The Five Kingdom Classification system, proposed by R.H. Whittaker in 1969, uses several criteria to categorize organisms: cell structure (prokaryotic vs. eukaryotic), body organization (unicellular vs. multicellular), mode of nutrition, reproduction, and phylogenetic relationships.

Among these, the mode of nutrition is considered the main basis because it was the primary factor used to distinguish between the three higher kingdoms—Fungi (absorptive heterotrophs), Plantae (autotrophs), and Animalia (ingestive heterotrophs)—which often share similar levels of cellular and body complexity.

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