Here's why:

* Photosynthesis: The process of photosynthesis, carried out by plants, algae, and some bacteria, converts sunlight into chemical energy stored in the form of glucose.

* Food Chain: This glucose is then consumed by other organisms, forming the base of the food chain. Animals eat plants, and larger animals eat smaller animals, passing the energy from the sun down the chain.

* Cellular Respiration: Cells within organisms break down the glucose, extracting its stored energy through a process called cellular respiration. This energy is used to power all the activities of life, including growth, movement, and reproduction.

While there are some organisms that can obtain energy from inorganic sources (like chemosynthesis), the vast majority of life on Earth relies on sunlight as the ultimate source of energy.