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In biology, '''Archezoa''' is a term that has been introduced by several authors to refer to a group of organisms (a taxon). Authors include [[Maximilian Perty|Josef Anton Maximilian Perty]],<ref>Perty, M. 1852. Zur Kenntnis kleinster Lebensformen nach Bau, Funktionen, Systematik, mit Spezialverzeichniss der in der Schweiz beobachteten. Jent & Reinert, Bern </ref> [[Ernst Haeckel]]<ref>Haeckel, E. 1866. Generelle Morphologie der Organismen Vols I and II.  Reimer.</ref> and in the 20th century by [[Thomas Cavalier-Smith]] [[Cavalier-Smith's system of classification|in his classification system]].  Each author used the name to refer to different arrays of organisms. This reuse by later authors of the same taxon name for different groups of organisms is widely criticized in taxonomy because the inclusion of the name in a sentence (e.g. "Archezoa have no olfactory organs") does not make sense unless the particular usage is specified (e.g. "Archezoa ''sensu'' Cavalier-Smith (1987) have no olfactory organs").  Nonetheless, all uses of 'Archezoa' are now obsolete.
== Archezoa ''sensu'' Cavalier-Smith (1987) ==
Cavalier-Smith proposed the term 'Archezoa' for a paraphyletic (see [[Paraphyly]]) territory of [[eukaryotes]]  that primitively lacked mitochondria.  Like Margulis and others before (see [[Pelomyxa]]), Cavalier-Smith argued that the initial ancestor of eukaryotes emerged prior to the endosymbiotic acquisition (see [[endosymbiosis]]) of mitochondria.<ref name=Cavalier-Smith-1987>{{cite journal |last=Cavalier-Smith |first=T. |date=1987 |title=Eukaryotes with no mitochondria |journal=Nature |volume=326|issue=6111 |pages=332–333 |doi=10.1038/326332a0 |pmid=3561476 |bibcode=1987Natur.326..332C |s2cid=4351363 |issn=1476-4687 |url=https://www.nature.com/articles/326332a0 |lang=en}}</ref> The same paraphyletic territory was referred to as 'Hypochondria' by others.<ref>{{cite journal|pmid=16754614 |date=2006 |last1=Embley |first1=T. M. |title=Multiple secondary origins of the anaerobic lifestyle in eukaryotes |journal=Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences |volume=361 |issue=1470 |pages=1055–1067 |doi=10.1098/rstb.2006.1844 |pmc=1578728 }}</ref> The argument for Archezoa sensu Cavalier-Smith was never universally accepted because of conflicting information, and was dropped when the contrary argument, that amitochondriates were descendants of eukaryotes with mitochondria, became dominant.
Eukaryotes that eventually acquired a bacterial endosymbiont that became the mitochondria were placed in a taxonomic group which Cavalier-Smith called the Metakaryota, whereas the Archezoa represented an earlier [[paraphyletic]] group to which Cavalier-Smith variously assigned the diplomonads, ''Entamoeba'', [[Microsporidia]], oxymonads, parabasalids ([[Parabasalid|Parabasalids]]), pelobionts (see ''[[Pelomyxa]]''), retortamonads, trichomonads, and ''Trimastix''<ref>{{cite journal |last1=Gray |first1=Michael W. |last2=Burger |first2=Gertraud |last3=Lang |first3=B. Franz |date=1999-03-05 |title=Mitochondrial evolution |journal=Science |volume=283 |issue=5407 |pages=1476–1481 |doi=10.1126/science.283.5407.1476 |pmid=10066161 |pmc=3428767 |bibcode=1999Sci...283.1476G |issn=0036-8075 |lang=en}}</ref> (see [[Cavalier-Smith's system of classification]]). With the rejection of 'Archeozoa', the meaning of the term 'Metakaryota' became the same as 'Eukaryota' (see [[Eukaryote]]), and Metakaryota became superfluous.
