The basic body plans of all modern animals were set during the Cambrian Period, 542 - 488 million years ago. Your friends, family, and pet turtle may not look much like the creatures here. But we and our fellow animals are heirs of these ancient ocean dwellers. Changes in Earth’s climate and ocean oxygen levels may have fueled rapid diversification during this period. Another possible cause may have been the interactions among the increasingly complex animals. Competition and predation often spark innovation. Not every Cambrian body plan was successful. But those that did succeed set the pattern for every animal that followed—in the water and on land. Explore the ancient ocean in an image gallery or in our Ocean Over Time interactive.