The Cambrian explosion, or Cambrian radiation, was the relatively rapid appearance, around 542 million years ago, of most major animal phyla, as demonstrated in the fossil record.[1][2] This was accompanied by major diversification of other organisms.[note 1] Before about 580 million years ago,[note 2] most organisms were simple, composed of individual cells occasionally organized into colonies. Over the following 70 or 80 million years, the rate of evolution accelerated by an order of magnitude[note 3] and the diversity of life began to resemble that of today.[5] Ancestors of many of the present phyla appeared during this period,[6][7] with the exception of Bryozoa, which made its earliest known appearance in the Lower Ordovician.[8]

The Cambrian explosion has generated extensive scientific debate. The seemingly rapid appearance of fossils in the “Primordial Strata” was noted as early as the 1840s,[9] and in 1859 Charles Darwin discussed it as one of the main objections that could be made against his theory of evolution by natural selection.[10] The long-running puzzlement about the appearance of the Cambrian fauna, seemingly abruptly and from nowhere, centers on three key points: whether there really was a mass diversification of complex organisms over a relatively short period of time during the early Cambrian; what might have caused such rapid change; and what it would imply about the origin and evolution of animals. Interpretation is difficult due to a limited supply of evidence, based mainly on an incomplete fossil record and chemical signatures remaining in Cambrian rocks.