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A unicellular organism, also known as a single-celled organism, is an organism that consists of only one cell, unlike a multicellular organism that consists of more than one cell. Historically the simple single celled organisms have sometimes been referred to as Monads. The main groups of unicellular organisms are bacteria, archaea, protozoa, unicellular algae and unicellular fungi. Unicellular organisms fall into two general categories: prokaryotic organisms and eukaryotic organisms. Unicellular organisms are thought to be the oldest form of life, with early protocells possibly emerging 3.8-4 billion years ago. Prokaryotes, most Protista, and some fungi, are unicellular. Although some of these organisms live in colonies, they don't exhibit specialization. These organisms live together, and each cell in the colony is the same. However, each individual cell must carry out all life processes to survive. In contrast, even the simplest multicellular organisms have cells that depend on each other to survive. Most multicellular organisms have a unicellular life-cycle stage. Gametes, for example, are reproductive unicells for multicellular organisms. Additionally, multicellularity appears to have evolved independently many times in the history of life. Some organisms are partially uni- and multicellular, like ''Dictyostelium discoideum''. Additionally, unicellular organisms can be multinucleate, like Myxogastria and ''Plasmodium''. ''Candidatus'' Magnetoglobus multicellularis, related to Deltaproteobacteria, is a multicellular prokaryote. It is neither unicellular, nor a colony. == Evolutionary Hypothesis == Primitive cells, often referred to as protocells, are the precursors to todays unicellular organisms. Although the origin of life is largely still a mystery, in the currently prevailing theory, known as the RNA world hypothesis, early RNA molecules would have been the basis for catalyzing organic chemical reactions and self-replication. The RNA world hypothesis assumes that RNA molecules could form in abiotic conditions, which would require nucleic acids and ribose to be present. Theoretical and experimental findings show that nucleic acids and sugars could have been synthesized in early prebiotic conditions.〔 Compartmentalization was necessary for chemical reactions to be more likely as well as to differentiate reactions with the external environment. For example, an early RNA replicator ribozyme may have replicated other replicator ribozymes of different RNA sequences if not kept separate. When amphiphiles like lipids are placed in water, the hydrophobic (water fearing) tails aggregate to form micelles and vesicles, with the hydrophilic (water loving) ends facing outwards.〔〔 Primitive cells likely used self-assembling fatty-acid vesicles to separate chemical reactions and the environment.〔 Because of their simplicity and ability to self-assemble in water, it's likely that these simple membranes predated other forms of early biological molecules.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Unicellular organism」の詳細全文を読む スポンサード リンク
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