The Five Kingdoms Of Life

Archaebacteria also have introns in some genes, an advanced eukaryotic characteristic that was previously unknown among prokaryotes. In eukaryotic cells, the initial messenger RNA (M-RNA) transcribed from the DNA (gene) is modified (shortened) before it leaves the nucleus. Sections of the M-RNA strand called introns are removed, and the remaining portions called exons are spliced together to form a shortened (edited) strand of mature M-RNA that leaves the nucleus and travels to the ribosome for translation into protein. This process is known as "gene editing." Some authorities hypothesize that eukaryotic organisms may have evolved from ancient archaebacteria (archae = ancient) rather than from the common and cosmopolitan eubacteria. The archaebacteria could have flourished more than 3 billion years ago under conditions previously thought to be uninhabitable to all known life forms.

Although many conservative references place the archaebacteria in a separate division within the kingdom Monera, most authorities now recognize them as a 6th kingdom--The kingdom Archaebacteria. In fact, data from DNA and RNA comparisons indicate that archaebacteria are so different that they should not even be classified with bacteria. Systematists have devised a classification level higher than a kingdom, called a domain or "superkingdom," to accomodate the archaebacteria. These remarkable organisms are now placed in the domain Archaea. Other prokaryotes, including eubacteria and cyanobacteria, are placed in the domain Bacteria. All the kingdoms of eukaryotes, including Protista (Protoctista), Fungi, Plantae and Animalia, are placed in the domain Eukarya. The large molecular differences between the majority of prokaryotes in the kingdom Monera and the archaebacteria warrants a separation based on categories above the level of kingdom. In other words, the differences between the true bacteria and archaebacteria are more significant than the differences between kingdoms of eukaryotes.

Guillaume Lecointre and Hervé Le Guyader (2006) have published a remarkable book entitled The Tree of Life: A Phylogenetic Classification. The book includes the three major domains which are in turn subdivided into numerous branches (clades). An oversimplified 3-domain system of classification is shown in the following table. The number of subdivisions listed by G. Lecointre and H.L. Guyader for each domain are shown in parentheses.

2. The kingdom Protista includes a diverse array of organisms, from minute flagellated cells to macroscopic kelp. The smallest microscopic organisms are termed protists, consequently some biologists prefer to call this kingdom the Protoctista rather than Protista. All members of this vast phylum have nucleated cells and live in aquatic habitats (freshwater and marine). According to Lynn Margulis, K.V. Schwartz and M. Dolan (1994), the cells of all Protoctista originally formed by bacterial symbioses (symbiogenesis).

Members of the kingdom Protoctista are not animals, which develop from an embryo called a blastula; they are not plants, which develop from an embryo that is not a blastula but is retained in the mother's tissue; they are not fungi which develop from spores and lack cilia and flagella (called undulipodia) at all stages of development; they are not monerans, which have prokaryotic cells.

Fossil protoctists, with thick-walled resting stages or cysts, can be extracted from shale treated with hydroflouric acid. One of the richest sources of bizarre fossil protoctists was discovered in southern Australia during the late 1950s. Known as the Ediacaran biota, these deposits date back 600 million years ago. Some of these ancient protoctists may have been ancestral to certain animal and plant phyla. In fact, some flattened protoctists discovered in the Ediacaran biota had characteristics resembling lichens. [Lichens are organisms resulting from genetic mergers betweeen protists and fungi.] All the Ediacaran biota became extinct by about 530 million years ago and were replaced be shelled Cambrian animals.

Some general biology textbook authors place the microscopic, unicellular green algae (Division Chlorophyta) in the Kingdom Protista, and place the larger, multicellular (macroscopic) green algae (Division Chlorophyta) in the Kingdom Plantae. They also place the macroscopic, multicellular brown algae (Division Phaeophyta) and red algae (Division Rhodophyta) in the Kingdom Plantae. In fact, some authors place all of the algae divisions in the Kingdom Plantae. Although the Kingdom Protista includes mostly unicellular organisms, the WAYNE'S WORD staff feels that these algal divisions belong in the Kingdom Protista (Protoctista) rather than the Kingdom Plantae.

