The three domains of life are Archaea, Bacteria, and Eukarya/Unit_1%3A_Introduction_to_Microbiology_and_Prokaryotic_Cell_Anatomy/1%3A_Fundamentals_of_Microbiology/1.3%3A_Classification_-_The_Three_Domain_System). The three-domain system is a biological classification introduced by Carl Woese, Otto Kandler, and Mark Wheelis in 1990 that divides cellular life forms into these three domains. The classification system recognizes the fundamental divide between the two prokaryotic groups, insofar as Archaea appear to be more closely related to eukaryotes than they are to other prokaryotes – bacteria-like organisms with no cell nucleus. The three domains are defined as follows:

• Archaea: Single-celled microorganisms that lack nuclei. They are often found in extreme environments such as hot springs, salt lakes, and deep-sea hydrothermal vents/Unit_1%3A_Introduction_to_Microbiology_and_Prokaryotic_Cell_Anatomy/1%3A_Fundamentals_of_Microbiology/1.3%3A_Classification_-_The_Three_Domain_System).

• Bacteria: Single-celled microorganisms that lack nuclei. They are found in a wide range of environments, including soil, water, and the human body/Unit_1%3A_Introduction_to_Microbiology_and_Prokaryotic_Cell_Anatomy/1%3A_Fundamentals_of_Microbiology/1.3%3A_Classification_-_The_Three_Domain_System).

• Eukarya: Organisms with cells that contain a nucleus and other membrane-bound organelles. This domain includes all plants, animals, fungi, and protists/Unit_1%3A_Introduction_to_Microbiology_and_Prokaryotic_Cell_Anatomy/1%3A_Fundamentals_of_Microbiology/1.3%3A_Classification_-_The_Three_Domain_System).

The three-domain system adds a level of classification "above" the kingdoms present in the previously used five- or six-kingdom systems. This system is based on differences in the sequences of nucleotides in the cells ribosomal RNAs (rRNA), as well as the cells membrane lipid structure and its sensitivity to antibiotics/Unit_1%3A_Introduction_to_Microbiology_and_Prokaryotic_Cell_Anatomy/1%3A_Fundamentals_of_Microbiology/1.3%3A_Classification_-_The_Three_Domain_System).