In 1969, Whittaker described a system of classification that distinguished between kingdom according to cellular organization and mode of nutrition. According to this system members of kingdom monera are bacteria and cyanobacteria. They are prokaryotes members of kingdom protista are eukaryotic and consist of single cells or colonies of cells. Members of kingdom plantae are eukaryotic, multi-cellular and photosynthetic. Plants have walled cells and are usually non motile. Members of kingdom fungi are also eukaryotic and multi-cellular. They also have walled cells and are usually non motile. Fungi are decomposers. Members of kingdom animalia are eukaryotic and multi-cellular and they usually feed by ingesting other organisms or parts of other organisms. Their cells lack walls and they are usually motile.
For the first two billion years of life on the earth, living forms were prokaryotic microbes. Molecular studies of variations in base sequences of ribosomal RNA from more than two thousand organisms are providing evidence of relationship rooted within this two billion year period. Emerging picture is that five kingdoms do not represent distinct evolutionary lineages.
Evolutionary Conservation: Ribosomal RNA is excellent for studying the evolution of early life on earth. It is an ancient molecule and it is present and retains its function in virtually all organisms. In addition ribosomal RNA changes very slowly. This showness of change is called evolutionary conservation. It indicates that the protein producing machinery of a cell can tolerate little change and still retain its vital function. Closely related organisms (recently diverged from a common ancestory) are likely to have similar ribosomal RNAs. Distantly related organisms have ribosomal RNAs that are less similar but the differences are small enough that the relationship to some ancestral molecules are still apparent.
Method: Molecular systematic compare the base sequences in ribosomal RNA of different organisms to find the number of positions in RNAs where bases are different. They enter these data into computer programmes and examine all possible relationships among different organisms. The systematic then decide which arrangement of the organisms best explains the data.
Conclusion: Studies of ribosomal RNA have let systematic to the conclusion that all life shares a common ancestor and that there are three major evolutionary lineages. Each of these lineage is called domain. Archara are prokaryotic microbes that live in extreme environments such as high temperature rift valleys on the ocean floor or high salt or acidic environments. They environments may reflect conditions on the earth at the time of life’s origin. The archara are the most primitive life forms known. Ancient archaras gave rise to two other domains of organisms Eubacteria and Eukarya. Eubacteria are true bacteria and are prokaryotic micro-organisms. Eukarya include all eukaryotic organisms eukaraya diverged more recently related to Archara than Eubacteria to Archara.
Eubacteria to Archara:
Eukarya arose about 1.5 billion years ago. Photosynthetic accumulation of oxygen in the atmosphere probably resulted in the production of ozone which shielded the planet from ultra violet light. Eukarya then underwent late, but raid, evolutionary diversification into modern lineages of protests, fungi, plants and animals.
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