The Cambrian Explosion: The construction of animal biodiversity

The Cambrian explosion is one of the major evolutionary transitions in the history of life. From a meager and only dimly perceived beginning from single-celled ancestors nearly 800 million years ago (Ma), early animals spent over 200 million years in which their evolutionary changes chiefly involved the development of different cell types in relatively simple organisms, but patterned by increasingly sophisticated genetic programs. Beginning about 579 Ma, during the Ediacaran period, many novel clades of animals began to appear, radiated into distinctive forms, thus producing an increasingly diverse family tree of early animals. Unfortunately, only some of these lineages are represented by fossils. Finally, during a relatively narrow time span in the early Cambrian (after about 542 Ma), there is an explosive appearance of many different, morphologically distinctive fossils, including representatives of most of the major animal groups that are alive today.