Shots of two of the infamous catalyst comets - both appeared in the episode ‘Evergreen’ back in the Cretaceous period - the shot of the blue one was shown at the end of ‘Evergreen’ and the comet would hit in Finn and Jakes ‘present’ timeline. 

The green color of the comet that hits Ooo in the Cretaceous period (the period between 145.5 and 65.5 million years ago, ending with the extinction of the dinosaurs) could come from it containing radioactive materials. Evergreen says that it contains evil/wants to destroy all of us. These facts could suggest that the comet introduced the evil of the Lich in the form of material that could be used to produce the nuclear bomb which exploded during the mushroom war to cause the near extinction of humans, the creation of the Lich and the mutation of Ooo’s inhabitants. In the world of Ooo, the comet is allowed to strike (probably killing all the dinosaurs and the four elementals, Gunther etcetera) due to Gunther not using the wish to stop it from impacting. This would then allow the crown to be found in the future by Simon, who would eventually become the Ice King.

It is very possible that the Ice Age was caused by the crown after Gunther was killed by the comet. In “Finn the Human” Farmworld Marceline’s recount of the last 1000 years mentions the crown reacting to losing it’s bearer: “And the horrible crown, grieving its lost master, loosed a spasm of magic so wild and crazy the entire earth was buried in ice for four hundred boring years.” The crown, being new, possibly inadvertently cast a much more powerful spell than it would later on in the Alternate Wish World; thus starting the Pleistocene Epoch, the ice age.

Gathered by Shots of Ooo