eLife assessment
This important paper addresses the challenging problem of dating the origin of several groups of marine microorganisms. The analyses are solid, with various test of clock models and time calibrations used, however given the uncertainty of many of the dates used to anchor ancient geological events, further studies are needed to support or refute the hypotheses put forth in this paper. Despite some methodological concerns, this work is a commendable attempt at an extremely difficult problem and will be of broad interest to microbiologists, geologists, and evolutionary biologists.