Question: How was the atmosphere made?

Paul Brack answered on 18 Jun 2015:

Scientists believe that the Earth was formed about 4.5 billion years ago. Its early atmosphere was probably formed from the gases given out by volcanoes. It is believed that there was intense volcanic activity for the first billion years of the Earth’s existence. Gases like carbon dioxide, hydrogen sulphide and methane were around in large quantities, and there was pretty much no oxygen.

However, 2.7 billion years ago, bluish-green microscopic organisms called cyanobacteria began to flourish in the oceans. They made oxygen from carbon dioxide, water, and sunlight—the process called photosynthesis. Between 700 and 550 million years ago, oxygen levels in the oceans and atmosphere increased dramatically, and this continued as more and more plants appeared, until we got to our current atmosphere of 21% oxygen about 290 million years ago.

There’s much more detail on the website where I got most of this from http://forces.si.edu/atmosphere/02_02_00.html.