Triggers for the Cambrian explosion: Hypotheses and problems

Abstract

Abrupt appearance of major bilaterian clades in the fossil record during the first three stages of the Cambrian Period has puzzled the scientific world since 1830s. Many proposed causes including environmental, developmental, and ecological hypotheses, are reviewed. Nutrient availability, oxygenation, and change of seawater composition are commonly supposed to be environmental triggers. The nutrient input, e.g. the enrichment of phosphorus in an environment, would cause excess primary production, but it is neither directly linked with diversity nor disparity. Fluctuating abiotic conditions during the Snowball Earth and the associated oxygenation event may have stimulated the diversification of complex multicellular organisms including diverse of macroscopic and morphologically differentiated algae in the early Ediacaran, but did not lead to the ecological success of metazoan or bilaterian lineages. Further increase of oxygen level and change of seawater composition just before and during early Cambrian are suggested by the high weathering rate of the trans-Gondwana mountains, Great Unconformity, and decline of oceanic salinity. These are potential candidates of environmental triggers for the Cambrian explosion but require future more detailed geochemical studies to confirm. The molecular phylogeny calibrated with the molecular clock data suggested that the developmental system of bilaterians was established before their divergence. This, in turn, suggests that the Cambrian explosion require environmental triggers. However, there still exists the contention between deep or shallow divergence of bilaterian clades, which remains to be solved in the future. The deep divergence model is supported by a majority of molecular clock studies, but is challenged by the paucity of bilaterian fossils before and during the Ediacaran Period. The shallow model is generally consistent with the fossil record, but has to explain the rapidity of increase in diversity, disparity, morphological complexity, acquisition of biomineralized shells, etc. Regardless of deep or shallow model, the conservation of lineage-specific kernels within the gene regulatory networks (GRNs) provides an explanation for the long-term stability of body plans after the Cambrian explosion, and continuous addition of microRNAs into the GRNs seems to correspond well to the increase in morphological complexity. As for ecological causes, some hypotheses (e.g. adaptive radiation after mass extinction, cropping, and geosphere–biosphere feedbacks) cannot explain the uniqueness of the event, some others (such as Cambrian substrate revolution, predator–prey pressure, evolution of zooplankton, and roughening of fitness landscapes) fall into the trap of chicken-and-egg problem because of considering the consequence as a cause. Expansion of ecosystem engineering in the early Cambrian might also be caused by the Cambrian explosion. However, ecosystem engineering associated with Ediacaran ecosystems is likely a pivotal ecological prerequisite for the later ecological success of bilaterian clades, particularly the engineering effect by Ediacaran sponges that ventilated seawater by sponge pumping and removing organic material from the water column. However, the ecological abundance of Ediacaran sponges needs to be further investigated. Finally, a working plan is proposed for future research. For paleontologists, searching for ancestors of early Cambrian faunas is crucial to testify the earlier divergence of bilaterian lineages. Environmentally, precise values on the oxygen level and seawater composition are required during the Ediacaran–Cambrian transition.