Causes and consequences of the Cambrian explosion

The Cambrian explosion has long been a basic research frontier that concerns many scientific fields. Here we discuss the cause-effect links of the Cambrian explosion on the basis of first appearances of animal phyla in the fossil record, divergence time, environmental changes, Gene Regulatory Networks, and ecological feedbacks. The first appearances of phyla in the fossil record are obviously diachronous but relatively abrupt, concentrated in the first three stages of the Cambrian period (541-514 Ma). The actual divergence time may be deep or shallow. Since the gene regulatory networks (GRNs) that control the development of metazoans were in place before the divergence, the establishment of GRNs is necessary but insufficient for the Cambrian explosion. Thus the Cambrian explosion required environmental triggers. Nutrient availability, oxygenation, and change of seawater composition were potential environmental triggers. The nutrient input, e.g., the phosphorus enrichment in the environment, would cause excess primary production, but it is not directly linked with diversity or disparity. Further increase of oxygen level and change of seawater composition during the Ediacaran-Cambrian transition were probably crucial environmental factors that caused the Cambrian explosion, but more detailed geochemical data are required. Many researchers prefer that the Cambrian explosion is an ecological phenomenon, that is, the unprecedented ecological success of metazoans during the Early Cambrian, but ecological effects need diverse and abundant animals. Therefore, the establishment of the ecological complexity among animals, and between animals and environments, is a consequence rather than a cause of the Cambrian explosion. It is no doubt that positive ecological feedbacks could facilitate the increase of biodiversity. In a word, the Cambrian explosion happened when environmental changes crossed critical thresholds, led to the initial formation of the metazoan-dominated ecosystem through a series of knock-on ecological processes, i.e., ecological snowball effects.