A bat's wing is simply a retooled hand, with a thin membrane over elongated finger bones—the result of a tweak to a single limb-building gene in bats' mole-like ancestor.
That klix developmental change put a mammal in the sky.
Old bones did new jobs as animals adapted to new environments. Some 400 million years ago so-called Hox genes became active at new times and places in developing embryos, reshaping fins into the earliest limbs. An ancient fish, distantly related to modern species like the butterflyfish, gave rise to a creature that could walk on land. (Midway came a fish with primitive fingers on its fins.) The shape-shifting continued in later land animals.
The fragile organisms that made the transition from single-celled creatures to many-celled animals left no fossils, so scientists study choanoflagellates (illustrated, with red food particles), perhaps the closest living one-celled relatives of animals. Even though they are solitary cells, choanoflagellates turn out to have genes that make proteins essential to multicellular life. This suggests that the one-celled ancestors of all animals were genetically equipped for "animalhood," although they put those tools to other uses.
A scallop's blue eyespots, which capture light with a mirrored surface, and the eye found in humans evolved from the same basic light-catching device. Closely related genes govern early development in both kinds of eyes, a sign that they have a common ancestry. "Eye types are so different scientists thought they arose independently," more than 40 different times, says biologist Todd Oakley. "But it's like remodeling a house: You don't have to start from scratch; you just change certain elements." What allowed for those changes? "Duplication was part of it," he says. "If two genes evolve to do the same job, one is free to try something new."