Cambrian Explosion

CORE DEFINITION

The Cambrian Explosion, also known as the Cambrian radiation, is an evolutionary radiation event that occurred in a relatively short period at the beginning of the Phanerozoic Eon, starting in the early Cambrian period 541 million years ago and lasting 20 to 25 million years. It is characterized by a great increase in species diversity of multicellular organisms with mineralized tissues capable of forming body fossils, especially macroscopic motile eumetazoans. The fossil record shows that most animal phyla appeared during this period, and fossil assemblages found around the world, such as the Burgess Shale in Canada and the Yuanshan Formation (Chengjiang fossil site) and Qingjiang biota in China, collectively attest to this spectacular event in the history of life. The evidence for the Cambrian Explosion also puzzled Darwin, who wrote in On the Origin of Species: "There is another and allied difficulty, which is much more serious. I allude to the manner in which species belonging to several of the main divisions of the animal kingdom suddenly appear in the lowest known fossiliferous rocks."