As the Earth's oceans filled with single-celled bacteria, these organisms found new ways to exploit the available resources. For hundreds of millions of years these simple prokaryotic (without a nucleus) cells adapted to every available water habitat, developing new capabilities to take advantage of all conditions and resources. At about 2.5 billion years ago some of these bacteria took one of the most important steps in the history of life: they developed the ability to absorb sunlight and use the energy from it to make food from the carbon dioxide in the atmosphere. This process, called photosynthesis, has been one of the most important life processes on Earth, changing the atmosphere and surface of the entire planet. The oceans very quickly filled with bacteria that could perform photosynthesis, called cyanobacteria or blue-green bacteria. One of the waste products of photosynthesis is oxygen, and therefore oxygen, which was previously almost nonexistent, began to fill the atmosphere.
Before long, enough oxygen had accumulated in the atmosphere that it began to have a significant effect on living things and the physical structure of the Earth's surface. Some species of bacteria evolved the ability to not only tolerate the oxygen, but to use it to their advantage. They developed the ability to use oxygen to help convert food into energy. This process is called respiration and turned out to be another life process which drastically changed the course of events on Earth.
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