The Cambrian Explosion: How tectonics drove evolution

If life had been around on the planet for three billion years already, why did multi-cellular organisms (metazoa) only appear 540 million years ago? But when multi-cellular organisms appeared on the plan, they radiated quickly into very many species. The radiation happened so fast, it was dubbed the "Cambrian Explosion".

In a paper in print at Molecular Biology and Evolution Xavier Fernàndez-Busquets et al. looked at sponges to understand what holds them together in a way that is different to bacterial colonies. Sponges are the oldest extant Precambrian metazoan phylum and thus a valid model to study factors that could have unleashed the rise of multicellular animals. One such factor is the advent of self/non-self recognition systems, which would be evolutionarily beneficial to organisms to prevent germ cell parasitism or the introduction of deleterious mutations resulting from fusion with genetically different individuals.

The molecules responsible for self/non-self recognition systems probably evolved gradually before the Cambrian period. Sponge cells associate through calcium-dependent, multivalent carbohydrate-carbohydrate interactions of the g200 glycan found on extracellular proteoglycans. Fernàndez-Busquets et al. propose that stronger cell adhesion allowed the integrity of genetically uniform animals composed only of "self" cells, facilitating genetic constitutions to remain within the metazoan individual and be passed down inheritance lines.

Thus, the Cambrian explosion might have been triggered by the coincidence in time of primitive animals 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. The increase of sea water calcium would be the product of increased tectonic activity and resulting chemical erosion.

Fernandez-Busquets, Xavier, Andre Kornig, u. a. (2009), Self-recognition and Ca2+-dependent Carbohydrate-carbohydrate Cell Adhesion Provide Clues to the Cambrian Explosion, Molecular Biology and Evolution, msp170, doi:10.1093/molbev/msp170.