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 mode