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Roughly 545 million years ago, at the beginning of the Cambrian period, there was rapid evolution of new animal groups known as the Cambrian explosion. A. Biologists want to find out why the rapid diversification of body forms did not happen soon after the appearance of eukaryotes around 1.2 billion years ago. B. Biologists classify animals according to their body plans into phyla such as Chordata, animals with a backbone, and Arthropoda, animals with segmented exoskeletons. C. The Cambrian explosion was a unique episode in the history of evolution that produced nearly all of the 30 or so animal body plans that have ever been seen. D. The Cambrian explosion may have been aided by genes that could yield a variety of body forms and the inability of early predators to eliminate the new forms. E. Once predators became efficient after the Cambrian explosion, they were able to eliminate any animals that began to evolve a new body plan. F. At the beginning of the Cambrian, an increase in oxygen needed for animal growth and the return of a hospitable climate may have contributed to the Cambrian explosion.