When philosophers first described the plants and animals that they they found around them, they described them as just that - plants and animals. It was an easy division to make and it was found to be useful for nearly two thousand years. Until recently all living organisms were placed in either the Plant Kingdom or the Animal Kingdom.

During the 20th century, mankind’s knowledge and understanding of the living world became much better and the idea that you could place every life-form into one or other of these kingdoms had to be abandoned. There had always been a few problems on deciding where to place some species, and it often meant that they were to be found in both the plant and animal kingdoms! It is generally agreed that plants are green, don’t move about much and feed themselves using sunlight in a process known as photosynthesis. Animals can be green, but this is not because they are using the green pigment chlorophylll to carry out photosnthesis. Animals must eat some plant or animal substances in order to stay alive and they usually move about or move parts of their bodies.

This is as true today as it was when the first classifications of life were drawn up. Now though, we have 3 other kingdoms in which to put those species that are not so clearly defined. Consider mushrooms and toadstools for instance. They are broadly speaking plant like, but they don’t feed by photosynthesis. Nor do they catch and eat their food, they soak up the remains of plants and animals from around the meshwork of fibres that make up their bodies. For them, a new kingdom was created - the Fungi Kingdom.

Bacteria used to be grouped along with the single-celled plants. Improved techniques of microscopy showed that the cells of bacteria were distinctly different from plant cells and such a fundamental difference would require a new kingdom for the bacteria. The same cell-type is present in a group that was known as the blue-green algae, and so they had to go with the bacteria into the new division - the Monera Kingdom. To avoid confusion with the remaining algae, the blue-green algae have been renamed Cyanobacteria.

The fifth kingdom has been created for the simple organisms, many of which consist of just a single cell, although amongst the Algae are some seaweeds that grow as big as trees. The reason that they are not plants, is that they lack the internal organisation of tissues. This kingdom also contains the Protozoa, a group exemplified by the Amoeba and formerly classified as animals. Some of the Protozoa, such as Euglena, displayed dual standards by having chlorophyll but also eating like an animal. All these simple types are now in the Protoctista Kingdom.

Hence 5 kingdoms to describe all life on earth - except for viruses?

Biodiversity

Toadstools

Ciliate protozoan

Green alga