Hydrogen hypothesis
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The hydrogen hypothesis is a model proposed by William Martin and Miklos Muller 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 methanogen archea which used hydrogen and carbon dioxide, producing methane - and a primitive eubacteria, the future mitochondrion, which produced hydrogen and carbon dioxide as byproducts of anaerobic respiration, started a symbiotic relantionship due to this byproducts.
The idea originated when Martin asisted a talk by Muller on hydrogenosomes, these ocurr in anaerobic eukariotic cells to replace mitochondria's ATP production role, and they produce large amounts of hyrogen and carbon dioxide. One of Muller's slides presented a cluster of methanogens around a hydrogenosome inside a eukariotic cell they had invaded.
If the hypothesis is correct it would imply that eukaryotes are very close to archea and appeared relatively late in contrast with the current view that states that archea and eukarya branched off before the modern groups of archea appeared.