The aliens showed up and nobody was here to freak out.
The nameless animals that had arisen since we collectively left or died off or got taken up to Heaven or dropped into Hell floated around eating toxins and shitting out clean water. The visitors collected several samples for study, and after a series of tests, shockingly few of which involved anal probing, determined them to be unintelligent. A breeding pair was retained for documentation, and was stored in the ship.
Crystal formations, covered in plant growth, clustered in certain areas. Most often, these were located at the convergence of flowing bodies of freshwater. Others seemed placed with no clear indicator linking them to the others beyond shape.
In a small cavern beneath one such structure was a box. In this box were approximately one hundred perfectly preserved discs of polymers blended from oils and plant matter. The discs contained culture, art, history, drama and the sheer aspiration of mankind.
One sample was taken, studied, disposed of. The rest, left behind, never watched. A simple child’s Jack-in-the-box, wind up spring and handle and lid and long since rotted cloth dummy inside, sat in the captain’s room. This could not have occurred in nature. Someone must have visited before.