Here's why:

* Photosynthesis: This process is the primary way oxygen is produced. Photosynthetic organisms, like plants, algae, and some bacteria, use sunlight, carbon dioxide, and water to create their own food (sugars) and release oxygen as a byproduct.

* Evolutionary Impact: The rise of oxygen in the Earth's atmosphere was a major turning point in evolution. Early forms of life were anaerobic (didn't need oxygen). The emergence of photosynthetic organisms led to a significant increase in atmospheric oxygen, paving the way for the development of more complex life forms that use oxygen for respiration.