In the almost four billion years since life on earth oozed into existence, evolution has generated some marvelous metamorphoses. One of the most spectacular is surely that which produced terrestrial creatures bearing limbs, fingers and toes from water-bound fish with fins. Today this group, the tetrapods, encompasses everything from birds and their dinosaur ancestors to lizards, snakes, turtles, frogs and mammals, including us. Some of these animals have modified or lost their limbs, but their common ancestor had them--two in front and two in back, where fins once flicked instead.
The replacement of fins with limbs was a crucial step in this transformation, but it was by no means the only one. As tetrapods ventured onto shore, they encountered challenges that no vertebrate had ever faced before--it was not just a matter of developing legs and walking away. Land is a radically different medium from water, and to conquer it, tetrapods had to evolve novel ways to breathe, hear, and contend with gravity--the list goes on. Once this extreme makeover reached completion, however, the land was theirs to exploit.