We as a race always knew it would happen, we just didn't know when. We knew that we would eventually deplete the natural resources of the planet Earth. It was inevitable that we would take more from her than we could give back. So when it finally happened we were not suprised or even really sorry. We just packed our bags and left.
What humanity left behind were those few hundred thousand people that believed they could help to heal the damage that was left behind. They lived without the benefit of most modern technologies. None of the mind numbing electronics were allowed to remain. Those that stayed wanted to work with the land, they had no time for mindless games or pointless studies.
These remnants separated into small groups that lived on every continent but one. They toiled and worked and built a new civilization and the Earth slowly healed.
But as luck would have it, when all is going well something will inevitably happen to screw it all up. So it was with the growing populations of Earth.
One day the sun shone bright and the day was fresh and inviting. Unfortunately an unexpected guest accepted the invitation. They were the Winiks. They were running from an enemy across the stars. They had engine trouble and stopped off at planet Earth looking for respite.
Their persuers, the Zalens also had engine trouble but they also had braking problems. They crashed onto the planet.