This interactive module explores how the changes in oxygen levels throughout Earth’s history have impacted the planet’s environment and types of life.

Today, oxygen (O₂) makes up about 21% of Earth’s atmosphere, but oxygen levels in both the atmosphere and ocean were much lower in Earth’s past. In this Click & Learn, students explore an interactive graph of atmospheric oxygen levels over the last 3.8 billion years, which covers three geologic eons: the Archean, the Proterozoic, and the Phanerozoic. The graph includes descriptions of the biological and geological factors that influenced oxygen levels in different time periods, and how life on Earth was affected.