What gas made up the largest portion of earth's atmosphere for most of earth's history

Gases in Earth's Atmosphere. Nitrogen and Oxygen are by far the most common. Dry air is composed of about 78% Nitrogen (N2) and about 21% Oxygen (O2). Argon, Carbon Dioxide (CO2), and many other gases are also present in much lower amounts; each makes up less than 1% of the atmosphere's mixture of gases.

So therefore Oxygen is the gas that makes up the largest portion of the Earths Atmosphere.