A major scientific milestone was Carl Woese's discovery that the evolution of life on Earth has resulted in three distinct types of living systems:Archaea, Bacteria and Eukarya1. Organisms that belong to the eukaryal domain are clearly different from the former two because of their more complex structural organization: without exception, they possess intracellular compartments (at least a nucleus, often with additional organelles), and in addition they often have a multicellular composition. The Archaea and the Bacteria are prokaryotes: they have neither a nucleus nor any other cytoplasmic compartments, and are generally unicellular.

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