The First Organisms

Close to 2.5 billion years ago, the earth's surface and atmosphere were stable enough to support primitive life. Single-cell organisms began to develop in the seas that covered the planet. Most of them were very simple single-cell bacteria that fed on chemicals in the ocean's waters.
A simple organism known as blue-green algae appeared and spread across the seas. Blue-green algae are still alive today. It was very important to the future of the planet because blue-green algae used sunlight and water to make food, and in the process, created oxygen. As the blue-green algae grew in the earth's seas, they began to fill the atmosphere with oxygen.
The oxygen blue-green algae produced made it possible for other types of organisms to develop. These organisms needed oxygen to carry out their life processes of growth, feeding, responding and reproducing. Unlike the blue-green algae, these organisms could not produce their own food. They needed oxygen to perform their life processes of growth, feeding, responding, and reproducing. In return, they produced CO2, which the algae needed to perform its life processes. A precise balance between plants and animals was established.
Adapted By:http://www.blueplanetbiomes.org/animals.htm