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.