Where the air came from

Scientists think the earth's atmosphere has changed during the billions of years since the earth was formed. At first, the atmosphere was probably very different from what it is now. It began to change as some of the gases went off into space.

For many millions of years the earth was fiercely hot. Clouds of hot gases from inside the earth came out of cracks and volcanoes. As these clouds spread out, they became part of the atmosphere. These gases changed the atmosphere, but, you couldn't have breathed the air. It had no oxygen to keep you alive. It was made up mostly of steam, and gases called nitrogen and carbon dioxide.

But as the earth grew cooler, some of the steam broke up into the two gases steam is made of - and one of those gases is oxygen. So, some oxygen became part of the air. Much of the steam became water, and formed the first ocean.

After millions of years there were billions of tiny green plants in the ocean. These plants, called diatoms, made their own food out of water, sunlight, and carbon dioxide gas, just as green plants do today. When plants make food, they give off oxygen, so for millions of years the little diatoms put oxygen into the air, helping to make the kind of atmosphere we have now.