Since people first began to classify living things in a systematic way in the 18th century the name 'kingdom' has been used to separate out the primary categories. To begin with two kingdoms: the animal kingdom and the plant kingdom were defined; the animals generally feeding on other organisms, and the plants generally producing their energy by photosynthesis. This division has become increasingly difficult to apply to many simple organisms, and some since the 1960s several schemes have been proposed based on a primary division according to the structure of the nucleus. This division defines:-

Eukaryotic organisms, with definite nuclei in their protoplasm, like most large plants and animals.

Prokaryotic organisms, with no compact nucleus, but nuclear material dispersed in the protoplasm.

The prokaryotes include some of the blue-green algae and the bacteria. The blue-green algae have chlorophyll and are capable of photosynthesis. The bacteria, in the main, lack chlorophyll and must obtain their energy from sources other than light; they have a large range of chemical powers. Both groups have been found in early Precambrian rocks which are much older than the rocks containing the first definite eukaryotes. The blue-green algae are considered to have been the world's first source of free oxygen produced by photosynthesis; they were thus necessary precursors to the development of the first animals.

Most stromatolites were (and are) formed by blue-green algae but some other groups can also form these mounds. Crowded, erect filaments on their upper surface trap mud particles, which become consolidated to form domes with a layered internal structure. Similar algae can also form oncoliths, spherical layered structures like large ooliths.

Eukaryotes can be divided roughly according to their methods of obtaining energy:

A divsion according to whether or not individuals exists as as a single cell-like organisms produced the division of protozoa.

These divisions become blurred in many single-celled organisms, most of which are microscopic. These uncertainties have resulted in some being classified as protozoans or members of the Animal and Plant Kingdoms.

Mainly from discoveries about the molecular structure of nucleic acids, it is now realised that the greatest division in the living world is not between plants and animals but within the microorgranisms between the eukaryotic bacteria and the eurkaryotic protozoa.

This has led to the classification of living things into two primary 'Domains' and five 'Kingdoms'. This 5-kingdom classification of life has been produced by Margulis and Schwartz to take account of differences and similarities in the structure of nucleic acids and features of the whole organism, molecular, morphological and developmental.

The domain of prokaryotes contains the Kingdom of the bacteria, all having a simple unicellular architecture that does not have nuclei and other organellesin the cytoplasm, and the other domain of eukaryotes, which comprises organisms with more complex cells, divided into four Kingdoms of the protozoans, animals, fungi and plants.

the Prokarya with their simple unicellular architecture lacking nuclei and other organelles, and forming the kingdom of Bacteria, the Eukarya made of more complex cells and including the other four kingdoms of

Classifications must also record degree and amount of diversity and complexity (while never violating the primary signal of phylogeny, or order of branching), as well as the timing of branch points. When these criteria are added, the breaking of the enormous eukaryote branch into four kingdoms, and the compression of the two prokaryote branches into one kingdom of Bacteria seems fully justified, if only for our legitimately parochial interest in the astonishing diversity of organisms in our visible range of size and complexity.

From an enlarged and less human-centred perspective, bacteria really are the dominant form of life on Earth—and always have been, and probably always will be. They are more abundant, more indestructible, more diverse in biochemistry (if not in complexity and outward form), and inhabit a greater range of environments than all the other four kingdoms combined.