3. Some members of the Kingdom Fungi (in the fungal classes Ascomycetes and Basidiomycetes) are associated with algal cells of the Kingdom Protista (in the algal division Chlorophtya) and/or prokaryotic cyanobacteria of the Kingdom Monera. This complex symbiotic, mutualistic relationship is called lichen. Lichens are essentially lichenized fungi containing unicellular monerans and/or protists.

4. There are approximately 1.6 million species of living organisms on earth. This number may be much higher because new species are continually being discovered each day, particularly insects and nematodes in remote tropical regions. However, at the present rate of destruction, most of the virgin tropical rain forest will be annihilated by the end of the 20th century, so many species will never be known to humans. It is estimated that 99 percent of all the species that have ever lived on earth were already extinct before humans ever walked on this planet. Although humans have a phenomenal impact on the ecology of earth, they are relative newcomers on this great planet. It is estimated that the earth is over 4.5 billion years old, and ancient life forms (such as the cyanobacteria) appeared about 2-3 billion years ago. If the geologic history of the earth is compared to a 24-hour time scale, the first multicellular organisms do not appear until just after 8:00 P.M. and humans are not on the scene until less than a minute before midnight.

5. There are more than one million species of animals (Kingdom Animalia), more than all the other kingdoms combined. More than half of all animal species are insects (800,000 species), and beetles (300,000 species) comprise the largest order of insects (one fifth of all species--using a total of 1.5 million). In fact, if all the species of plants and animals on earth were lined up at random, every 5th species would be a beetle.

6. Viruses: Viruses do not belong to the above 5 kingdoms of life. They are much smaller and much less complex than cells. They are macromolecular units composed of DNA or RNA surrounded by an outer protein shell. They have no membrane-bound organelles, no ribosomes (organelle site of protein synthesis), no cytoplasm (living contents of a cell), and no source of energy production of their own. They do not exhibit autopoiesis--i.e. they do not have the self-maintenance metabolic reactions of living systems. Viruses lack cellular respiration, ATP-production, gas exchange, etc. However, they do reproduce, but at the expense of the host cell. Like obligate parasites, they are only capable of reproduction within living cells. In a sense, viruses hijack the host cell and force it to produce more viruses through DNA replication and protein synthesis. Outside of their host cells, viruses can survive as minute macromolecular particles. Viruses may attack animals and plants. Infectious human viruses can be dispersed though the air (airborne viruses) or body fluids (HIV virus). Epidemic viruses (such as HIV) that are passed from person to person via sexual conjugation are remarkably similar to computer viruses. Unfortunately in humans there is no resident antivirus program to alert you of a potential infection, or to quickly scan your body and delete the invader once it has entered your system. Humans must rely on their amazing antibody and cell-mediated immune response, one of the most complex and remarkable achievements in the evolution of living systems.

1. Armstrong, W.P. 1978. "Southern California's Vanishing Cypresses." Fremontia 6 (2): 24-29.

2. Armstrong, W.P. 1977. "The Close-Cone Pines and Cypresses" (Chapter 9, pp. 295-358). In: Terrestrial Vegetation of California, John Wiley & Sons.

3. Hickman, J.C. (Editor). 1993. The Jepson Manual: Higher Plants of California. University of California Press, Berkeley.

4. Lecointre, G. and H.L. Guyader. [Illustrated by D. Visset & Translated by K. McCoy.] 2006. The Tree of Life: A Phylogenetic Classification. Harvard University Press, Cambridge, Massachusetts.

5. Margulis, L., K.V. Schwartz, and M. Dolan. 1994. The Illustrated Five Kingdoms: A Guide To The Diversity Of Life On Earth. HarperCollins College Publishers, New York.