4 billion years ago, the Earth's atmosphere contained large amounts of two toxic gases (methane and ammonia) which were emitted into the atmosphere through volcanic outgassing. There was also a small amount of water.
4 Billion Years ago
3.8 billion years ago, many intricate photochemical reactions occurred between all of the previous gases in the atmosphere and solar energy. The methane and ammonia were dissociated into new gases which were released into the atmosphere: nitrogen, carbon dioxide, and hydrogen. This added more variety to the composistion of gases in the atmosphere.
3.8 Billion Years ago
Carbon Dioxide (CO2)
3.6 billion years ago, the break down of the previous methane and ammonia molecules formed into hydrogen and oxygen atoms. So now, a completely new gas has been introduced to the atmosphere (oxygen). This will play a key role in the developement of Earth's atmosphere as time goes on.
3.6 Billion Years ago
Hydrogen atom (H2)
Oxygen atom (O2)
3.5 billion years ago, the hydrogen in Earth's atmosphere escaped into space since it was too light to be held down by gravity. After that, the water vapor was broken down into oxygen and hydrogen due to solar energy. The oxygen interacted with each other and formed ozone. Once enough of this gas built up, the ozone layer formed which absorbed most of the UV rays emitted from the sun and protected life below. There was also an abundant amount of nitrogen and carbon dioxide.
3.5 Billion Years ago
(Shields from UV rays)
700 million years ago, a miroorganism formed in the water but went out to land later. This bluish green bacteria used photosynthesis to convert the suns light, carbon dioxide, and water into oxygen and food. This would change the planet forever since now oxygen from this bacteria would settle near the surface of the Earth and give way for potential life.
700 million Years ago
600 million years ago, the levels of oxygen and carbon dioxide in the atmosphere begin to average out to about what the levels are today.