Over the years, many models for classifying organisms have been touted as the next best one. Linnaeus's scheme of classifying everything into two kingdoms was the first real attempt, and it lasted for more than 200 years! However, because living organisms are so diverse, Linnaeus's classification system is considered inadequate today as a means of accurately representing the degree of relationship between organisms. More advanced thinking created first a five-, then a six-, then a three-kingdom classification system to include all the recent discoveries related to kinship. It is likely that as more discoveries are made and more data processed, the three-kingdom model may be in jeopardy.
Linnaeus's model classified everything as either a plant (kingdom Plantae) or animal (kingdom Animalia), which was sufficient for that era. However, even Linnaeus's followers struggled with the classification of fungi. Most placed them in the plant kingdom because they appeared to have roots and certainly did not belong in the animal kingdom! As additional species were discovered, the two-kingdom classification gave way to other models.
American ecologist Robert Whittaker created a five-kingdom model that accounted for prokaryote and eukaryote distinctions. In this schemata, all prokaryotes were placed in the kingdom Monera. The remaining eukaryotes were separated by differences mostly in structure. Plants and animals were easily separated into their own kingdoms. It was decided that because fungi were neither plant nor animal but subsisted by decaying or decomposing once-living matter, they also become their own kingdom. Everything else was clumped into the kingdom Protista. The protists included all eukaryotes that did not clearly fit into the plant, animals, or fungi kingdoms. With the advent of sophisticated collecting and microscopy techniques, the question arose as to where the “new” types of bacteria belong.
Linnaeus created his model during the eighteenth century, which proved useful for scientific knowledge at that time. However, recent discoveries of archaebacteria in deep-ocean thermal vents, hot springs in Yellowstone, and brine marine environments justified another look at the five-kingdom model. Analysis of archaebacteria indicates that they are more similar to eukaryotes than their bacterial prokaryotic cousins (see Origin of Prokaryotes and Eukaryotes). The six-kingdom model contains the same original four kingdoms, but splits the Moneran kingdom into kingdom Archaea bacteria to include the newly discovered bacteria type with internal membranes, and kingdom Eubacteria, which contains the more common forms of bacteria. Interestingly, cladistic analysis indicates that organisms in the kingdom Protista are actually more different from each other than originally suspected. In fact, most mammals are more closely related to certain archaebacteria than some protists are related to each other!
The five- and six-kingdom models are based on the scheme of two fundamentally different groups of organisms: prokaryote and eukaryote. They divided and sorted all living things into the various kingdoms based on similarities of evolutionary history. The three-kingdom model makes a fundamental change. Instead of classifying organisms into prokaryote or eukaryote, it sorts them into three categories by splitting the prokaryotes into two kingdoms similar to the six-kingdom model: Archae bacteria and Eubacteria. All the eukaryotes, which is virtually everything else, are clumped into the Eukarya kingdom. Modern chemical and cellular evidence currently supports the three-kingdom model. It is believed that bacteria were among the first type of life, and the eubacteria separated leaving the archaebacteria and eukarya to continue as one in evolutionary struggles. The fact that modern archaebacteria contain many eukaryotic features indicates their separation occurred after the eubacteria and thereby establishes greater kinship between archaebacteria and eukarya. Archaebacteria are established as a midpoint between eubacteria and eukaraya. Most modern taxonomists favor the three-kingdom model.
Perhaps as more species are discovered, additional anomalies will occur that force the reorganization of the model. We may end up with a four-kingdom classification model!
Excerpted from The Complete Idiot's Guide to Biology © 2004 by Glen E. Moulton, Ed.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.