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 byproduct of  oxygenic photosynthesis, in the atmosphere.

 Archea

 Bacteria

 Cell

 Eukarya

 Multicellular Organisms

 Oxygenic Photosynthesis

 Prokaryote