Abstract
All extant organisms are thought to be classified into three primary kingdoms, eubacteria, eukaryotes, and archaebacteria. Based on the analyses of a pair of duplicated genes, elongation factors EF-Tu/l α and EF-G/2, and catalytic and noncatalytic subunits of ATPase (ATPsynthase), archaebacteria are more closely related to eukaryotes than eubacteria. While this relatedness is certified, phylogenetic relationship among several major groups of archaebacteria (extreme thermophiles, extreme halophiles, and methanogens) and eukaryotes is still unsettled because the phylOgenetic tree topologies among them vary with the genes analysed. The ambiguous situation suggests that archaebacterial major groups and eukaryotes were diverged during very short period in the long course of evolution.
