In Doomsday Clock (which admittedly I've yet to actually read), when Manhattan is observing the various continuity shifts that occur around Superman, he talks about the various forces that have triggered continuity changes - Krona, Anti-Monitor, Extant, and himself.

We know that COIE caused the shift from Silver Age/Bronze Age to Post COIE. That Manhattan himself caused the shift to the New 52. The return of Wally West and the re-merging of the two Supermen caused the latest ongoing shift.

But my question is - what caused the original continuity shift that Manhattan observed? The one which caused the transition from Golden Age to Silver Age. Or in other words, the one which caused Superman to go from showing up in 1938 to showing up in 1956.

I know the traditional explanation has been that there was no 'continuity shift' but the comics simply started following another earth at that point. But DDC now establishes the fact that on the 'mainstream' earth (or Prime Earth or whatever you want to call it), Superman did once debut in 1938, and the transition to the Silver Age version was yet another timeline shift/reboot.

So what caused this timeline shift?

Could it be Krona's original experiment that created the Multiverse? Could it be that in reaching back to observe creation, he inadvertantly not only triggered the birth of the Multiverse, but wiped out the JSA from earth's history and moved forward Superman's origin (with something akin to the 'original timeline' continuing to exist on the parallel earth we call 'Earth Two')?

Or was it something else?

What's interesting about this continuity shift is that, unlike other shifts, its had a permanent impact on DC continuity and on the DC brand at large! Changes from COIE, IC and now the New 52 have been undone, but the original shift from Golden Age to Silver Age has persisted throughout every subsequent incarnation of the DCU...in the comics and across media.

Superman, Batman and Wonder Woman are widely considered to be contemporaries (if not founders) of the Justice League. The 'iconic' GL and Flash are Hal and Barry, not Alan and Jay, and the GL and Flash mythologies have largely been based on the Silver Age interpretations. The Silver Age remains the 'base template' of the DCU, and has since the late 50's/early 60's. Silver Age characters and concepts have thrived, while Golden Age ones, by and large, have been relegated to the sidelines.

Even the new DC timeline, which otherwise represents continuity as it originally happened, is slaved to this original continuity shift - ensuring that Superman and Batman are contemporaries of the JLA, not JSA (Wonder Woman is the one exception, and in her case, it might have more to do with movie synergy that an attempt to honor the Golden Age continuity).