There's been enough UFO insiders (the most recent one being Matthew Brown) that have hinted at there having been multiple iterations/cycles of humanity throughout earth's history. And what happens (allegedly, via Tom DeLonge and Corso, for example) is that once any one cycle reaches a critical point (where maybe we develop the tech to legitimately "fight back"), the Others then wipe us out. If true, we really are a Petri Dish. And the Others... the Grand Scientists.

UAPs in China and Extraterrestrial Revelation of Earth’s History a Myste...