What happened in the Cambrian explosion?
Life began more than 3 billion years before the Cambrian, and gradually diversified into a wide variety of single-celled organisms. Toward the end of the Precambrian, about 570 million years ago, a number of multicelled forms began to appear in the fossil record, including invertebrates resembling sponges and jellyfish, and some as-yet-unknown burrowing forms of life. As the Cambrian began, most of the basic body plans of invertebrates emerged from these Precambrian forms. They emerged relatively rapidly, in the geological sense — over 10 million to 25 million years. These Cambrian forms were not identical to modern invertebrates, but were their early ancestors. Major groups of living organisms, such as fish, amphibians, reptiles, birds, and mammals, did not appear until millions of years after the end of the Cambrian Period.
