Current understanding on the Cambrian Explosion: questions and answers

Abstract

The Cambrian Explosion by nature is a three-phased explosion of animal body plans alongside episodic biomineralization, pulsed change of generic diversity, body size variation, and progressive increase of ecosystem complexity. The Cambrian was a time of crown groups nested by numbers of stem groups with a high-rank taxonomy of Linnaean system (classes and above). Some stem groups temporarily succeeded while others were ephemeral and underrepresented by few taxa. The high number of stem groups in the early history of animals is a major reason for morphological gaps across phyla that we see today. Most phylum-level clades achieved their maximal disparity (or morphological breadth) during the time interval close to their first appearance in the fossil record during the early Cambrian, whereas others, principally arthropods and chordates, exhibit a progressive exploration of morphospace in subsequent Phanerozoic. The overall envelope of metazoan morphospace occupation was already broad in the early Cambrian though it did not reach maximal disparity nor has diminished significantly as a consequence of extinction since the Cambrian. Intrinsic and extrinsic causes were extensively discussed but they are merely prerequisites for the Cambrian Explosion. Without the molecular evolution, there could be no Cambrian Explosion. However, the developmental system is alone insufficient to explain Cambrian Explosion. Time-equivalent environmental changes were often considered as extrinsic causes, but the time coincidence is also insufficient to establish causality. Like any other evolutionary event, it is the ecology that make the Cambrian Explosion possible though ecological processes failed to cause a burst of new body plans in the subsequent evolutionary radiations. The Cambrian Explosion is a polythetic event in natural history and manifested in many aspects. No simple, single cause can explain the entire phenomenon.

Fossil evidence unveils an early Cambrian origin for Bryozoa

“…This aligns the origin of phylum Bryozoa with all other skeletonized phyla in Cambrian Age 3, pushing back its first occurrence by approximately 35 million years. It also reconciles the fossil record with molecular clock estimations of an early Cambrian origination and subsequent Ordovician radiation of Bryozoa following the acquisition of a carbonate skeleton [10][11][12][13] .…”

“…The Cambrian fossil record chronicles in exceptional detail the emergence of major bilaterian clades and continues to provide chronological constraints on the evolutionary diversification of disparate metazoans from a common ancestor [12][13][14][15] . Nearly all animal phyla, including soft-bodied Deuterostoma 14 , Entoprocta 16 , Phoronida 17 and Priapulida 12 , made their first appearance during the Cambrian evolutionary radiation 12,13,18 .…”

“…The Cambrian fossil record chronicles in exceptional detail the emergence of major bilaterian clades and continues to provide chronological constraints on the evolutionary diversification of disparate metazoans from a common ancestor [12][13][14][15] . Nearly all animal phyla, including soft-bodied Deuterostoma 14 , Entoprocta 16 , Phoronida 17 and Priapulida 12 , made their first appearance during the Cambrian evolutionary radiation 12,13,18 . A key exception is the 'missing' colonial lophotrochozoan phylum Bryozoa, in which six of the eight recognized orders belonging to the classes Stenolaemata and Gymnolaemata appear abruptly with considerable diversity during the early Ordovician period 6,7,19,20 .…”

Supply-driven evolution explains the locking-in of structures that open new evolutionary possibilities, such as higher hierarchies of organization

“…Another important phenomenon can be made sense of in the light of this theory: the empirical insight that the most evolutionarily impactful innovations are actually quite silent in the moment of their invention [65]. There is evidence, for example, that a long period of cryptic evolution has been present before the Cambrian explosion [66]. Closer to home, this phenonmenon was seen in the Long Term Evolution Experiment (LTEE) when aerobic citrate utilization evolved among lineages of E. coli .…”

The Origin of Immunity by Means of Prior Infection and Vaccination with Implications for the Origin of Species

“…Such sudden appearance of forms which are as complex as the animals that exist today, 34 has since been referred to as the Cambrian Explosion 35 and Evolution's big bang. 36,37 And for as long as the Cambrian Explosion has been seen as a consequence of rapid manner of animal diversification in the light of Darwin's inference that the evolutionary law and the biological law according to which the species appeared on the planet are one and the same, the event has appeared to be one that is far more complicated than any historical event 38 and therefore incapable of being accounted for by any single cause which can be easily tested. 38,39, But the results that predict the event enable us to see that it is not one of rapid diversification at all as we have assumed since the time of Darwin but rather one in which the diverse species of animals, which originated independently of one another long before the Cambrian, according to a biological law which is different from the evolutionary law, were changed from forms which could not be preserved in rocks into mineralized forms which could be preserved within a short period so that their fossils appeared suddenly in the rocks of the Cambrian as though they had just appeared on the planet during the period.…”