The Cambrian explosion, or less commonly Cambrian radiation, was the relatively short evolutionary event, beginning around in the Cambrian Period, during which most major animal phyla appeared, as indicated by the fossil record. Lasting for about the next 20–25 million years, it resulted in the divergence of most modern metazoan phyla. Additionally, the event was accompanied by major diversification of other organisms. Prior to 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 by an order of magnitude and the diversity of life began to resemble that of today. Many of the present phyla appeared during this period, with the exception of Bryozoa, which made its earliest known appearance in the Lower Ordovician.