Linnaeus' original hierarchy of organism classification included only two kingdoms. Later, this would be expanded to three by Ernst Haeckel, as some single-celled organisms couldn't be classified as animal or plant. Later still, Edouard Chatton's idea for dividing between single-celled organisms with and without a distinct nucleus was popularized, leading to four kingdoms. Finally, Robert Whittaker addressed the ambiguous classification of fungi between plantae and Protista by making them their own, fifth kingdom.
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