While the evolution of increasing respiratory sophistication was going on among the bacteria, the nucleated eukaryotes
(or, at least, their predecessors) survived seemingly without access to ETSs, presumably employing SLP pathways as their source of ATP. However, these 'sluggish' eukaryotes were colonizable within their cytoplasms by other cells, including by bacteria that served as proto-mitochondria (Figures 9 and 10). The advantages to eukaryotes of acquiring mitochondria were particularly strong within a well-oxygenated environment, since oxygen could serve as an easily diffused (into the big eukaryotic cell) and highly
effective final electron acceptor for ETSs, and because this allowed these organisms to acquire cellular respiration without actually evolving the necessary cellular machinery! Endosymbiotic mutualism set the stage for the acquisition, by eukaryotes, of a lifestyle that was less cell replication oriented. This was one in which cells were more energetic, more capable gatherers of food, more complex in their morphology, and, of course, larger. Eventually, single-celled eukaryotes would give rise to multicellular eukaryotes, such as animals, that would take these tendencies to extremes.
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