Hydrogen hypothesis

The hydrogen hypothesis is a model proposed by William Martin and Miklós Müller in 1998 that describes a possible way in which the mitochondrion arose as an endosymbiont within a prokaryote (an archaebacterium), giving rise to a symbiotic association of two cells from which the first eukaryotic cell could have arisen.

According to the hydrogen hypothesis:
* The host that acquired the mitochondrion was a prokaryote, a hydrogen-dependent archaebacterium, possibly similar in physiology to a modern methanogenic archaebacterium which uses hydrogen and carbon dioxide to produce methane;
* The future mitochondrion was a facultatively anaerobic eubacterium which produced hydrogen and carbon dioxide as byproducts of anaerobic respiration;
* A symbiotic relationship between the two started, based on the host's hydrogen dependence (anaerobic syntrophy).

The hypothesis differs from many alternative views within the endosymbiotic theory framework, which suggest that the first eukaryotic cell 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 evolutionary significance to hydrogenosomes and provides a rationale for their common ancestry with mitochondria. Hydrogenosomes are anaerobic mitochondria that produce 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 chimeras with genes of archaebacterial and eubacterial ancestry. It posits that eukaryotes appeared later in evolution than prokaryotes. This contrasts with some views that assume eukaryotes to be older than prokaryotes. Most theories within the endosymbiotic theory framework predict that some eukaryotes never possessed mitochondria. The hydrogen hypothesis predicted that no primitively mitochondrion-lacking eukaryotes ever existed. In the 10 years following the publication of the hydrogen hypothesis, that specific prediction has been tested many times and supported.Fact|date=August 2008

References

* cite journal
author=López-Garćia P, 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
* 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
* 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

ee also

*symbiogenesis