The Cambrian period began about 540 million years ago. Life had existed for about three billion years before the Cambrian period. The fundamental processes of life had evolved before the Cambrian period: metabolism, prokaryotic cells, eukaryotic cells, and multicellularity. Protists, including seaweeds, existed before the Cambrian. Fossilized animal embryos and burrows indicated the existence of at least simple animal life before the Cambrian. Immediately prior to the Cambrian period, the Ediacaran organisms dominated fossil deposits worldwide. But at the beginning of the Cambrian, a rapid diversification of complex animals, particularly animals with hard skeletons, occurred in the oceans of the world. Because much of this evolution occurred right at the beginning of the Cambrian period, it has been called the Cambrian explosion. Paleontologist Andrew Knoll wrote that the Cambrian explosion looked like a "brief moment … when all things were possible in animal evolution." The appearance of new animal forms was rapid, but not sudden; it occurred over a period of about five million years.

Some scientists explain the Cambrian explosion as the appearance of animals that had previously existed but had not been fossilized. According to this view, complex animals without hard parts had previously existed although complex animals with hard skeletons had not. Hard skeletons are much more readily preserved in the fossil record. However, organisms without hard parts have often been preserved in the fossil record, including the Precambrian Ediacaran organisms and animal embryos mentioned previously. The Cambrian explosion, then, must be explained as the rapid evolution of new animals, not just the evolution of hard skeletons. Some observers have classified Ediacaran organisms as relatives of modern animal groups such as cnidarians. But even if this is true, the Cambrian explosion represents spectacular diversification within those invertebrate groups, as well as the origin of new animal groups such as the arthropods. . .

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