A meteor crashed into earth carrying a strange alien gass. The gas wasn't poisonous, but it did remove all the cholorplasts from EVERY plant cell on earth.
If there was no cholorplasts, plants would not be able to make oxygen, plants and animals, including humans, likely could not exist. In addition the atmosphere would have very little oxygen because photosynthesis releases a large amount of oxygen into the air. Otherwise, Earth would be a pretty barren lifeless place without plants having chlorplasts
