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Since archaea and bacteria are no more related to each other than they are to eukaryotes, the term prokaryote may suggest a false similarity between them. [68] However, structural and functional similarities between lineages often occur because of shared ancestral traits or evolutionary convergence.
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.
Metagenomic analyses recover a two-domain system with the domains Archaea and Bacteria; in this view of the tree of life, Eukaryotes are derived from Archaea. [ 57 ] [ 58 ] [ 59 ] With the later gene pool of LUCA's descendants, sharing a common framework of the AT/GC rule and the standard twenty amino acids, horizontal gene transfer would have ...
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. [1] [2] [3] It emerged from development of knowledge of archaea diversity and challenges the widely accepted three-domain system that classifies life into Bacteria, Archaea, and Eukarya. [4]
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.
Due to the similarities found between eukaryotes and both archaea and bacteria, it is thought that a major source of the genetic variation is through horizontal gene transfer. [39] Horizontal gene transfer explains why archaeal sequences are found in bacteria and bacterial sequences are found in archaea. [39]
[20] [21] By sequencing the ribosomal RNA, it was found that the Archaea most likely split from bacteria and were the precursors to modern eukaryotes, and are actually more phylogenetically related to eukaryotes. [21] As their name suggests, Archaea comes from a Greek word archaios, meaning original, ancient, or primitive. [22]
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.