1) Many of the major phyla of animals arose during the Cambrian period, in what is called the Cambrian Explosion. Prior to the Cambrian period, simple one-celled organisms had slowly evolved into primitive multicellular creatures. Then, in a relatively rapid explosion during the period from 540 million years ago to 500 million years ago, there was a period of astonishing diversification in which quickly developing organisms became widely distributed and formed complex communities.

2) One theoretical explanation for the rapid diversification that occurred during the Cambrian period is known as the theory of polar wander. According to this theory, the rapid diversification occurred because of an unusually rapid reorganization of the Earth’s crust during the Cambrian period. This rapid change in the Earth’s crust initiated evolutionary change inasmuch as change in the environment serves to trigger evolutionary change.