by Piter Kehoma Boll
Since ancient times, living beings were classified as either plants or animals and Linnaeus retained this system in his great work Systema Naturae in the 18thcentury, where he divided nature in three kingdoms: Regnum Animale (animal kingdom), Regnum Vegetabile (plant kingdom) and Regnum Lapideum (mineral kingdom). This system was not intended to reflect natural relationships among living organisms, since Linnaeus was a Christian and believed that all life forms were created separately by God himself just as they are today, but was created to make the study of living beings easier.
Linnaeus and the two kingdoms of life. Painting by Alexander Roslin, 1775.
When the first unicellular organisms were discovered by Antoine van Leeuwenhoek in 1674, they were placed in one of the two kingdoms of living beings, according to their characteristics. It remained so until until 1866, when Ernst Haeckel proposed a third kingdom of life, which he called Protista, and included all unicellular organisms in it.
Haeckel and the three kingdoms. Photo by the Linnean Society, 1908.
Later, the development of optic and electronic microscopy showed important differences in cells, mainly according to the presence or absence of distinct nucleus, leading Édouard Chatton to distinguish organisms in prokaryotes (without a distinct nucleus) and eukaryotes (with a distinct nucleus) in a paper from 1925. Based on it, Copeland proposed a four-kingdom system, moving prokaryotic organisms, bacteria and “blue-green algae”, into the kingdom Monera. The idea of a ranking above kingdom came from this time and so life was separated in two empires or superkingdoms, Prokaryota (Monera) and Eukaryota (Protista, Plantae, Animalia).
Two empires and four kingdoms
Since Haeckel, the position of fungi was not well established, oscillating between kingdoms Protista and Plantae. So, in 1969, Robert Whittaker proposed a fifth kingdom to include them, the called Kingdom Fungi. This five-kingdom system remained constant for some time; Monera were prokaryotes; Plantae were multicellular autotrophs (producers); Animalia multicellular consumers; and Fungi multicellular saprotrophs (decomposers). Protista was like the trash bag, where anything that doesn’t fit in the other 4 kingdoms was placed in.
Whittaker and the five kingdoms. Photography source: National Academy of Sciences: Robert H. Whittaker (1920—1980) – A Biographical Memoir by Walter E. Westman, Robrt K. Peet and Gene E. Likens.
With the dawn of molecular studies around 1970, significant differences were found inside the Prokaryotes, regarded, for example, to the cell membrane structure. Based on those studies, Carl Woese divided Prokaryota in Eubacteria and Archaeobacteria, emphasizing that the differences between those two were as high as the ones between them and the eukaryotes. This later gave rise to a new higher classification of life in three domains, Bacteria, Archaea and Eukarya.
Woese and the three domains. Photo from Photo from News Bureau – University of Illinois, given by IGB (Institute for Genomic Biology).
By the end of the 20th century, Thomas Cavalier-Smith, after intense study of protists, created a new model with 6 kingdoms. Bacteria and Archea were put together in the same kingdom, called Bacteria. Protists were divided in two kingdoms: (1) Chromista, including Alveolates (Apicomplexa, parasitic protozoa like Plasmodium; Ciliates and Dinoflagellates), Heterokonts or Stramenopiles (brown algae, golden algae, diatoms, water moulds, etc) and Rhizarians (like Radiolaria and Foraminifera), among others; and (2) Protozoa, including Amoebozoa (amoebas and slime moulds), Choanozoa (choanoflagellates) and a set of flagellated protozoa called Excavata. Glaucophytes, red and green algae were classified inside the kingdom Plantae.
Cavalier-Smith and his two new kingdoms. Photo from Department of Zoology – University of Oxford.
From the 21th century on, a phylogenetic approach to classify living beings has gained strength. After a lot of molecular analyses using different genes, the real evolutionary relationship among Eukaryotes is still not clear. However, the following groups are supported by most phylogenetic trees:
(1) Archaeoplastida (or Plantae): glaucophytes (Glaucophyta), red algae (Rodophyta) and green plants and algae (Viridiplantae)
(2) Chromalveolata: Stramenopiles or Heterokonta, haptophytes (Haptophyta), cryptomonads (Cryptophyta) and Alveolata.
(3) Rhizaria: Foraminifera, Radiolaria and some amoeboid protozoa
(4) Amoebozoa: amoebas and slime moulds
(5) Opisthokonta: animals, fungi, choanoflagelates
(6) Excavata: many flagellate protozoa. This group, however, isn’t as well supported as the other ones.
The current (maybe not so) well-established groups of organisms
So, as we can see, the Eukaryotes’ case is yet to be solved, but we hope that further molecular studies will help us understand better how the tree of life branches.
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Baldauf, S. L. et al. 2000: A Kingdom-Level Phylogeny of Eukaryotes Based on Combined Protein Data. Science 290, 972-977.
Cavalier-Smith, T. 2004: Only six kingdoms of life. Proceedings of the Royal Society B 271, 1275-1262.
Rogozin, I. B. et al. 2009: Analysis of Rare Genomic Changes Does Not Support the Unikont–Bikont Phylogeny and Suggests Cyanobacterial Symbiosis as the Point of Primary Radiation of Eukaryotes. Genome, Biology and Evolution 1, 99-113.
Wikipedia. Kingdom (Biology). Available on-line in: <en.wikipedia.org/wiki/Kingdom_(biology)>. Acess on December 5th, 2011.