Estimated minimal divergence times of the major bacterial and archaeal phyla

Previous calibrations of prokaryotic phylogenetic events were based on the vertebrate fossil record because a detailed microbial fossil record does not exist. Recently, compounds (2-methylhopanoids) that are found in cyanobacterial membranes were identified among compounds extracted from late Archean sedimentary rocks. These lipids establish a minimum time, 2.65 Ga (Billion Years Ago), in the geologic record for the existence of cyanobacteria. We have used this new information to calibrate a Neighbor Joining distance tree generated from SSU rRNA sequences of major prokaryotic lineages and to estimate the dates for significant events in the history of life on Earth. The results suggest that the Last Common Ancestor occurred at about 4.29 Ga and that the individual Bacterial and Archaeal Domains began radiating about 3.46 Ga.