If the eukaryotes arose in an endosymbiosis between two prokaryotes, an archaeal host cell and a bacterial endosymbiont, which went on to become mitochondria, then we can explore the question from a more conceptual point of view. Can we think of a good reason why one cell getting inside another cell should transform the prospects of prokaryotes, unleashing the potential of eukaryotic complexity? Yes. There is a compelling reason, and it relates to energy.
