 Viruses are generally regarded as not living and therefore not considered as microorganisms, although a subfield of microbiology is virology, the study of viruses.Single-celled microorganisms were the first forms of life to develop on Earth, approximately 3–4 billion years ago.Further evolution was slow, and for about 3 billion years in the Precambrian eon, (much of the history of life on Earth), all organisms were microorganisms.