الفهرس 
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Abstract
Archaea constitute the third domain of life, but were previously classified and considered as Bacteria
along the history of the phylogenetic system. In 1977, Woese and Fox introduced the usage of a new tool
for comparative analysis of phylogenetic relationships, which was the sequence of small subunit rRNA
(SSU rRNA),the 16S subunit in prokaryotes and the 18S subunit in eukaryotes (Woese and Fox, 1977).
In this study, Woese and Fox stated that the living world is structured into a tripartite way rather than the
traditional bipartite one. At that time, the proposed three distinct taxonomic divisions were eubacteria,
archaebacteria and urkaryotes. In 1990, these divisions were called domains: Bacteria, Archaea and
Eucarya (Figure 2.1) (Woese et al, 1990.