Question

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

Answer 1

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.