Two prokaryotes turning into a eukaryotic cell began with a process called
endosymbiosis. One large prokaryote engulfs (a process called
endocytosis) one small, aerobically respiring prokaryote. The small
prokaryote turned into a membrane-bound organelle, which prokaryotes do
not have, and like we know, only eukaryotes have membrane-bound
organelles. So that is how the eukaryotic cell structure evolved from
prokaryotic cells, meaning that before this evolution, they were most likely symbiotic.
