I had never heard of the two-space rule until I looked at my boyfriend's old college papers and then again recently when I proofread a friend's essay. I was an English major and I just finished a graduate thesis, and none of the comments ever mentioned that I should have used two spaces. I've studied several publishing style guides and have worked as an editor for the military and for K-12 standardized tests, and double spaces were always removed. Also, I've been looking through submission guidelines to academic journals and magazines/newspapers, and I haven't seen anything mention a preference one way or the other.
It really does seem like the idea originated with the typewriter, though at 28 I'm old enough to have used them and to have been around people who regularly used them, and I still don't remember hearing about the rule. Maybe I stopped remembering or caring once computers took over my life
Jam it back in, in the dark.