    Sentence'1: ‘To what type of organisms‘is oxygen  toxic?    Sentence’ 2: Oxygen’. is. toxic ‘to obligately  anaerobic: organisms, which’ were.the domiriant  form: of early. life on Earth uatil-O.2 began to  accumulate in the atmosphere about 2.5 billion  years ago during the Great Oxygenation’ Event,  about a billion years after the first appearance of  these organisms.