Stromatolites are layered bio-chemical accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms (microbial mats) of microorganisms, especially cyanobacteria. Fossilized stromatolites provide ancient records of life on Earth by these remains, some of which may date from 3.7 billion years ago. This is remarkable by itself but even more so when you consider that the oldest rocks that have been dated are 3.8 billion years old. Billions of cyanobacteria producing oxygen, constantly, over many millions of years made an atmosphere that could support life as we know it. The beginning of the Cambrian Period Marked a time of decrease for the stromatolites but oxygen breathing life forms exploded in the oceans of that period.