CAMBRIAN EXPLOSION ~542 million years
The Cambrian explosion or Cambrian radiation was the seemingly rapid appearance of most major groups of complex animals around 530 million years ago, as evidenced by the fossil record .
This was accompanied by a major diversification of other organisms, including animals, phytoplankton, and calcimicrobes. Before about 580 million years ago, 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 (as defined in terms of the extinction and origination rate of species) and the diversity of life began to resemble today’s.
The Cambrian explosion has generated extensive scientific debate. The seemingly rapid appearance of fossils in the “Primordial Strata” was noted as early as the mid 19th century, and Charles Darwin saw it as one of the main objections that could be made against his theory of evolution by natural selection.
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 left in Cambrian rocks. (click to see all the article on Wikipedia)
(譯者註：major groups 主要群體，指的是大的分類項目，找了生物界分類，group不在分類中，無從對應過去，有錯請指正。)
(譯者註：1.calcimicrobes 沒找到中文翻譯，我是把字拆成兩部份翻的，正確性完全不知XD 2.一個數量級：有好幾種，2，10，10^3，2^10，一般來說是10。 3.看來這式子應該是：物種誕生/物種滅絕)