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 developed in the first eukaryotic cell within the endosymbiotic theory framework.

According to the hydrogen hypothesis the first eukaryotic cell did not appear as a consequence of a primitive host cell engulfing a primitive bacteria, which wasn't fully digested and eventually became the mitochondrion as the current endosymbiotic theory suggests. It claims instead that the host - a methanogenic archaebacterium which used hydrogen and carbon dioxide, producing methane - and a facultatively anaerobic eubacterium, the future mitochondrion, which produced hydrogen and carbon dioxide as byproducts of anaerobic respiration, started a symbiotic relationship based on the host's hydrogen dependence (anaerobic syntrophy).

The idea originated when Martin attended a talk by Müller on hydrogenosomes. These are anaerobic mitochondria that produce ATP and large amounts of hydrogen and carbon dioxide. One of Müller's slides presented a cluster of methanogens around a hydrogenosome inside a eukaryotic cell they had invaded.

If correct, this hypothesis would imply that eukaryotes are chimeras with both archaebacterial and eubacterial ancestry. It would furthermore imply that eukaryotes appeared later in evolution than prokaryotes. This contrasts with some current views that assume archaea and eukarya to have split before the modern groups of archaea appeared. The hydrogen hypothesis predicts that no primitively mitochondrion-lacking eukaryotes ever existed, in contrast to many current views, which predict the existence of such cells.