In this article we will discuss about the classification of living organisms.
With the advancement of knowledge on living world, scientists classified the living organisms into two groups: Plantae, i.e., Plant kingdom and Animalia, i.e., Animal kingdom.
The Plantae is characterised by:
1. Nutrition depends on light energy,
2. Presence of rigid cell wall, and
3. Most of them remain rooted in the soil.
The Animalia is characterised by:
1. Ingestion of food, and
2. Members are usually motile.
Later on, with the advancement of knowledge on microorganisms, i.e., single-celled microscopic organisms, it was thought that the above system is inadequate.
In the earlier system, scientists placed bacteria, microscopic algae and fungi under plant kingdom, because:
1. They are relatively immobile, and
2. They are not able to ingest food.
Difficulty arose due to the discovery of certain microorganisms having characteristics shared with both plants and animals. With the advancement of more knowledge on microorganisms, many scientists proposed to place them in a separate kingdom.
Based on the above concept, Earnest Haeckel (1834-1919), a German zoologist proposed (1866) the name of 3rd kingdom, the Protista (Gr. Protistos – first or primitive), includes all microscopic, unicellular, motile and non-motile, photosynthetic or non- photosynthetic, free and autonomously living organisms (neither typical plant nor animal).
Thus all unicellular microscopic organisms of algae, fungi, protozoa and also the bacteria are included under the Kingdom Protista. The viruses were known since the end of 19th century. Due to their non-cellular nature, they were not included in the classification of living organisms.
Based on the more detailed knowledge on cell structure during late 1940s with the aid of powerful magnification provided by electron microscopy, it was found that living organisms are comprised of two types of cells:
1. Prokaryotic (Gr. Pro – primitive; karyon – nucleus), characterised by:
i) Cell has a non-membrane bound DNA,
ii) Semi-rigid cell wall composed of murine complex.
This group includes Bacteria and Blue green algae.
2. Eukaryotic (Gr. Eu – true; karyon – nucleus), characterized by:
i) Cell has a well-organised nucleus,
ii) Well-developed mitochondria, endoplasmic reticulum.
iii) Flagella consist of 9 + 2 fibrillar structure.
This group includes green, brown and red algae, protozoa, fungi, higher plants and animals.
The prokaryotes are considered more primitive than the eukaryotes.
Based on the above idea, the Kingdom Protista was divided into two divisions:
1. Lower Protists i.e., Prokaryotic Protista, and
2. Higher Protists i.e., Eukaryotic Protista.
With the event of above heterogeneity of protista, Copeland (1956) proposed the four-kingdom system. In his system, lower protists were placed under a separate kingdom, Monera, while higher protists were retained in the kingdom Protista. Thus, the four kingdoms are Monera, Protista, Metaphyta (green plants) and Metazoa (animals).
The four, kingdom system, though highly developed, suffers with several deficiencies.
Important shortcomings are:
1. Lack of the fundamental difference between fungi and other eukaryotic protists.
2. Based on various characteristics, such as histone distribution and chromosomal organisation, ecological and nutritional role, cytochrome c, amino acid sequence, lack of phagocytosis and pinocytosis, etc., it is difficult to place the fungi (at least the higher ones) in any one of the above kingdoms.
Later, the four-kingdom system was also followed by Barkley (1968).
Above knowledge on Prokaryotes and Eukaryotes shows many differences and it was thought that different life forms (eukaryotes) have evolved polyphyletically from a variety of primitive ancestral forms (prokaryotes). From the above knowledge, many scientists decided to give the status of Kingdom to the different life types.
Based on the above idea, Robert Whittaker (1969), an American ecologist, proposed a Five- Kingdom System (Fig. 1.9) of classification based on at least two major criteria:
i) 3 levels of cellular structure: Prokaryotes, unicellular eukaryotes and multicellular eukaryotes, and
ii) 3 principal modes of nutrition: Photosynthesis (in plants), absorption (in fungi) and ingestion (in animals).
The kingdoms are:
Includes all prokaryotes and their hypothetical ancestors.
Includes unicellular eukaryotes, such as algae, protozoa and slime molds.
3. Plantae (plants):
Includes multicellular eukaryotes, i.e., higher algae and green plants.
Includes only fungi.
Includes multicellular animals.
Knowledge about the cells became enriched during the 1970s, with the advancement of different technologies such as electron microscope etc. Later on, Woese and Fox (1977) observed that there are two types of prokaryotes.
1. Archaebacteria and
Archaebacteria have the following unique characteristics:
i) Unique wall composition,
ii) Unique DNA and RNA sequence,
iii) Plasma membrane is unlike any other living organism’s.
But it becomes difficult to place some of the organisms in either of the above two groups.
Later, Edwards (1976), and Whittaker and Margulis (1978) divided the living organisms into two Super-Kingdoms – the Prokaryonta and Eukaryonta. The Prokaryonta includes one kingdom Monera, which includes Bacteria and Blue green algae. The other Super-kingdom Eukaryonta is divided into 4 kingdoms: Protista, Plantae, Fungi and Animalia.
The classification reflects the evolutionary development from primitive to advanced organisms and is also compatible with the hypothesis that eukaryotes have been evolved by the symbiotic association of two or more types of prokaryotic organisms. In this type of symbiotic association, one remains inside the other.
Due to many distinctive characteristics, the fungi are placed in a separate Kingdom.
Due to the presence of differentiation of different algal groups in respect of cellular organisation, physiology and biochemistry; the unicellular eukaryotic algae are placed under the Kingdom Protista, along with Protozoa and, on the other hand, the higher algae (eukaryotic algae: members of Chlorophyceae, Phaeo- phyceae, Rhodophyceae etc.) have been included under Kingdom Plantae.
Based on biochemical and electron microscopic studies, Stanier et al. (1971) and Stanier and Chohen-Brazire (1977), stated that blue green algae are more similar to bacteria, therefore they should be considered as blue green bacteria (Cyanobacteria).
Though virus is not a cellular organism, it has been included under the Super-kingdom Prokaryonta for easy handling.
Later on, 5-kingdom system was not accepted by many biologists for the following reasons:
i) Lack of differentiation between Archaea and Bacteria, and
ii) The boundaries among the Kingdoms – such as Protista, Plantae and Fungi – are not well-demarcated.
For the above problems with five-kingdom system of Whittaker, alternative suggestion has been made and six-kingdom system came in focus. In six-kingdom system, Kingdom Monera or Prokaryotae is divided into two kingdoms – the Eubacteria and Archaebacteria. Other kingdoms remain same as suggested by Whittaker. So, the six kingdoms are Eubacteria, Archaebacteria, Protista, Plantae, Fungi and Animalia.
Attempts have been made by different scientists to divide Protists into several well-defined kingdoms. In this regard, the eight-kingdom system of Cavalier-Smith is an ideal one.
According to him, the cell structure and genetic organisation are very much important to study phylogeny. He considered the ultra- structural characteristics as well as r-RNA sequence and also other molecular characteristics in his classification. He divided the living organisms into 2 Empires (Bacteria and Eucaryota) and 8 Kingdoms.
The first Empire Bacteria is divided into two kingdoms:
i) Eubacteria, and
The second Empire Eucaryota (the eukaryotic organisms) is divided into six kingdoms:
i) Archaezoa (Unicellular organisms having 70S ribosomes and devoid of golgi apparatus, chloroplast, mitochondrion and peroxisomes), e.g., Gigardia etc.,
ii) Chromista (Photosynthetic organs having chloroplast within the lumen of the rough endoplasmic reticulum), e.g., Diatoms, Cryptomonads, Brown algae and Oomycetes,
v) Fungi, and