The Evolution of Eukaryotes
Everything you’ll ever need to know is within you; the secrets of the universe are imprinted on the cells of your body. ~ American author Dan Millman
As the Earth and its atmosphere were transformed by cyanobacteria, viruses promoted prokaryotic evolution by providing genetic uploads.
~2.5 bya, eukaryotes arose. The first step to eukaryotic life was through unification: one prokaryote incorporated another. The host was an archaeon.
An intracellular bacterial parasite gave rise to the mitochondria found in all eukaryotic cells. At some point, the bacterium that beget mitochondria became benign, then mutualistic. The mitochondrial bacterium went from stealing ATP to providing it.
Transition to accommodation is not unusual. Viruses go from devastating to their hosts to being tolerable or even beneficial, as they learn to prolong their residency by not inflicting untold damage, and thereby benefit from host longevity.
Mitochondrial incorporation came late in the evolution of eukaryotes. Already many eukaryotic hallmarks, including complex subcellular organization, were in place prior to added a dedicated power plant facility to the works.