If you could hop onboard a time machine and visit the earth as it was 500 million years ago, during the Paleozoic era, you'd be forgiven for thinking you had traveled not to another time period but to another planet altogether. In essence, you would have. The continents mostly sat in the Southern Hemisphere, the oceans had vastly different configurations and currents, the Alps and the Sahara had yet to form. Land plants had not even evolved. Perhaps the most dramatic difference, however, would lie in the animals that inhabited this primeval earth. Back then, most of the world's multicellular creatures lived in the sea. Clamlike creatures called brachiopods and trilobites—those extinct cousins of today's lobsters and insects, with their hard exoskeletons, long antennae and compound eyes—reigned supreme.
The diversity of marine animals grew substantially over the next 250 million years, until the so-called Permian extinction event snuffed out more than 90 percent of ocean species and brought the Paleozoic to a close. The loss of life was staggering. But change was on the horizon, and while life on land underwent a radical transformation with the rise of dinosaurs and mammals, life in the sea entered a dramatic phase of reorganization that would establish the dominance of many of the animal groups that prevail in the marine realm today, including modern groups of predatory fish, mollusks, crustaceans, sea urchins and sand dollars, among others.