Answer:
oxygen
Explanation:
Earth's early atmosphere before life began was devoid of oxygen. Intense volcanic activity occurred for around a billion years and the earth's surface was in molten state. When it started to cool early atmosphere was formed from the gases emitted by volcano. It contained carbon dioxide, methane, hydrogen sulphide, ammonia and small amounts of water vapor. But this atmosphere had no trace of oxygen.
The water vapor condensed and formed oceans. Photosynthesis by early plants finally released oxygen which became a part of atmosphere. Carbon dioxide was also used up in various cell processes and trapped in rocks and fossil fuels which decreased its share in atmosphere.
