The alternate dimensions of Earth Prime-Time did not come into existence at the same time. Each of them shared a common existence as one dimension until one event caused an off-shoot from the original plane of existence. It would then be in its own dimension and begin a new timeline of its unfolding history.
But before that point in Time where the divergence took place, the Event Hour, its timeline would be exactly the same as that of Earth Prime-Time. (The one exception may be that of the Evil Mirror Universe which is the most popular of the alternate TV dimensions, as seen in 'Star Trek', 'Deep Space Nine', 'Enterprise', 'Buffy The Vampire Slayer', and 'Hercules: The Legendary Journeys' among others. That may have been inherently evil from the get-go.)
It doesn't have to take a catastrophic event to cause a dimensional divergence either; it can be something quite trivial. For instance, in an episode of 'Sliders', Quinn's Quartet thought they made it home to their own dimension's San Francisco - until they discovered that the Golden Gate Bridge was painted blue and called the Azure Gate Bridge instead. That's all it took.)
This all may be known already to faithful readers of Inner Toob, or just obvious to fans of science fiction and fantasy. But it's nice to get the basics down and locked in for future use....
Plus it's a good lead-in for the next story.....