The Cambrian Explosion is one of the most fascinating periods in Earth’s prehistory, as here we first find animals in the fossil record. With the discovery of the Burgess Shale, we can get a sense of the enormous diversity in animals evolving in a relatively short time, as the appearance of complex multicellular organisms provided evolution with an open canvas. The chapter devotes some space to exploring the inferences made about trilobites, given the morphological diversity, social inferences, and informative anatomical structures we see in this group. With the evolution of animals, the chapter introduces the phylogeny of the large taxonomic groups, and describes the major transitions in the evolution of the animals, starting with the sponges and ending with the vertebrates. It reviews the evolution of the nervous system, contrasting the distributed neural network seen in cnidarians (i.e. jellyfish) with the central nervous system of vertebrates and arthropods. Finally, it goes into detail on the evolution of the notochord in the earliest known chordates.