Kingdom of Life
Earth originated about 4500 to 5000 million years ago, and life originated on earth about 3500 to 4000 million years from now. So, the 1000 million years, in between, did not see any trace of life, and therefore this period is referred to as the azoic era( without remains of organisms or their products). But at present, we come across more than 2.75 million species of living organisms, out of which more than one million species to the animal group, and the rest belong to the plant groups. Many new from are also added every year to the existing number of animals and plants, then the existing ones were there in the long past. Still, they gradually became extinct due to change in environmental conditions during the last several million years. By studying the fossils, present-day scientists can know about the origin and evolution of different plants and animals to some extent. Several scientists tried to classify living organisms into various groups systematically.
Linnaeus (1758) gave the two kingdom classification of living organisms. He divided the living organisms into two influential groups such as (1) PLANTAE, which includes all the plant and plants like organisms, and (2) ANIMALIA, which includes all the animals. Then Earnst Haeckel, a German biologist (1866), gave the three kingdom classification of living organisms, introducing a third kingdom called PROTOCTISTA into living organisms. The group includes all the eukaryotic, colonial, and simple multicellular organisms that do not fall easily into plants or animal kingdoms. Protoctista is a novel kingdom found in some modern taxonomic schemes. It is based on the Kingdom Protista but includes the acellular slime mold, water models, large ( including large forms such as multicellular seaweeds), and certain other groups like macroalgae and diatoms protozoa. Later, the four kingdom classification was given by Herbert F. Copeland (1938) when he divided the Protoctista into two groups such as (a) Monera( for prokaryotic unicellular organisms) and (b) Protista ( for eukaryotic organisms). R.Y. Stanier, C.B. van Niel, and their colleagues also formally proposed dividing all living things into two great groups, the Prokaryotic and the Eukaryotic.
Two kingdom classification
Two kingdom classification
Linnaeus (1758) presented a two-kingdom classification of living organisms. He divided the living organisms into two significant groups such (i) PLANTAE includes all plant and plant-like organisms, and (ii) ANIMALIA, including all animals based upon the locomotion, mode of nutrition, and basic form and structure; he could easily differentiate between plants.
And animals. The unicellular eukaryotes, such as Amoeba, etc., are placed under Animalia. Plants differ from animals in various respects. Plants are autotrophs, i.e., They synthesize their food or organic requirements from inorganic compounds with the help of sunlight, whereas animals are heterotrophs depending on plants for food. More precisely, a heterotrophic organism has an inorganic source of carbon, namely carbon dioxide. Animal typically search their food, and so show locomotion. They require a nervous system for coordination in the more complex animals, whereas plants are stationary and do not show travel or possess a nervous system. However, according to Linnaeus plant kingdom includes prokaryotes ( bacteria and cyanobacteria), photosynthetic Eukaryotes ( all green unicellular and multicellular plants), and some nonphotosynthetic Eukaryotes (e.g., Fungi). The animal kingdom includes unicellular protozoans and multicellular metazoans.
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