Answer:
The correct answer is "photosynthesis".
Step-by-step explanation:
One of the major changes that Earth's atmosphere experienced in ancient times was the addition of oxygen due to photosynthesis. The Earth’s early atmosphere was toxic for most organisms, since it consisted mostly of water, carbon dioxide and ammonia. However, over million of years, the photosynthetic activity of ancient cyanobacteria and blue-green algae changed the atmosphere into favored conditions for life.