The microorganism term is used instead of microbes; they are very small organisms size belonging to various groups: Bacteria, Fungi, Protozoa, one can add Viruses, characterized by their noncellular structure. Microorganisms were observed for the first time in 1674 by Van Leeuwenhoek but they are only identified 200 years later.
Although those organisms are incredibly small (their size ranges from 0.03 µm for the virus of the foot-and-mouth disease to 300 µm for Paramecium), they play a significant role to keep the planet running. Indeed, they carry out 90% of the biochemical reactions produced on earth, without them the life on earth would be destroyed.
The micro-organisms are probably the first alive beings appeared on ground, they would be thus the common ancestors of all the living being. Although microorganisms appeared 3.8 billion years ago, they are observed by Pasteur and koch through their microscope way after their apparition. Pasteur delivered the fatal blow to the doctrine of spontaneous generation thanks to his examination and Koch discovered revealed the existence of bacilla at the origin of tuberculosis. Those discoveries changed radically the perception of the human beings on the world.