A unicellular organism is any life form that is composed of a single cell; they live and carry out all life processes as a single cell. Based on their complexity, unicellular organisms can be placed in one of two categories: eukaryote and prokaryote. Most unicellular organisms are microscopic, but some are visible to the naked eye.
Eukaryotes have a simple cell structure compared to prokaryotes. The prokaryotic unicellular organism does not have a cell nucleus, while the eukaryotic unicellular organism has a nucleus within the cell. The evolution theory suggests that unicellular organisms were the first living things on earth, existing approximately 3.8 billion years ago.
Unicellular organisms pursue different strategies for survival, including heterotrophy (amoeba), chemotrophy and photosynthesis (cyanobacteria). Unicellular organisms include most life on earth and can be found in virtually every habitat, including the least hospitable conditions. Common examples of unicellular organisms include: cyanobacteria, bacteria, paramecium and amoeba. Most single-celled organisms live in colonies, but each organism remains self-sufficient.
The opposite of a unicellular organism is a multicellular organism. Multicellular organisms are typically larger and require specialized organ systems. They require complex organ systems to facilitate communication between cells, control exchanges with the external environment and supply the cells with the required nutrients.