Here's why:

* Photosynthesis is the primary source of oxygen on Earth. Plants, algae, and some bacteria use sunlight to convert carbon dioxide and water into glucose (sugar) and oxygen. This process releases oxygen as a byproduct.

* Early Earth's atmosphere had very little oxygen. It was primarily composed of gases like carbon dioxide, methane, and nitrogen. The evolution of photosynthesis and the subsequent production of oxygen transformed the atmosphere over millions of years.

* The vast majority of oxygen in our atmosphere is a direct result of photosynthesis. While some oxygen is produced by other geological processes, they are dwarfed by the amount generated by photosynthetic organisms.

However, there are a few nuances:

* A small fraction of oxygen comes from geological sources. This includes processes like the breakdown of water by sunlight (photolysis) and the release of oxygen from rocks through weathering.

* Oxygen is constantly being used up. Breathing, combustion, and rusting all consume oxygen. Photosynthesis replenishes this oxygen, maintaining a balance.

In essence, the oxygen we breathe is almost entirely a product of photosynthesis. It's the foundation of life as we know it.