Explanation:
Without oxygen, aerobic respiration cannot occur, which means eukaryotes cannot produce the energy they need to survive. The rise of oxygen levels in Earth's atmosphere occurred between 2.5 and 2.7 billion years ago, a significant shift often referred to as the "Great Oxygenation Event." At this time, photosynthetic bacteria began to release vast amounts of oxygen as a waste product of photosynthesis. As oxygen levels accumulated, it became possible for aerobic respiration to occur, setting the stage for the evolution of complex life forms on land.
