The following points highlight the top four concepts that classify living organisms. The concepts are: 1. Linnaeus’s Two Kingdom Concept 2. Hackel’s Three Kingdom Concept 3. Copeland’s Four Kingdom Concept4. Carl Woese’s Concept.
Living Organisms: Concept # 1. Linnaeus’s Two Kingdom Concept:
As early as 1758 Linnaeus had scientifically divided living beings into two major (or Primary), kingdoms; the Planate and the Animalia. Linnaeus was a great Swedish naturalist and for his original contributions in Systematics he has been called the father of Taxonomy.
The binomial method of nomenclature (a name having two parts, first the genus and second the species) was for the first time used by him for animals in the tenth edition of his book Systema Naturae (1758).
Living Organisms: Concept # 2. Hackel’s Three Kingdom Concept:
In 1866, a German biologist E. Hackel (a disciple of great naturalist Charles Darwin) proposed a third kingdom ‘Protista’ to classify all the organisms composing microbial forms of life.
As the system proposed by Linnaeus incorporated developed animals and plants but was not just towards microbes that either possessed “animal like” or “plant like”, or both the characteristics or other forms like slime molds (earlier classified as protozoans under animalia) but were found to be phagotrophic and amoeboid and later turned to be true fungi.
The incorporation in the third kingdom ‘Protista’ was relied on morphological features, tissue level of organization including division of labour and also used a variety of modes of nutrition. It included algae, fungi, protozoa and bacteria. It addressed all existing microorganisms of that time as protists and not as plants or animals.
Living Organisms: Concept # 3. Copeland’s Four Kingdom Concept:
Copeland (1959) put forth a four kingdom concept to classify living organisms, by creating a new kingdom Monera to include all the lower protists (i.e., the prokaryotic bacteria and blue green algae). The other higher protists including algae, fungi and protozoa, were then classified under the kingdom Protoctista.
In order to address the remaining kingdom ‘Plantae’ (eukaryotic Plants) and ‘Animalia’ (eukaryotic animals), the terms kingdom Metaphyta (incorporating all Bryophytes, Pteridophytes, Gymnosperms and Angiosperms) and Kingdom Metazoa (Classifying all the invertebrates and vertebrates) were used respectively.
Five Kingdom System:
The so called five kingdom system was suggested in 1969 by Robert Whittaker.
The five kingdoms were: Monera, Protista, Plants, Animals and Fungi, with a primary differentiation between prokaryotes and eukaryotes.
Living Organisms: Concept # 4. Carl Woese’s Concept:
In 1977, Woese and Wolfe using 16S ribosomal RNA sequences (Olsen and Woese, 1993) overturned one of the major dogmas of biology. Woese, an American microbiologist, applied the concept of molecular phylogeny to 16 S r RNA and pioneered modem taxonomy.
He had used this technique to identify a third form of life, the Archaea, whose genetic make-up is distinct from but related to both bacteria and Eucarya. It was in 1982 that, Kark Stetter, isolated hydrothermophilic microbes (Archaea) that could grow at 105° C. The discovery redefined the upper temperature limits at which life can exist.
In 1994, Garry Olsen, Carl Woese and Ross Overbeek summarized the state of Phylogeny in Prokaryotes. Then Woese et. al. used the protein sequencing information for making the roots of the phylogenetic tree. These attempts led to the beginning of a hypothetical ancestor (progenote) concept.