Comment Re:...as far as we know. (Score 2) 49
The endosymbiotic theory for the origin of the nucleus started with Mereschkowsky [13]. He postulated that the nucleus evolved from a prokaryote (mycoplasma), which was engulfed by an amoeboid cell homologous to the eukaryotic cytosol (figure 1a; [15]).
The Mereschkowsky paper they reference is from 1905. In their further discussion of the idea, they reference a number of papers from the 1990s and 2000s, including Was the nucleus the first endosymbiont?, Selective forces for the origin of the eukaryotic nucleus, and A new fusion hypothesis for the origin of Eukarya: better than previous ones, but probably also wrong.
So it has been thought of many times, there just isn't strong enough evidence to convince everybody yet.