The Cambrian explosion is a brief time when most major groups of animals that have bilateral symmetry first appear in the fossil record. Do you know what a bilateral animal is? It is one whose body has two mirror-image halves. Modern examples are lobsters, people, dogs, and butterflies. The event is referred to as an ‘explosion’, because a rich diversity of species appeared in a relatively short amount of time. There is growing evidence that these different groups had a common ancestor that lived during the Cambrian Period. Evidence is growing to support this theory, at least from the fossils that date back to this period. After the Cambrian Period, very few additional animal phyla, or large animal categories, arose.
