Came across this house ad for what was then titled DC Universe: Crisis on Infinite Earths in Green Lantern #186, cover dated March 1985:
As the first issue of Crisis was cover dated April ’85, just one month later, I can only guess that the ad was done months in advance which is why it still had the original series title.
Sill, it’s sort of nice to see that even at that early stage, the tag-line of “The DC Universe will never be the same” was in place.



Okay, maybe not the origin, but in the introduction to the Collected Edition, Marv Wolfman talks about a letter he received when he was writing Green Lantern in the early 80s. That letter prompted him to think about DC continuity and set him off on the path towards what would become Crisis on Infinite Earths.