AT FIRST GLANCE, the diversity of the animal kingdom appears remarkable—from ants to alligators, bees to baboons, and crustaceans to cats, animals seem varied in countless ways. But if you pondered this further, you could just as easily conclude that what is remarkable about the animal kingdom is how little diversity there is. Almost all animals on Earth have the same body plan. They all have a front that contains a mouth, a brain, and the main sensory organs (such as eyes and ears), and they all have a back where waste comes out.
