Taxa rare as fossils are not always included in the following classification.
Until recently, it has been the custom to divide organisms into two kingdoms: the animal kingdom and the plant kingdom; 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 biologists (e.g. Whittaker, 1969) think that the primary division should be defined on the nucleus:
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.
PROKARYOTA (Earlier Precambrian to Present)
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 (Walter, 1972). 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.
Classification of organisms EUKARYOTA (Precambrian to Present)
Eukaryotes can be divided roughly according to their methods of obtaining energy:
plant-like: photosynthesis using chlorophyll, light and carbon dioxide;
animal-like: devouring plants or other animals (dead or alive) and oxidizing their substance; fungus-like: absorbing and degrading organic substances (rare as fossils).
These three divisions become blurred in many single-celled organisms, most of which are microscopic; these have been artificially grouped in the Protista Kingdom. Some of the varied groups or protists are members of the Animal and Plant Kingdoms.
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