd coincidence. “What can be more curious,” he asked, “than that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern?”

For Darwin, there was a straightforward answer: We are cousins to bats and to all other animals with hands, and we all inherited our hands from a common ancestor.

In exploring how hands have evolved, researchers over the past 150 years have dug up fossils on every continent. They’ve compared the anatomy of hands in living animals. They’ve studied the genes that build hands. Again and again, they’ve found support for Darwin’s contention.

Our hands began to evolve at least 380 million years ago from fins–not the flat, ridged fins of a goldfish but the muscular, stout fins of extinct relatives of today’s lungfish. Inside these lobe fins were a few chunky bones corresponding to the bones in our arms. Over time the descendants of these animals also evolved smaller bones that correspond to our wrists and fingers. The digits later emerged and became separate, allowing the animals to grip underwater vegetation as they clambered through it.

Early hands were more exotic than any hand today. Some species had seven fingers. Others had eight. But by the time vertebrates were walking around on dry land 340 million years ago, the hand had been scaled back to only five fingers. It has never recaptured the original exuberance of fingers–for reasons scientists don’t yet know.

Still, there is a great diversity of hands in living species, from dolphin flippers to eagle wings to the hanging hooks of sloths. By studying these living hands, scientists are beginning to understand the molecular changes that led to such dramatic variations–and to understand that despite the outward differences, all hands start out in much the same way. There is a network of many genes that builds a hand, and all hands are built by variations on that same network. Some sculpt the wrist; others lengthen the fingers. It takes only subtle shifts in these genes to make fingers longer, to make some of them disappear, to turn nails into claws.

The discovery of the molecular toolbox for hand building has given scientists a deeper understanding of Darwin’s great insight. As different as a vulture’s wing and a lion’s paw may look from the outside, the difference between them may come down to tweaks–a little more of one protein here, a little less of another protein there. Darwin could recognize only the outward signs that hands had evolved from a common ancestor. Today scientists are uncovering the inward signs as well.