Here's why:

* Energy Source: Photosynthesis is the process of using sunlight to convert carbon dioxide and water into glucose (sugar) and oxygen. This process provides the essential energy source for life.

* Early Earth Atmosphere: The early Earth's atmosphere lacked significant amounts of oxygen. Photosynthesis was crucial for generating oxygen and changing the Earth's atmosphere.

* Cellular Respiration Dependency: Cellular respiration, the process of breaking down glucose to release energy, requires oxygen. It evolved later as a way to utilize the oxygen produced by photosynthesis.

In summary: Photosynthesis provided the energy source and oxygen for life to evolve, making it a prerequisite for the development of cellular respiration.