Visceral

I'm thinking maybe Gaia created planet Mobius and the very basic creatures at the start of the evolutionary tree, but he put a lot of strain on himself by doing so. Being unable to properly watch over the world in his weakened state, he split itself into two gods, Light Gaia and Dark Gaia. Light would watch over the light side of the world and the creatures living in the day, while Dark would watch over the dark side and the creatures of the night. This lasted for billions of years, their powers as singular gods grew and eventually the two forgot they were part of the same being.

As ages passed, Dark Gaia grew jealous of Light Gaia. Light got to live in the daytime, playing with the people and being surrounded by sunshine. It lashed out at Light, bringing the whole world to disarray, and Light's only choice was to seal Dark away in the core of the planet they once created. Light ended sealing himself away in the process, which leads to the cycle we see in Sonic Unleashed.

But perhaps one day they will remember who they once were, and come back together as one...