ABSTRACT
Our theory is embarrassingly simple. What made today's prokaryotes and modern cyanobacteria so robust is the fact that in their origin, back in the Archean (3 billion years ago), selection did not play a central role in evolution, it had only a transitory role. Asexual reproduction, mutation, drift and sampling variance in local demes were more important especially when they were accompanied by population catastrophes, where millions perished. Metazoans are generally macroscopic, sexually reproducing, ecologically specialized organisms whose history is full of extinctions and radiations leading to morphological change. On the other hand, prokaryotes, thanks to their origin, avoid extinction because as a group they have slowly evolved as generalists. Specialization appears to be less important than ecological versatility and metabolic unspecialization. Modern cyanobacteria keep on using that strategy