Thanks for the great question.

To answer your question, we have to go back to the formation of the Earth itself. Around 4.54 billion year ago, the Earth formed through a process called accretion. This is when dust and gas in space are pulled together by their own gravity, which in turn pulls in more dust and gas until the planet is formed.

The Earth’s first atmosphere was made of the same gas that was present in the solar nebula, or the cloud of gas and dust in which the sun formed. This first atmosphere was made of water vapor, methane, and ammonia. You would not want to breathe this air.

Next, the eruption of many volcanoes put huge amounts of carbon dioxide and nitrogen into the air. Then, 3.5 billion years ago, the first life on Earth evolved. This is very important to the future of the atmosphere, especially when bacteria 2.5 billion years ago evolved to convert the carbon dioxide into oxygen, which had not been a part of the atmosphere until this point.

Today our atmosphere is still about 80% nitrogen, with about 20% oxygen, and trace amounts of other gases like carbon dioxide, which plants still use. The atmosphere is still changing, however, especially as humans put back a lot of the carbon dioxide that previously locked up in oil and coal. This is what’s causing global warming.

Good question. The different gasses in our atmosphere probably have different sources. Most of the Earth's atmosphere is nitrogen, and it is difficult to imagine how that got to Earth other than by impact from comets. Most of the rest of the atmosphere is oxygen, which could have come from a variety of different sources, including comets, but also water-containing minerals in asteroids, and so on.