The primary source of energy for most living organisms is the sun.

Here's how it works:

* Photosynthesis: Plants and other photosynthetic organisms capture sunlight and convert it into chemical energy stored in the bonds of glucose (a type of sugar). This process is called photosynthesis.

* Food Chain: Animals obtain energy by consuming plants or other animals that have eaten plants. This energy transfer continues up the food chain.

Therefore, the sun is the ultimate source of energy for almost all life on Earth.