The microbial world, generally invisible to the naked eye, has largely shaped our environment and has been instrumental in the emergence and evolution of all other living organisms on Earth. These microscopic unicellular organisms were for 3 billion years the only forms of life on our planet.Their most spectacular action was the modification of the primitive atmosphere: the dioxygen certainly not present initially reached its present concentration (21 % of the gas content of the atmosphere) through the action of microorganisms that are able of oxygenic photosynthesis. For the evolution of life, it is now widely accepted that multicellular life forms extremely complex have emerged from eukaryote microorganisms classified in the kingdom Plantae and in the Stramenopiles and Opisthokonta (especially metazoans which includes humans). These life forms are still dependent on the activity of microorganisms.If a disaster, whether natural or caused by humans, should annihilate all nonmicrobial living species, it is likely that some microorganisms that have colonized all oceans (from the surface to the abyssal domain) and the earth’s crust (to a depth of hundreds of meters) would be spared and would allow the initiation of a new evolution process, whatever the new environmental conditions at the end of this disaster, except in the absence of liquid water.