Anaerobic metabolism is usually considered to be an old and rather primitive way of life, but the scenario described here would imply that anaerobically functioning mitochondria are an adaptation of the traditional type of mitochondria to anaerobic environments; hence, these anaerobic mitochondria are in fact a further evolution of aerobic mitochondria.

Similar to the observed diversity in hydrogenosomes (Fig. 5.1), it is now known that not all mitochondria are the same (Scheffler 1999; Tielens et al 2002, Burger et al. 2003; Gabaldon and Huynen 2004). Over the past few years many different variants have been identified, and we might be only at the beginning of the discovery of many more interesting variants of ATP-producing organelles. Mitochondria are no longer the once-thought uniform ATP-producing organelles, identical in all organisms, but are rather diverse and adapted to distinct conditions. Recent analyses suggest that all mito-chondrial variants, including anaerobically functioning ones, have evolved from a single endosymbiotic ancestor. The mitochondrial diversity has evolved by multiple mitochondrial adaptations that occurred independently in multiple nonrelated lineages.

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