Final answer:
The early Earth's atmosphere lacked oxygen for the first two billion years. This is supported by low atmospheric oxygen concentration, absence of an ozone layer, presence of carbon dioxide, and studies of ancient rock chemistry.
Step-by-step explanation:
The early Earth's atmosphere did not contain oxygen for the first two billion years of its existence. This is supported by several pieces of evidence. First, the absence of oxygen is indicated by the low atmospheric oxygen concentration and the lack of an ozone layer to filter UV radiation. Second, the presence of carbon dioxide in the early atmosphere suggests that photosynthetic organisms had not yet evolved to produce oxygen. Lastly, studies of ancient rock chemistry show that despite the existence of plants releasing oxygen through photosynthesis, chemical reactions with the Earth's crust removed oxygen as quickly as it formed.