<Tr> <Td> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> </Td> </Tr> <Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> The Cambrian explosion or Cambrian radiation was an event approximately 541 million years ago in the Cambrian period when most major animal phyla appeared in the fossil record . It lasted for about 20--25 million years . It resulted in the divergence of most modern metazoan phyla . The event was accompanied by major diversification of other organisms . </P> <P> Before the Cambrian explosion, most organisms were simple, composed of individual cells occasionally organized into colonies . Over the following 70 to 80 million years, the rate of diversification accelerated, and the variety of life began to resemble that of today . Almost all present animal phyla appeared during this period . </P>

What is meant when scientists refer to the cambrian explosion