The organisms whose bodies consist of single cell are called unicellular organisms.
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. Unicellular organisms fall into two general categories: prokaryotic organisms and eukaryotic organisms. Prokaryotes include bacteria and archaea. Many eukaryotes are multicellular, but the group includes the protozoa, unicellular algae, and unicellular fungi. Unicellular organisms are thought to be the oldest form of life, with early protocells possibly emerging 3.8–4 billion years ago.
Although some prokaryotes live in colonies, they are not specialised into 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.
Most multicellular organisms have a unicellular life-cycle stage. Gametes, for example, are reproductive unicells for multicellular organisms.[3] Additionally, multicellularity appears to have evolved independently many times in the history of life.
Some organisms are partially unicellular, like Dictyostelium discoideum. Additionally, unicellular organisms can be multinucleate, like Myxogastria and Plasmodium.
Evolutionary hypothesis
Primitive protocells were the precursors to today's 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.[4] 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.[4]
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.[5]
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.[2][5] Primitive cells likely used self-assembling fatty-acid vesicles to separate chemical reactions and the environment.[5] 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