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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 eukaryotes (/ j uː ˈ k ær i oʊ t s,-ə t s / yoo-KARR-ee-ohts, -əts) [4] constitute the domain of Eukaryota or Eukarya, organisms whose cells have a membrane-bound nucleus. All animals , plants , fungi , seaweeds , and many unicellular organisms are eukaryotes.
The opisthokonts (from Ancient Greek ὀπίσθιος (opísthios) 'rear, posterior' and κοντός (kontós) 'pole, i.e. flagellum') are a broad group of eukaryotes, including both the animal and fungus kingdoms. [5] The opisthokonts, previously called the "Fungi/Metazoa group", [6] are generally recognized as a clade.
According to the domain system, the tree of life consists of either three domains, Archaea, Bacteria, and Eukarya, [1] or two domains, Archaea and Bacteria, with Eukarya included in Archaea. [3] [4] In the three-domain model, the first two are prokaryotes, single-celled microorganisms without a membrane-bound nucleus.
Combined with the five-kingdom model, this created a six-kingdom model, where the kingdom Monera is replaced by the kingdoms Bacteria and Archaea. [16] This six-kingdom model is commonly used in recent US high school biology textbooks, but has received criticism for compromising the current scientific consensus. [ 13 ]
Phylogenetic and symbiogenetic tree of living organisms, showing the origins of eukaryotes and prokaryotes. The two-empire system (two-superkingdom system) was the top-level biological classification system in general use from the early 20th century until the establishment of the three-domain system (which itself is currently being challenged by the two-domain system).
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.
The last eukaryotic common ancestor (LECA) is the hypothetical last common ancestor of all living eukaryotes, around 2 billion years ago, [3] [4] and was most likely a biological population. [8] It is believed to have been a protist with a nucleus, at least one centriole and cilium , facultatively aerobic mitochondria, sex ( meiosis and syngamy ...