Introduction | A LONG LONG LONG time ago... | A LONG LONG time ago...
A LONG time ago... | Not so long ago...

A LONG LONG LONG time ago...

When the Earth formed about 4.5 billion years ago, it was a very different place than it is today. Volcanoes erupted all the time, spewing chemicals like methane, ammonia, and carbon dioxide into the air, there were no oceans, and the climate was very hot.

It took a very long time--more than a billion years—but Earth cooled down eventually. Rain took the chemicals from the air, forming lakes and oceans. There was still not much oxygen in the air until bacteria developed in the oceans that got their energy from the Sun about 3 billion years ago. Just like plants today, these bacteria were able to use the Sun’s energy to make their food by a process called photosynthesis. This process releases oxygen into the atmosphere and takes carbon dioxide out of the atmosphere. So, over about 3 billion years, tiny bacteria changed the climate of the world as they collected greenhouse gases and released oxygen. The climate cooled and the Earth became a better place for living things to survive.