Cambrian Explosion

Abstract

The Cambrian explosion of life was a relatively short period c. 540 Mya when a sudden acceleration in evolution led to the rise of multicellular animals, but the cause of this key biological event remains elusive. Diverse environmental, developmental and ecological causes have been put forward as potential triggering events, but none of them has so far succeeded in obtaining widespread acceptance. Probably the correct answer has to be sought in a unifying theory capable of taking into account several of the most likely candidate factors and using adequate physiological experimental models such as marine sponges, the oldest extant Precambrian metazoan phylum. The Cambrian explosion might have been unleashed by the coincidence in time of primitive metazoans endowed with self‐/non‐self‐recognition and of a surge in sea water calcium that increased the binding forces between their calcium‐dependent cell adhesion molecules. Key Concepts: The Cambrian explosion is widely regarded as one of the most relevant episodes in the history of life on Earth, when about half of living animal phyla first appear in the fossil record. The proposals to explain the occurrence of the Cambrian explosion in terms of why and when it happened have been various and usually fall within environmental, developmental and ecological reasons. The triggering event of the Cambrian explosion likely has not only environmental, developmental or ecological causes, but a mixture of them all. The development of self‐/nonself‐recognition in primitive metazoans was a necessary innovation for the consolidation of multicellularity. The molecular basis of the Cambrian explosion should be understood based on mechanisms which could generate organismal diversity by utilising pre‐existing genes but not by creating new genes with novel function. Marine sponges are an ideal candidate model to provide answers dealing with molecular, cellular, organismal and populational aspects of the Cambrian explosion. Calcium‐based cell adhesion is essential for the integrity of marine sponges. There was a well‐referenced substantial increase in sea water calcium levels around the beginning of the Cambrian period. The calcium increase in Cambrian oceans coincided with the existence of primitive sessile metazoans, endowed with allogeneic recognition and carrying cell adhesion molecules which had significantly longer dissociation times at the new calcium concentrations. The Cambrian explosion might have been triggered by the coincidence in time of primitive animals endowed with self‐/nonself‐recognition and of a surge in sea water calcium that increased the binding forces between their calcium‐dependent cell adhesion molecules.

Evolution of the Calcium-Based Intracellular Signaling System

“…The evolution of the toolkit in animals and plants appears to be substantially different, with plants showing a greater degree of coevolution between the major functions and animals expanding mechanisms for decoding and signal relay rather than creating new mechanisms for generating signatures. This abundance and diversity of calcium-signaling proteins coincided with the diversification of complex animals ( Fernandez-Busquets 2010 ; Verkhratsky and Parpura 2014 ), during which the toolkit consolidated under selection. In the plant lineage there is an independent example of environmental calcium concentration affecting the size of the toolkit during evolution; we observe a transient increase in protein diversity when Chlorophyta and Streptophyta diverged in freshwater and seawater habitats ( Becker and Marin 2009 ) leading to a subsequent loss of diversity in decoding and relay mechanisms of Streptophyta ( fig.…”

Adhesion of freshwater sponge cells mediated by carbohydrate–carbohydrate interactions requires low environmental calcium

“…Sponges (Phylum Porifera) likely emerged in Precambriam 800 Mya, which makes these sessile filter-feeding animals the oldest extant metazoans (Fernàndez-Busquets 2010). Although marine sponge body fossils are found in shales dated between 660 and 635 Ma (Cryogenian Period), most of freshwater sponges´ fossil records (> 99.5%) are much more recent, dating back up to the Miocene epoch (~23 Ma) (Maloof et al 2010;Pronzato et al 2017).…”

Carbohydrate-Carbohydrate Interactions Mediated by Sulfate Esters and Calcium Provide the Cell Adhesion Required for the Emergence of Early Metazoans

“…The Cambrian explosion was a relatively short period ϳ540 Ma that marked a general acceleration in the evolution of most animal phyla; however, the trigger of this key biological event remains elusive. We hypothesized that the Cambrian explosion may have been unleashed by the coincidence in time of primitive sponge-like metazoans endowed with 1) self/non-self-recognition, 2) adhesive sulfated polysaccharides, and 3) a surge in seawater calcium that increased the binding forces and stabilities between their calcium-dependent cell adhesion molecules (1,5).…”

“…Molecular clock estimates, paleobiogeochemical evidence, and the most widely accepted phylogenetic inferences place sponges (phylum Porifera) at the root of the metazoan tree as the oldest extant multicellular animals (1)(2)(3)(4). Such ancestry makes modern sponges suitable organisms for studying the cellular and molecular aspects of the evolution of multicellularity (5).…”

“…About 20 mg of DaSP were dissolved in 0. H TOCSY spectra were run as previously described (30,31) with 4046 ϫ 400 points with a spin-lock field of 10 kHz and a mix time of 80 ms. 13 C, 1 H HSQC spectra were run with 1024 ϫ 256 points and globally optimized alternating phase rectangular pulses for decoupling. 13 C, 1 H HMBC spectrum was recorded with 1024 ϫ 256 points, with a 60-ms delay for evolution of long-range couplings and was set with no decoupling during acquisition time.…”