A common plot device to many science fiction stories (and more than a few comic book story arcs) is one of an alternate timeline. Something happens... usually involving time travel... which causes established events to veer from their intended path, thus creating a new timeline.
One of my favorite uses of this is in Back to The Future:
Lately I've been feeling like I'm trapped in an evil alternate timeline where somebody has changed the path that we're all supposed to be on. And do you blame me? The amount of horrendous shit happening in the world just keeps escalating, and there really doesn't seem to be any end in sight.
Alas, something tells me that Marty McFly ain't going to show up and magically fix everything this time.
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I things went way off the rails when they started firing the Hadron Collider.
OR when the wheel was invented. Mankind has been in a downward spiral ever since!
OR when we make the mistake of coming down from the trees in the first place!
This is probably more accurate!