When they first arrived at Earth, they examined the whole planet and quickly became disgusted.
Not with violence, not with injustice, but with sex and the mating rituals of all the species on the planet.
Soon after, their will was to genetically modify every single being on Earth in order to make sure those primitive rituals would be abandoned, and so it began.
Drones scouted the land and the oceans to make sure no being was left untouched.
Then they left without a trace, resuming their journey across the stars, and leaving a changed Earth behind them.
Only a few beings had been able to elude the drones and be left untouched, and then only a few of those were able to resume and propagate their rituals and make them prevail.
Those species are still alive today, such as the praying mantis and a few spiders, like the black widow.