what is meant when scientists refer to the cambrian explosion
Cambrian explosion The Cambrian explosion or Cambrian radiation[1] was an event approximately 541 million years ago in the Cambrian period when most major animal phyla appeared in the fossil record.[2][3] It lasted for about 20[4][5]–25[6][7] million years. It resulted in the divergence of most modern metazoan phyla.[8] The event was accompanied by major diversification of other organisms.[note 1]