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A speculatively rooted tree for RNA genes, showing major branches Bacteria, Archaea, and Eukaryota The three-domain tree and the eocyte hypothesis (two-domain tree), 2008. [7] Phylogenetic tree showing the relationship between the eukaryotes and other forms of life, 2006. [8] Eukaryotes are colored red, archaea green, and bacteria blue.
The three-domain system adds a level of classification (the domains) "above" the kingdoms present in the previously used five- or six-kingdom systems.This classification system recognizes the fundamental divide between the two prokaryotic groups, insofar as Archaea appear to be more closely related to eukaryotes than they are to other prokaryotes – bacteria-like organisms with no cell nucleus.
The three main early branches of the tree of life have been intensively studied by microbiologists because the first organisms were microorganisms. Microbiologists (led by Carl Woese) have introduced the term domain for the three main branches of this tree, where domain is a phylogenetic term similar in meaning to biological kingdom.
In 1990, a novel concept of the tree of life was presented, dividing the living world into three stems, classified as the domains Bacteria, Archaea, Eukarya. [1] [50] [51] [52] It is the first tree founded exclusively on molecular phylogenetics, and which includes the evolution of microorganisms.
They suggested and formally defined the terms Bacteria, Archaea and Eukarya for the three domains of life. [22] It was the first tree founded on molecular phylogenetics and microbial evolution as its basis. [23] [24] The model of a tree is still considered valid for eukaryotic life forms.
A phylogenetic tree based on rRNA data showing Woese's three-domain system. All smaller branches can be considered kingdoms. All smaller branches can be considered kingdoms. Based on RNA studies, Carl Woese thought life could be divided into three large divisions and referred to them as the "three primary kingdom" model or "urkingdom" model.
Phylogenetic tree showing the relationship between the Archaea and other domains of life. Eukaryotes are colored red, archaea green and bacteria blue. Adapted from Ciccarelli et al. (2006) [80] The relationships among the three domains are of central importance for understanding the origin of
Lake hypothesised the tree of life as having only two primary branches: prokaryotes, which include Bacteria and Archaea, and karyotes, that comprise Eukaryotes and eocytes. Parts of this early hypothesis were revived in a newer two-domain system of biological classification which named the primary domains as Archaea and Bacteria. [3]