точно те са си извършвали поначало цикъла, преди да бъдат включени в еукариотите.
Ето нещо интересно по темата:
The traditional view posits that the host that acquired the mitochondrion was an anaerobic nucleus-bearing cell, a full-fledged eukaryote that was able to engulf the mitochondrion actively via phagocytosis (Figure 2). This view is linked to the ideas that the mitochondrial endosymbiont was an obligate aerobe, perhaps similar in physiology and lifestyle to modern Rickettsia species; and that the initial benefit of the symbiosis might have been the endosymbiont's ability to detoxify oxygen for the anaerobe host. Because this theory presumes the host to have been a eukaryote already, it does not directly account for the ubiquity of mitochondria. That is, it entails a corollary assumption (an add–on to the theory that brings it into agreement with available observations) that all descendants of the initial host lineage, except the one that acquired mitochondria, went extinct. The oxygen detoxification aspect is problematic, because the forms of oxygen that are toxic to anaerobes are reactive oxygen species (ROS) like the superoxide radical, O2-. In eukaryotes, ROS are produced in mitochondria because of the interaction of O2 with the mitochondrial electron transport chain. In that sense, mitochondria do not solve the ROS problem but rather create it; hence, protection from O2 is an unlikely symbiotic benefit. This traditional view also does not directly account for anaerobic mitochondria or hydrogenosomes, and additional corollaries must be tacked on to explain why anaerobically functioning mitochondria are found in so many different lineages and how they arose from oxygen-dependent forebears.
An alternative theory posits that the host that acquired the mitochondrion was a prokaryote, an archaebacterium outright. This view is linked to the idea that the ancestral mitochondrion was a metabolically versatile, facultative anaerobe (able to live with or without oxygen), perhaps similar in physiology and lifestyle to modern Rhodobacteriales. The initial benefit of the symbiosis could have been the production of H2 by the endosymbiont as a source of energy and electrons for the archaebacterial host, which is posited to have been H2 dependent. This kind of physiological interaction (H2 transfer or anaerobic syntrophy) is commonly observed in modern microbial communities. The mechanism by which the endosymbiont came to reside within the host is unspecified in this view, but in some known examples in nature prokaryotes live as endosymbionts within other prokaryotes. In this view, various aerobic and anaerobic forms of mitochondria are seen as independent, lineage-specific ecological specializations, all stemming from a facultatively anaerobic ancestral state. Because it posits that eukaryotes evolved from the mitochondrial endosymbiosis in a prokaryotic host, this theory directly accounts for the ubiquity of mitochondria among all eukaryotic lineages.
http://www.nature.com/scitable/topicpage/the-origin-of-mitochondria-14232356