When the Earth first formed its atmosphere contained hydrogen, helium, methane, ammonia and water vapor, but no free oxygen! That atmosphere was so hot (130-300oC) and produced so much pressure (256 atmospheres) that all the water on the planet was water vapor. By comparison, Earth's atmosphere today has 1.0 atmosphere of pressure at sea level and an average temperature of about 15oC. So, getting back to our story...
About 100 million years later Earth's atmosphere had changed and was made mainly of water vapor, carbon monoxide, carbon dioxide, ammonia, nitrogen, sulfur dioxide, and methane. It was still too hot for liquid water, and there was no free oxygen.
Around 4by (billion years) ago the atmosphere cooled that it rained, and rained and rained and rained, for 1000s of years...and filled the oceans. Within "only" another half a billion years, life showed up - prokaryotes (bacteria), and a group called cyanobacteria could do photosynthesis!
These are living species of cyanobacteria:
As I'm sure everyone recalls from those heady, exciting days in biology class, during photosynthesis sugar is made combining carbon dioxide and water, oh, and a waste product is also produced - oxygen. The equation looks like this CO2 + H2O = C6H12O6 + O2. (Apologies to the chemical purists out there - I didn't balance the equation, heh heh).
Cyanbacteria sat there doing their thing (photosynthesis and making more cyanobacteria) for a LONG time, and oxygen gradually accumulated in the atmosphere. The presence of oxygen changed things...lots of things. For one thing, oxygen is a toxic element and it killed most of the bacteria of the day. Doesn't it seem weird that oxygen probably caused Earth's first major extinction event?
It took from 3.5by until about 1.8by for oxygen to make up 2-5% of the atmosphere. While oxygen concentrations increased other other gases decreased: carbon dioxide, carbon monoxide, and methane. The atmosphere wasn't the only thing changing either. By about 1.5by other kinds of life show up - eukaryotic life. Eukaryotic cells have a nucleus and membrane-bound organelles, like mitochondria and chloroplasts. Plus about this time the oceans reached their current degree of salinity (saltiness).
The atmosphere stayed pretty much the same between 1.8by and 0.8by ago, and this time period is referred to as "The Boring Billion" years. Then, however, things really started to happen! Between 0.8by and 0.5by oxygen levels climbed to 10-20% of the atmosphere, and plants and animals showed up. The temperature of the Earth, however, fluctuated between hot-house conditions and ice-ball conditions.
Around 350 million year ago there was a massive spike of oxygen in the atmosphere. It made up about 35% of the atmosphere. This spike occurred during the time when Earth's first massive forests 400-300 appeared. These forests then died off and formed some of the huge coal reserves that we use today.
This is an artist's rendition of those forests. Image courtesy of the Smithsonian [http://www.mnh.si.edu/highlight/riola/].
Around 300 million years ago atmospheric oxygen made up about 21% of the atmosphere, and it's been pretty constant ever since.
So what's the bottom line about Earth's atmosphere? Like everything else on Earth...it changes!
These two links take you to parts one and two of a fantastic video based on The National Geographic Society's production, "The Story of Earth". It has background music but no narration. The video depicts our current scientific interpretation of the history of this planet we call home. Enjoy.
And here's part 2, just in case you wanted to see the rest...I know I did!