Nature

===Microbes===
[[File:Yellow mite (Tydeidae) Lorryia formosa 2 edit.jpg|thumb|upright|A microscopic mite [[Mite|''Lorryia formosa'']]]]
{{Main|Microbe}}
The first form of life to develop on the Earth were microbes, and they remained the only form of life until about a billion years ago when multi-cellular organisms began to appear.<ref>{{cite journal |
author = Schopf J|title = Disparate rates, differing fates: tempo and mode of evolution changed from the Precambrian to the Phanerozoic|journal = Proc Natl Acad Sci USA|volume = 91|issue = 15|pages = 6735–42|date = 1994|pmid = 8041691|doi = 10.1073/pnas.91.15.6735 |
pmc = 44277
|bibcode = 1994PNAS...91.6735S|doi-access = free}}</ref> Microorganisms are single-celled organisms that are generally [[microscopic]], and smaller than the human eye can see. They include [[Bacteria]], [[Fungus|Fungi]], [[Archaea]], and [[Protist]]a.

These life forms are found in almost every location on the Earth where there is liquid water, including in the Earth's interior.<ref>{{cite journal|author = Szewzyk U|author2 = Szewzyk R|author3 = Stenström T|title = Thermophilic, anaerobic bacteria isolated from a deep borehole in granite in Sweden|doi= 10.1073/pnas.91.5.1810|journal = Proc Natl Acad Sci USA|volume = 91|issue = 5|pages = 1810–1813|date = 1994|pmid = 11607462|pmc = 43253|bibcode = 1994PNAS...91.1810S|doi-access = free}}</ref>
Their reproduction is both rapid and profuse. The combination of a high mutation rate and a [[horizontal gene transfer]]<ref>{{cite journal|author = Wolska K|title = Horizontal DNA transfer between bacteria in the environment|journal = Acta Microbiol Pol|volume = 52|issue = 3|pages = 233–243|date = 2003|pmid = 14743976}}</ref> ability makes them highly adaptable, and able to survive in new environments, including [[outer space]].<ref>{{cite journal|author = Horneck G|title = Survival of microorganisms in space: a review|journal = Adv Space Res|volume = 1|issue = 14|pages = 39–48|date = 1981|pmid = 11541716|doi = 10.1016/0273-1177(81)90241-6}}</ref> They form an essential part of the planetary ecosystem. However, some microorganisms are [[pathogen]]ic and can post health risk to other organisms.