Ooops. Another false statement. Sorry, but there were so many I overlooked this one. No, this theory is not "required" by evolution. How the heck do you figure that? The endosymbiotic theory is of fairly recent origin. Off the top of my head I believe it was first suggested (but never tested or developed into an actual theory) in the mid-20's or thereabouts. It was only developed as a serious theory in the last twenty years (if IIRC), and generally accepted even more recently.
The mitochondria and cholorplasts developed from symbiots is only the way (apparently) that it happened to work out. There is nothing in the theory of evolution that requires that it happened this way. Similar organelles might have been evolved within the cell, or their functions might have evolved in some other way. After all bacteria evolved them in the first place.