I don’t buy the story of the symbiosis of prokaryotic cells to produce eukaryotic cells in the evolution of Life on earth. It makes no sense.
If one cell merged into another, eventually, the larger cell would burst, since the smaller cell would continue to divide. Also, you would need some sort of genetic similarity/compatibility for the symbiosis to continue.
I suggest that the mitochondrial cell within a eukaryotic cell is rather descended from an RNA strand or virus whose source was the cellular DNA of the “parent”/larger cell. For example, a strand of RNA “budded out” from the nucleus, and, eventually, the DNA of both made it a permanent condition because of the division of labor: mitochondria produce ATP for energy, the nucleus takes care of protein production. I mean, think about it. The mitochondria produces ATP for energy. What could it possibly have needed from another prokaryotic cell?
Lynn Margulis’s idea, although accepted canon in biology, simply strains the imagination.