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Archaea and bacteria have generally similar cell structure, but cell composition and organization set the archaea apart. Like bacteria, archaea lack interior membranes and organelles. [68] Like bacteria, the cell membranes of archaea are usually bounded by a cell wall and they swim using one or more flagella. [119]
The eukaryotes seemingly emerged within the Asgard archaea, and are closely related to the Heimdallarchaeia. [5] This implies that there are only two domains of life, Bacteria and Archaea, with eukaryotes incorporated among the Archaea. Eukaryotes first emerged during the Paleoproterozoic, likely as flagellated cells.
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.
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.
Major eukaryotic organelles; Organelle Main function Structure Organisms Notes chloroplast : ... all eukaryotes, all archaea, and some bacteria ribosome (80S)
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.
Bacteria do not contain organelles in the same sense as eukaryotes. Instead, the chromosome and perhaps ribosomes are the only easily observable intracellular structures found in all bacteria. There do exist, however, specialized groups of bacteria that contain more complex intracellular structures, some of which are discussed below.
However, molecular systematics showed prokaryotic life to consist of two separate domains, originally called Eubacteria and Archaebacteria, but now called Bacteria and Archaea that evolved independently from an ancient common ancestor. [5] The archaea and eukaryotes are more closely related to each other than either is to the bacteria.