Encyclopedia of Astrobiology

Living Edition
| Editors: Muriel Gargaud, William M. Irvine, Ricardo Amils, Henderson James Cleaves, Daniele Pinti, José Cernicharo Quintanilla, Michel Viso

Unicellular Organisms

  • Ricardo Amils
Living reference work entry
DOI: https://doi.org/10.1007/978-3-642-27833-4_1628-2


A unicellular organism is any life form that consists of just a single cell. Most forms of life are unicellular. They can be found in the three domains of life, Bacteria, Archaea, and Eukarya, although most prokaryotes are unicellular organisms. Unicellular organisms are ubiquitous; they can be found in many different types of environments. The oldest forms of life were unicellular. Multicellular organisms appeared later in the fossil record, and some authors correlate their appearance with an increase in oxygen, a by-product of oxygenic photosynthesis, in the atmosphere.

See Also


Single Cell Bioorganic Chemistry Life Form Fossil Record Multicellular Organism 
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Copyright information

© Springer-Verlag Berlin Heidelberg 2014

Authors and Affiliations

  1. 1.Departamento de Biologia MolecularUniversidad Autónoma de MadridMadridSpain