Contribution of Scientists :
Carl Woese : 

Explanation

Since the time of civilisation there have been many attempts to classify living organisms.
Classification was done instinctively not using criteria that were scientific but borne out of the need to use organisms for our use for food, shelter and clothing.
Aristotle was the earliest to attempt a more scientific basis for classification
Aristotle has used simple morphological characters to classify plants into trees, shrubs and herbs.
Aristotle divided animals into two groups: those which had red blood and those that did not.
Linnaeus has developed Two Kingdom system of classification with plantae and animalia that included all plants and animals respectively.
This system was used till very recently.
This system did not distinguish between the eukaryotes and prokaryotes, unicellular and multicellular organisms and photosynthetic (green algae) an non-photosynthetic (fungi) organisms.
Classification of organisms into plants and animals was easily done and was easy to understand but a large number of organisms did not fall into either category.
Hence the two kingdom classification used for a long time was found inadequate.