Dan Slott could have produced the smoking gun for the 1961 appearance of the superheroes at Marvel. In the FF #14-19 storyline, it is admitted that the Overseer from a race of people at Alpha Centauri, sabotaged the FF’s first flight by bombarding their craft with excessively more cosmic rays, to dissuade them going there. Now, it could just be a coincidence that other super heroes just happened to appear after that event, and they were all orphans (more or less). But I like to think, if we ever did find the smoking gun, that it would be related to the explanation why all superhumans began appearing after 1961.
Maybe Slotts story arc only relates to the FF getting their powers? I don’t know. Would the editors at Marvel intended this story to be restricted just to the FF’s formation?