Where did air come from? Origin of the atmosphere
Early air had almost no oxygen for 2 billion years until microbes raised oxygen to 21 percent and nitrogen to 78 percent.
First air from volcanoes, not oxygen
Four billion years ago volcanoes released steam, carbon dioxide and nitrogen that built a thick air with almost zero oxygen. Surface pressure may have reached tens of bars, while oceans condensed as steam cooled and rained for thousands of years.
For over 2 billion years oxygen stayed below 0.001 percent because iron and sulfur swallowed it. Rocks from 3.5 billion years ago show no red rust, proving air lacked free oxygen while methane kept early Earth warm enough for liquid water.
Cyanobacteria filled air with oxygen
About 2.4 billion years ago cyanobacteria in shallow seas released oxygen that rusted iron oceans and formed red beds. Oxygen then jumped to several percent during the Great Oxidation, poisoning many microbes yet powering larger cells with breathing.
Today air holds 78 percent nitrogen, 21 percent oxygen and 1 percent argon, with ozone shielding land life. Without cyanobacteria and later plants, oxygen would vanish in 5,000 years, so forests and plankton still refill every breath you take.
- First air: volcano steam and carbon dioxide, zero oxygen
- No oxygen for 2 billion years, below 0.001 percent
- Great Oxidation 2.4 billion years ago from cyanobacteria
- Today: 78 percent nitrogen, 21 percent oxygen
- Ozone shield formed, life reached land 500 million years ago