ORIGIN OF SKELETONIZED PROTISTS AND METAZOANS: GENOMIC AND FOSSIL EVIDENCE

The origins of the major groups of eukaryotes is critical to understanding the development of life and environments in early earth history. The records of these organisms, so far, are based on sparse and commonly misinterpreted fossils or on inferred molecular clocks. Various eukaryotic algae first appear in the Mesoproterozoic (over 1000my or more) as suggested by molecular analyses and the fossil record. For foraminifera, radiolaria, ciliates, and metazoans, older molecular data also indicate a long Precambrian presence back to at least 800my, but true representatives of these groups do not appear as fossils until much later. Skeletal metazoans and heterotrophic protists, including purported foraminifera, tintinnids, and radiolaria reported from rocks as old as 1600mya, are either algal or mineral in origin and are not early heterotrophic protists or animals. Foraminifera, radiolaria, tintinnids and skeletal metazoan fossils do not occur in rocks older than the latest Ediacaran, early Cambrian or even the Mesozoic. Thus, a misfit between fossils and molecules is apparent. Hypotheses to account for the later appearance of fossils compared with the molecular clock data include low populations of these taxa spread widely geographically, the absence of hard parts, the failure of hard parts to preserve or to be discovered, and incorrect age assignments for either the fossils, the molecular clocks, or both. In particular, insufficient taxonomic sampling and fewer gene datasets make molecular clock estimations problematic. Herein, we test the molecular clock hypotheses of early evolution of these taxa, focusing on foraminifera and radiolaria. Reevaluation of the first fossil appearances and robust sampling of modern taxa for 157-gene analysis yields dates of origination for those taxa in close agreement and near the Ediacaran/Cambrian boundary (~540-550ma). Shortly after their appearances with skeletons, the Cambrian radiation began populating the oceans with early representatives of the Phanerozoic biotas.