Aristotle devised the first scientific method of classification, based on morphological characters, where he classified plants into trees, shrubs and herbs and animals into two groups - those that had red blood and those that didn’t. Later, Carl Linnaeus came up with the two kingdom classification, Kingdom Plantae that included all the plants and Kingdom Animalia that included all the animals. However, this system did not distinguish between unicellular and multicellular organisms, prokaryotes and eukaryotes and photosynthetic and non-photosynthetic organisms. There were also a large number of living organisms such as bacteria, algae and fungi that did not fall into any category under this system.
In 1969, R. H. Whittaker proposed a five kingdom classification system where three more kingdoms, Monera, Protista and Fungi were added to the existing kingdoms of Plantae and Animalia. The main criteria that Whittaker used for this classification system were cell structure, thallus organisation, mode of nutrition, reproduction and phylogenetic relationships. He grouped all heterotrophic organisms based on the differences in cell wall composition, under Kingdom Fungi; and all prokaryotic organisms under kingdom Monera; and unicellular eukaryotic organisms under kingdom Protista.
Previously, unicellular Chlamydomonas and multicellular Spirogyra were placed together under algae in Kingdom Plantae. However, in the new classification system, Chlamydomonas moved to Kingdom Protista, while Spirogyra remained in Kingdom Plantae. Similarly, Kingdom Protista brought together both Chlamydomonas and Chlorella with other eukaryotes like Amoeba and Paramoecium, which were initially placed in Kingdom Animalia.