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Reference.org
Unicellular organism
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<p>A unicellular organism, also known as a single-celled organism, is an <a href="/facts/Organism/j8JkL09u">organism</a> that consists of a single <a href="/facts/Cell_(biology)/bLM9UQvy">cell</a>, unlike a <a href="/facts/Multicellular_organism/rk8QBF7o">multicellular organism</a> that consists of multiple cells. Organisms fall into two general categories: <a href="/facts/Prokaryotic/oYyvaDDH">prokaryotic</a> organisms and <a href="/facts/Eukaryotic/SkwyPLMA">eukaryotic</a> organisms. Most prokaryotes are unicellular and are classified into <a href="/facts/Bacteria/wC8xSCsU">bacteria</a> and <a href="/facts/Archaea/Adxh0aHw">archaea</a>. Many eukaryotes are multicellular, but some are unicellular such as <a href="/facts/Protozoa/JczTf25H">protozoa</a>, unicellular <a href="/facts/Algae/U1ftVKU3">algae</a>, and unicellular <a href="/facts/Fungi/BHo5DP59">fungi</a>. Unicellular organisms are thought to be the oldest form of life, with early organisms emerging 3.5–3.8 billion years ago. </p><p>Although some prokaryotes live in <a href="/facts/Colony_(biology)/b5QfltwY">colonies</a>, they are not specialised cells with differing functions. These organisms live together, and each 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. </p><p>Most multicellular organisms have a unicellular life-cycle stage. <a href="/facts/Gamete/j8e8fls6">Gametes</a>, for example, are reproductive unicells for multicellular organisms. Additionally, multicellularity appears to have <a href="/facts/Convergent_evolution/dd0L8z4R">evolved independently</a> many times in the history of life. </p><p>Some organisms are partially unicellular, like <i><a href="/facts/Dictyostelium_discoideum/A3wL6lx6">Dictyostelium discoideum</a></i>. Additionally, unicellular organisms can be <a href="/facts/Multinucleate/mvaHLA0V">multinucleate</a>, like <i><a href="/facts/Caulerpa/jfFEUt9u">Caulerpa</a></i>, <i><a href="/facts/Plasmodium/Je91qK4F">Plasmodium</a></i>, and <a href="/facts/Myxogastria/PIRX1Cyu">Myxogastria</a>. </p>