A microorganism, or microbe may be a microscopic organism, which can exist in its single-celled form or during a colony of cells. The possible existence of unseen microbial life was suspected from past , like in Jain scriptures from 6th century BC India and therefore the 1st century BC book On Agriculture by Marcus Terentius Varro. The scientific study of microorganisms began with their observation under the microscope within the 1670s by Antonie van Leeuwenhoek. In the 1850s, Pasteur found that microorganisms caused food spoilage, debunking the idea of abiogenesis. In the 1880s, Koch discovered that microorganisms caused the diseases tuberculosis, cholera and anthrax.  Microorganisms include all unicellular organisms then are extremely diverse. Of the three domains of life identified by Carl Woese, all of the Archaea and Bacteria are microorganisms. These were previously grouped together within the two domain system as Prokaryotes, the opposite being the eukaryotes. The third domain Eukaryota includes all multicellular organisms and lots of unicellular protists and protozoans. Some protists are associated with animals and a few to green plants. Many of the multicellular organisms are microscopic, namely micro-animals, some fungi and a few algae, but these aren't discussed here. They sleep in almost every habitat from the poles to the equator, deserts, geysers, rocks and therefore the deep sea. Some are adapted to extremes like extremely popular or very cold conditions, others to high and a couple of like Deinococcus radiodurans to high radiation environments. Microorganisms also structure the microbiota found in and on all multicellular organisms. There is evidence that 3.45-billion-year-old Australian rocks once contained microorganisms, the earliest evidence of life on Earth..  

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