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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 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.
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 ]
By 1998, Cavalier-Smith had reduced the total number of kingdoms from eight to six: Animalia, Protozoa, Fungi, Plantae (including red and green algae), Chromista, and Bacteria. [44] Five of Cavalier-Smith's kingdoms are classified as eukaryotes as shown in the following scheme: Eubacteria; Neomura. Archaebacteria; Eukaryotes. Kingdom Protozoa
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.
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.
A second merger, 1.6 billion years ago, added chloroplasts, creating the green plants. [5] According to the theory of symbiogenesis (also known as the endosymbiotic theory) championed by Lynn Margulis, a member of the archaea gained a bacterial cell as a component. The archaeal cell was a member of the Asgard group.
1938 [5] [6] Whittaker 1969 [7] Woese et al. 1977 [8] [9] Woese et al. 1990 [10] Cavalier-Smith 1993 [11] [12] [13] Cavalier-Smith 1998 [14] [15] [16] Ruggiero et al. 2015 [17] — — 2 empires: 2 empires: 2 empires: 2 empires: 3 domains: 3 superkingdoms 2 empires: 2 superkingdoms: 2 kingdoms 3 kingdoms — 4 kingdoms: 5 kingdoms: 6 kingdoms ...