Editing Hydrogen hypothesis (section)

==Mechanism==

The hypothesis differs from many alternative views within the [[endosymbiotic theory]] framework, which suggest that the first eukaryotic [[Cell (biology)|cells]] evolved a nucleus but lacked mitochondria, the latter arising as a eukaryote engulfed a primitive [[bacterium]] that eventually became the mitochondrion. The hypothesis attaches [[evolution]]ary significance to [[hydrogenosome]]s and provides a rationale for their common ancestry with mitochondria. Hydrogenosomes are anaerobic mitochondria that produce [[adenosine triphosphate|ATP]] by, as a rule, converting [[pyruvate]] into hydrogen, carbon dioxide and [[acetate]]. Examples from modern biology are known where methanogens cluster around hydrogenosomes within eukaryotic cells. Most theories within the [[endosymbiotic theory]] framework do not address the common ancestry of mitochondria and hydrogenosomes. The hypothesis provides a straightforward explanation for the observation that eukaryotes are genetic [[Chimera (genetics)|chimeras]] with genes of archaeal and eubacterial ancestry. Furthermore, it would imply that archaea and eukarya split after the modern groups of archaea appeared. Most theories within the [[endosymbiotic theory]] framework predict that some eukaryotes never possessed mitochondria. The hydrogen hypothesis predicts that no primitively mitochondrion-lacking eukaryotes ever existed. In the 15 years following the publication of the hydrogen hypothesis, this specific prediction has been tested many times and found to be in agreement with observation.<ref name=EmbleyMartin>{{cite journal | author=Embley TM and Martin W | title=Eukaryotic evolution, changes and challenges | journal=Nature | year=2006 | volume=440 | pages=623–630 | doi=10.1038/nature04546 | pmid=16572163 | issue=7084| bibcode=2006Natur.440..623E | s2cid=4396543 | url=https://zenodo.org/record/897869 }}</ref><ref name=Lane2005>{{cite book |author=Lane, Nick |authorlink=Nick Lane |title=Power, Sex, Suicide: Mitochondria and the Meaning of Life |year=2005 |publisher=Oxford University Press |isbn=978-0-19-920564-6|title-link=Power, Sex, Suicide: Mitochondria and the Meaning of Life }}</ref><ref name=LopezMoreira>{{cite journal |author=López-Garćia P and Moreira D | title=Metabolic symbiosis at the origin of eukaryotes | journal=Trends Biochem Sci | year=1999 | volume=24 | pages=88–93 | issue=3 | doi=10.1016/S0968-0004(98)01342-5 | pmid=10203753}}</ref><ref name=MartinMuller>{{cite journal | author=Martin W and Müller M | title=The hydrogen hypothesis for the first eukaryote | journal=Nature | year=1998 | volume=392 | pages=37–41 | issue=6671  | doi=10.1038/32096 | pmid=9510246| bibcode=1998Natur.392...37M | s2cid=338885 }}</ref><ref name=PoolePenny>{{cite journal | author=Poole AM and Penny D | title=Evaluating hypotheses for the origin of eukaryotes | journal=BioEssays | year=2007 | volume=29 | pages=74–84 | issue=1  | doi=10.1002/bies.20516 | pmid=17187354}}</ref>

In 2015, the discovery and placement of the [[Lokiarchaeota]] (an archaeal lineage possessing an expanded genetic repertoire including genes involved in membrane remodeling and [[actin]] cytoskeletal structure) as the sister group to eukaryotes called into question particular tenets of the hydrogen hypothesis, as Lokiarchaeota appear to lack methanogenesis.<ref name="SpangSaw2015">{{cite journal |last1=Spang |first1=Anja |last2=Saw |first2=Jimmy H. |last3=Jørgensen |first3=Steffen L. |last4=Zaremba-Niedzwiedzka |first4=Katarzyna |last5=Martijn |first5=Joran |last6=Lind |first6=Anders E.|last7=van Eijk |first7=Roel |last8=Schleper|first8=Christa |last9=Guy |first9=Lionel |last10=Ettema |first10=Thijs J. G. |title=Complex archaea that bridge the gap between prokaryotes and eukaryotes |journal=Nature |year=2015 |doi=10.1038/nature14447 |volume=521 |issue=7551 |pages=173–179 |pmid=25945739 |pmc=4444528|bibcode=2015Natur.521..173S }}</ref>