Introduction: From Aristotle to the Five Kingdoms

Since the earliest days of civilisation, people have tried to sort the living world into groups. At first this was not done scientifically at all -- it grew out of the practical need to use organisms for food, shelter and clothing.

Aristotle made the earliest attempt at a more scientific basis for classification. He used simple morphological (visible, external) characters: plants were sorted into trees, shrubs and herbs, and animals were divided into just two groups -- those with red blood and those without.

By Linnaeus' time, biologists had settled on a Two Kingdom system: every living thing was placed into either Plantae or Animalia, covering all plants and all animals respectively. This system was simple and easy to apply, but as more organisms were studied closely it turned out to be inadequate, because it never actually distinguished between:

• eukaryotes and prokaryotes -- bacteria, which have no true nucleus, were lumped in with organisms that do
• unicellular and multicellular organisms
• photosynthetic organisms such as green algae and non-photosynthetic ones such as fungi

A large number of organisms simply did not sit comfortably under either "plant" or "animal", and it became clear that classification needed to look beyond gross external shape to characters such as cell structure, the nature of the cell wall, mode of nutrition, habitat, methods of reproduction and evolutionary relationships.

R.H. Whittaker (1969) answered this by proposing the Five Kingdom Classification -- Monera, Protista, Fungi, Plantae and Animalia -- built on exactly these criteria: cell structure, body organisation, mode of nutrition, reproduction and phylogenetic (evolutionary) relationships. (A further three-domain system, splitting Monera itself into two domains and leaving the eukaryotic kingdoms in a third -- giving a six-kingdom picture overall -- has also been proposed, though that is left for later study.)

The table below summarises how Whittaker's five kingdoms compare on the criteria that matter most -- the kind of cell each has, whether it carries a cell wall (and what that wall is made of), whether the nucleus has a membrane around it, how the body is organised, and how the organism gets its nutrition:

It helps to see exactly why the old cell-wall-based grouping broke down. Under the earlier system, anything with a cell wall was filed under "Plantae" -- bacteria, blue-green algae (cyanobacteria), fungi, mosses, ferns, gymnosperms and flowering plants all sat in the same kingdom for that one reason alone. That single criterion papered over huge differences: it placed prokaryotic bacteria and cyanobacteria alongside true eukaryotes, and it filed unicellular organisms next to multicellular ones -- for example, the single-celled Chlamydomonas and the multicellular, filament-forming Spirogyra were both simply called "algae" despite this difference in body plan. It also failed to separate fungi from green plants, even though the two are nutritionally opposite -- fungi are heterotrophs that absorb food from their surroundings, while green plants are autotrophs that photosynthesise -- and even though their cell walls are chemically distinct (chitin in fungi, cellulose in plants). Once this chemical and nutritional difference was recognised, fungi were pulled out into a kingdom of their own -- Kingdom Fungi -- rather than being kept as "plants" simply because they, too, had a wall around each cell.

The clearest illustration of why this change mattered is Kingdom Protista itself. Under the old classification, Chlamydomonas and Chlorella -- both single-celled organisms that happen to have a cell wall -- were filed under Algae, inside the plant kingdom, essentially because a cell wall was treated as a "plant" feature. Paramoecium and Amoeba, having no cell wall, were filed under the animal kingdom instead. Whittaker's Protista brought all four of these together into one kingdom, because what actually unites them has nothing to do with a wall or a resemblance to a "typical" plant or animal -- it is that each of them is a single-celled eukaryote. This is a concrete case of organisms that earlier systems had scattered across different kingdoms ending up side by side once the classifying criteria themselves changed, and it is exactly the kind of regrouping that keeps happening as our understanding of characteristics and evolutionary relationships improves.

This chapter looks in detail at Kingdoms Monera, Protista and Fungi from Whittaker's scheme; Plantae and Animalia are covered separately in the chapters that follow.