FOSSILS, MOLECULAR CLOCKS AND THE CAMBRIAN EXPLOSION

Recent inquiries into the relationships among the animal phyla have demonstrated that Bilateria can be subdivided into two groups, the protostomes and the deuterostomes, the latest common ancestor of which is referred to as PDA (Protostome-Deuterostome Ancestor). Comparative developmental and anatomical data suggests that PDA had all of the necessary attributes to leave both body and trace fossils. Paleontological data indicate that such a complex PDA could not be much older than about 555 Ma ago, which places the origin of crown-group Bilateria within about 20 million years of the subsequent Cambrian explosion. However, all molecular clock analyses estimate that PDA is minimally 100 Ma older, and most analyses estimate that it is 300-500 Ma older, than the first appearance of bilaterians in the fossil record. Given that most clock analyses rely on a single well-dated calibration point and use almost exclusively vertebrate, fly and nematode sequences where significant rate heterogeneity exists, these analyses may significantly overestimate the age of PDA. We chose to use echinoderms to investigate the question of the age of PDA because their excellent fossil record allows for the utilization of multiple well-dated calibration points, and they show several qualities that may minimize the degree of rate heterogeneity. Analyzing the concatenated sequence from three different proteins (900 amino acids) from seven taxa (demosponge, polychaete and five echinoderms) and three calibration points we find that the minimum age of PDA is 540 Ma ago, and the minimum age of crown-group Metazoa is just over 600 Ma ago. These data indicate that the fossil record accurately represents bilaterian evolution, with the Cambrian explosion reflecting the diversification of bilaterian phyla soon after the origin of crown-group Bilateria. Moreover, because of the apparent temporal coincidence between the origin of crown-group Metazoa and the Marinoan glaciation(s), "Snowball Earth" may have played a profound selective role upon the origin and early evolution of metazoans.