As cyanobacteria kept releasing oxygen, Earth changed. At first, oxygen did not simply fill the air. It reacted with iron, sulfur, methane, volcanic gases, and other oxygen-hungry materials. But eventually the old chemical buffers filled up. Oxygen began to linger. Then it began to spread.

A catastrophe for life. But evolution is rarely just disaster. It is also opportunity.

Most early microbes lived without oxygen. Many were anaerobes, meaning oxygen was not just unnecessary; it was harmful. When oxygen levels rose, many anaerobic lineages died out, shrank back, or retreated into oxygen-poor refuges: deep mud, sediments, hydrothermal systems, buried mats, and later, animal guts. Others survived by evolving ways to tolerate or use oxygen.

This is where oxygen-respiring bacteria enter the story. The oxygen-makers created the crisis; the oxygen-users learned to live inside it.

Among the bacterial winners were major proteobacterial lineages. Proteobacteria became especially important in oxygen-linked metabolism, chemical cycling, and later evolution. Much later, one alphaproteobacterial lineage would become the ancestor of mitochondria, the energy organelles inside eukaryotic cells. Inside your cells.

The Great Oxidation Event is not just “oxygen appears.” It is deeper. It is the moment life began turning poison into power.

Some bacteria evolved the ability to use oxygen as a terminal electron acceptor in respiration. That sounds technical, but the idea is simple: oxygen is excellent at helping cells extract energy from food. Aerobic metabolism opened a much richer energy pathway. Once oxygen became available, organisms that could use it gained a powerful advantage. Yes, when you think of aerobic exercise, you need to thank the bacteria in each of your cells! Likewise, when you feel the pain of anaerobic exercise, like too many squats, think of the pre-oxygen world.