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The Cambrian explosion (also known as the Cambrian radiation or Cambrian diversification) is an interval of time beginning approximately 538.8 million years ago in the Cambrian period of the early Paleozoic, during which a sudden radiation of complex life occurred. The fossil record of this period contains the first appearances of practically every animal phyla, implying that nearly all animal life has its origin during this radiation. It lasted between 13 and 25 million years and resulted in the divergence of most modern metazoan phyla. The event was accompanied by major diversification in other groups of organisms as well.
Before early Cambrian diversification, most organisms were relatively simple, composed of individual cells or small multicellular organisms, occasionally organized into colonies. As the rate of diversification subsequently accelerated, the variety of life became much more complex and began to resemble that seen today. Almost all present-day animal phyla appeared during this period, including the earliest chordates.
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