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Phylogenetic tree showing the relationship between the archaea and other forms of life. Eukaryotes are colored red, archaea green and bacteria blue. Adapted from Ciccarelli et al. [44] Woese argued that the bacteria, archaea, and eukaryotes represent separate lines of descent that diverged early on from an ancestral colony of organisms.
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.
It was also found that the eukaryotes are more closely related to the Archaea than they are to the Eubacteria. Although the primacy of the Eubacteria-Archaea divide has been questioned, it has been upheld by subsequent research. [22] There is no consensus on how many kingdoms exist in the classification scheme proposed by Woese.
The tree of life. Two domains of life are Bacteria (top branches) and Archaea (bottom branches, including eukaryotes). The two-domain system is a biological classification by which all organisms in the tree of life are classified into two domains, Bacteria and Archaea.
Halophiles (organisms that thrive in highly salty environments) and hyperthermophiles (organisms that thrive in extremely hot environments) are examples of Archaea. [1] Archaea are relatively small. They range from 0.1 μm to 15 μm diameter and up to 200 μm long, about the size of bacteria and the mitochondria found in
Bacteria and archaea are generally similar in size and shape, although a few archaea have very strange shapes, such as the flat and square-shaped cells of Haloquadratum walsbyi. [94] Despite this morphological similarity to bacteria, archaea possess genes and several metabolic pathways that are more closely related to those of eukaryotes ...
Spiral bacteria are another major bacterial cell morphology. [2] [30] [31] [32] Spiral bacteria can be sub-classified as spirilla, spirochetes, or vibrios based on the number of twists per cell, cell thickness, cell flexibility, and motility. [33] Bacteria are known to evolve specific traits to survive in their ideal environment. [34]
In this scheme they reintroduced the classification with the division of prokaryotes superkingdom into two kingdoms, Bacteria (=Eubacteria) and Archaea (=Archaebacteria). This is based on the consensus in the Taxonomic Outline of Bacteria and Archaea (TOBA) and the Catalogue of Life .