The Original Earth

"Every time an important decision, conscious or not, is made, the universe splits into two parts... One is if the decision was one way, the one if the other option was chosen. All of these universes coexist inside the Multiverse, and are all connected by one thing...

Each universe has, at some place or another on the unique planet Earth, a small cavern. On a full moon, the mouth of the cavern will begin to glow bluish, and a strange portal-like wall will suddenly appear. If one were to pass through this portal, they all land in the same place, The Original Earth.

The Original Earth is the core of its universe, and its universe is the core of the Multiverse."

This is a story told throughout the Multiverse, on nearly every Earth that has evolved sentient lifeforms.
Said lifeforms, although each could theoretically exist in each other's universes, are extremely varied. Some, in fact, are barely recognizable as lifeforms at all.
So how is it possible that the same tale (As many have decided it is) is told in each of these universes?
And why does nobody know anything about The Original Earth?

The year is, on our Earth, 1294. Across the Multiverse, a few people-- Most of them teenagers-- have decided that perhaps there is some merit to this story after all, and have started off to find this cavern.