Once upon a time, the beings who called themselves the human race had existed on just one small blue and green planet they called earth. That’s where they evolved and grew to become sentient, over time inventing all kinds of technology and eventually developing space flight.
By now humanity has spread across the universe, scattered among the stars, colonising every suitable planet they can find and mixing their gene pool with compatible species. Purebred humans are rare, but that’s just fine, the hybrids are more adaptable.
Earth itself is empty; everyone has left. This is the day the world ends.