When you're standing in a museum surrounded by fossils, you can almost imagine drifting through time to when long-extinct creatures swam the ocean. Found all over the world, these fossils can be read by scientists like pages from the book of the past. They tell not only an evolutionary story — the family tree of life — but also how these creatures used to live. Fossils can be found throughout the geologic history of earth, going back billions of years. Here we explore one of the most exciting of times, the Cambrian, when many of the major groups of animals familiar to us today first appear as conspicuous fossils. This is often referred to as the Cambrian Explosion.