National Geographic : 2012 May
• BAT A bat wing may look like a sheet of skin, but within its flesh are five fingers. The bones act something like tent poles, stretching out the membranous wing so that it can catch the air and lift the bat's body. By adjusting each finger, a bat can dodge through a forest, hover in front of a flower to feed, or slip into the mouth of a cave. 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. , researchers over the past 150 years have dug up fossils on every continent. ey've compared the anatomy of hands in living animals. ey'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 ns--- not the at, ridged ns of a gold sh but the muscular, stout ns of extinct relatives of today's lung sh. Inside these lobe ns 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 ngers. e 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 ngers. 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 ve ngers. It has never recaptured the original exuberance of ngers---for reasons scientists don't yet know. Still, there is a great diversity of hands in living species, from dolphin ippers 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 di erences, all hands start out in much the same way. ere 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; oth- ers lengthen the ngers. It takes only subtle shi s in these genes to make ngers longer, to make some of them disappear, to turn nails into claws. e discovery of the molecular toolbox for hand building has given scientists a deeper understanding of Darwin's great insight. As di er- ent as a vulture's wing and a lion's paw may look from the outside, the di erence between them may come down to tweaks---a little more of one protein here, a little less of another protein there. Darwin could rec- ognize only the outward signs that hands had evolved from a common ancestor. Today scientists are uncovering the inward signs as well. j EARLY HANDS WERE MORE EXOTIC THAN ANY HAND TODAY. SOME SPECIES HAD SEVEN FINGERS. OTHERS HAD EIGHT. Carl Zimmer's latest book, Evolution: Making Sense of Life, co-authored with Douglas Emlen, will be published this summer. Bryan Christie is an award- winning artist and illustrator whose passion is documenting the innate wonder and beauty of the physical world.