== Original mitochondria lost ==
Eukaryotic protists lacking mitochondria were discovered to have experienced secondary mitochondrial loss, meaning that their ancestors once possessed mitochondria but that these mitochondria had, over time, been transformed, reduced, or lost. In some of these organisms, mitochondria had degraded into simpler double-membrane bound organelles known as [[mitosomes]] and [[hydrogenosomes]]. Some of both types of organelles are known to have fully lost their genome.<ref>{{Cite journal |last1=Embley |first1=T. Martin |last2=Hirt |first2=Robert P. |date=1998 |title=Early branching eukaryotes? |journal=Current Opinion in Genetics & Development |volume=8 |issue=6 |pages=624–629 |doi=10.1016/S0959-437X(98)80029-4 |pmid=9914207 |lang=en |url=https://linkinghub.elsevier.com/retrieve/pii/S0959437X98800294}}</ref><ref>{{cite journal |last=Roger |first=Andrew J. |year=1999 |title=Reconstructing early events in eukaryotic evolution |journal=The American Naturalist |volume=154 |issue=S4 |pages=S146–S163 |doi=10.1086/303290 |pmid=10527924 |bibcode=1999ANat..154S.146R |s2cid=32138852 |issn=0003-0147 |lang=en |url=https://www.journals.uchicago.edu/doi/10.1086/303290}}</ref>
Initial discoveries found that amitochondriate organisms appeared to express mitochondrial Hsp60 and Hsp70 proteins from the nuclear DNA of the organism. This indicated that the ancestors of these organisms once possessed mitochondria which expressed these proteins, but that these genes had migrated to their nuclear DNA over time as a result of endosymbiotic gene transfer.<ref>{{cite journal |last1=Clark |first1=C.G. |last2=Roger |first2=A.J. |year=1995 |title=Direct evidence for secondary loss of mitochondria in Entamoeba histolytica |journal=Proc. Natl. Acad. Sci. USA |volume=92 |issue=14 |pages=6518–6521|doi=10.1073/pnas.92.14.6518 |pmid=7604025 |pmc=41549 |bibcode=1995PNAS...92.6518C |doi-access=free }}</ref><ref>{{cite journal |last=Roger |first=A.J. |year=1999 |title=Reconstructing early events in eukaryotic evolution |journal=Am. Nat. |volume=154 |issue=S4 |pages=S146–S163|doi=10.1086/303290 |pmid=10527924 |bibcode=1999ANat..154S.146R |s2cid=32138852 }}</ref>
As a result, the argument that some extant eukaryotes lacking mitochondria had emerged from the eukaryotic lineage before mitochondria were acquired was falsified.<ref>{{cite journal |last1=Embley |first1=T.M. |last2=Martin |first2=W. |year=2006 |title=Eukaryotic evolution, changes and challenges |journal=Nature |volume=440 |issue=7084 |pages=623–630|doi=10.1038/nature04546 |pmid=16572163 |bibcode=2006Natur.440..623E |s2cid=4396543 |url=https://zenodo.org/record/897869 }}</ref>
== Long branch attraction ==
An argument for the Archezoa group was that amitochondriate protists appeared to branch off early on from the eukaryotic lineage in phylogenetic analyses. This corroborated the supposition that Archezoa were more closely linked to primitive eukaryotes that evolved prior to the endosymbiotic process that generated the mitochondria.<ref name=Cavalier-Smith-1987/> However, this early divergence later turned out to be a class of systematic errors in phylogenetic analysis called "[[long branch attraction]]".<ref>{{cite book |last1=Brinkmann |first1=Henner |last2=Philippe |first2=Hervé |title=Eukaryotic Membranes and Cytoskeleton |chapter=The Diversity of Eukaryotes and the Root of the Eukaryotic Tree |year=2007 |series=Advances in Experimental Medicine and Biology |volume=607 |pages=20–37 |place=New York, NY |publisher=Springer New York |lang=en |doi=10.1007/978-0-387-74021-8_2 |pmid=17977456 |isbn=978-0-387-74020-1 |chapter-url=http://link.springer.com/10.1007/978-0-387-74021-8_2 |access-date=2022-02-01}}</ref><ref>{{cite journal |last=Gray |first=M.W. |date=2012-09-01 |title=Mitochondrial evolution |journal=Cold Spring Harbor Perspectives in Biology |volume=4 |issue=9 |pages=a011403 |issn=1943-0264 |pmc=3428767 |pmid=22952398 |doi=10.1101/cshperspect.a011403 |lang=en}}</ref>
== References ==
{{reflist|22em}}
{{Eukaryota classification}}
[[Category:Obsolete eukaryote taxa]]
[[Category:Biological hypotheses]]
[[Category:Mitochondria]]
[[Category:Kingdoms (biology)]]
{{Eukaryote-stub}}