From Populous Wiki
The Solar System is the level selection on Single Player. It shows every planet/level and how they orbit another, the ring colour shows weather that level or group of levels have been done or not.
The last level: The Beginning does not show up as a hole indent on the level selection panel, it is also the only level not on a planet. Instead, it takes part in the heavens, where the tribes are first told to be 'moving through the heavens like a plague'