Simplify the following text for a Grade 13 native English-speaking person.
Original: It was once thought that appreciable amounts of molecular oxygen were present in the prebiotic atmosphere , which would have essentially prevented the formation of organic molecules ; however , the current scientific consensus is that such was not the case .
Simplified: It was once thought that a large amount of molecular oxygen was present in the prebiotic atmosphere , which would have stopped the formation of organic molecules ; however , this is not the case .