Cambrian Explosion

The Cambrian Explosion is the radiation of animal phyla that started about 570 million years ago, which is 30 million years after the beginning of the Cambrian geologic period, and lasted between 5 and 17 million years through much of the early Cambrian. The extinctions connected with the Varangian glaciation that preceded this radiation (popularized as 'Snowball Earth'), along with subsequent greenhouse warming of the Earth, is theorized to have provided the evolutionary impetus. It is thought that the development of sexual reproduction increased the rate of evolutionary change.

Evidence for earlier multicellular animal forms, which may have been the precursors of this radiation, date from 600 million years ago. Notable among these are trace fossils in the form of imprints of animals and their activities, such as burrows in mud, produced by animals that paleontologists call the Ediacaran fauna. These organisms were soft-bodied and are found with various strange body forms. The so-called Small shelly fauna of the ensuing Tommotian period included Cloudinia and its kin. All phyla in existence today (and all others now extinct) except one were first found in this period.

The original and most widely-publicized source of fossils from the actual radiation period is the Burgess Shale in British Columbia. Some Burgess Shale organisms display strikingly unusual body plans that are not easily connected with any phyla known since.

A popular account of the paleontological analysis of the Burgess Shale is given in Wonderful Life by Stephen Jay Gould.