Question: when did significant amounts of oxygen appear in the earths atmosphere
[{'answer': '541 million years ago'}, {'provenance': [{'wikipedia_id': '24377796', 'title': 'Geological history of oxygen', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 2046, 'bleu_score': 0.961857475941935, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 9756828628479725311}, 'section': 'Section::::Abstract.'}]}]