The Cambrian Explosion: A Molecular Paleobiological Overview

The Cambrian (541–485 Millions of Years ago) is the first period in the Phanerozoic eon (the ‘age of visible life’), and it is characterized by the first appearance of fossil animals with bilateral body symmetry (the Bilateria). Bilateria include the majority of extant animal biodiversity, including Arthropoda (spiders, insects, and crustaceans), Chordata (e.g., vertebrates and amphioxus), and Mollusca (e.g., land snails and squids). The appearance of animals in Cambrian rocks is abrupt. This phenomenon has been described as the ‘Cambrian explosion’, and it has been historically considered as indicative of the origin of animals in the Cambrian. While the explosive nature of the appearance of animal fossils in Cambrian rocks is undeniable, the evolutionary significance of the Cambrian explosion has been debated. However, multiple lines of evidence (trace and body fossils as well as time-calibrated molecular phylogenies) suggest that it was a real diversification event, the radiation of the extant animal phyla, and not merely an increase in fossil abundance.