1. **Biological activities**: The appearance of photosynthetic microbes approxim

1. **Biological activities**: The appearance of photosynthetic microbes approximately 3,000 million years was crucial, since they began to produce oxygen, altering the atmospheric balance[1]. 2. **Geological changes**: The cooling of the Earth reduced the emission of volcanic gases, allowing oxygen to accumulate in the atmosphere[1]. 3. **Pollution and climate change**: Human activity, such as pollution and rising temperatures, has decreased oxygen levels in the oceans, affecting the solubility of oxygen and its flow into the sea[3].