Multicellular organisms arose because

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Unicellular organisms are still the majority on our planet today. They are evolutionarily a success. Its development for multicellular organisms, however, is, according to some researchers indicate, because of the advantages of "teamwork" carried out by the cells together. It is cheaper (in terms of energy) for cells to divide their work - different cells for each specific function. Thus, when a mutation originated a multicellular organism, it succeeded in its development, surviving and procreating.
Multicellular organisms arose because, among several reasons, they could suffer injuries in parts of their composition and continue living..
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