Question

Earth's early atmosphere contained mainly what gas?

Answer 1

Earth's atmosphere has changed considerably over geological time, so it depends on what you mean by "early".

Explanation:

Earth's first atmosphere, shortly after the planet formed 4.6 billion years ago, was mostly hydrogen, helium, ammonia and methane.

Between 4 to 1 billion years ago, the atmosphere consisted of nitrogen, water vapour, and CO2. In the last 1 billion years or so, life on the planet transformed the atmosphere into primarily nitrogen and oxygen (see pic).

http://ponce.sdsu.edu/plants_and_animals.html