Stromatolites as Evolutionary Forcing Houses
The success of cyanobacteria didn't just pump waves of oxygen into ocean water and the atmosphere. It also began to affect the anoxic bacteria living within stromatolites. Stromatolites would have become forcing houses of evolution, where massive bacterial populations were stressed by intermittent waves of oxygen that were weak at first but gradually became more intense and longer-lasting. Oxygen tolerance and then oxygen use probably evolved in several lines of bacteria in these microenvironments. Stromatolite microbiology was probably as complex at 2000 Ma as it is today.
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