For over half a million years, Earth has been a canvas for civilizations from distant stars. Each of these civilizations was part of a grand plan, and their arrival was carefully timed.
They succeeded in breaking through a protective barrier that had been keeping Earth isolated. After getting in, they thrived for certain periods – like 500 years, 5,000 years, or even 10,000 years – before the controlling forces of the planet either pushed them out or defeated them. These civilizations left behind hints and steps to unlock the planet's hidden library. They couldn't own Earth, but they paved the way for us to do so.