Here is some basic background information on our planet's atmosphere.
EARTH'S ATMOSPHERES
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| Layers of the atmosphere at sunset |
Most of us take our atmosphere, and the life forms that thrive in it, for granted, and think it’s the one and only atmosphere for life on Earth. But apparently Earth has had other atmospheres, other life forms. We are currently in the third atmosphere.
THE FIRST ATMOSPHERE, during the early days of the planet, was probably mostly hydrogen and helium. A hydrogen-rich atmosphere is called a reducing or reduced atmosphere. The universe in general has high hydrogen content.
THE SECOND ATMOSPHERE was produced by volcanic outgassing, a rich concoction that was probably mostly carbon dioxide with some stuff like water, sulfur, nitrogen, hydrogen, ammonia and methane. The oceans were forming...
THE THIRD ATMOSPHERE is our familiar oxygen atmosphere, called an oxidizing atmosphere. Actually it's only about 21% oxygen; the rest is nitrogen (79%) with bits of carbon dioxide, methane, and other gasses tossed in.
Before the Industrial Revolution in the late 18th century, the carbon dioxide (CO2) level was around 280 parts per million (ppm). Now it is much much higher - you can check the current level here.
