Answer:
The early atmosphere on Earth was mostly carbon dioxide and poisonous.
Step-by-step explanation:
The earth's atmosphere in pre-historic times was carbon dioxide with little or no oxygen. Other noxious gases included ammonia, methane, water vapor, and neon. Scientists believe that the atmosphere of the earth during its formation resembled the current atmosphere on Mars and Venus.
Much change was caused by the photosynthesis process from plants which consumed carbon dioxide and released oxygen. Furthermore, carbon dioxide dissolved in the ocean. The water vapor condensed and formed the oceans due to temperature reductions.