The atmosphere's oxygen content was itself the product of ancient pollution

Kean recounts how Earth's atmosphere originally contained essentially no free oxygen, and that the eventual buildup of breathable oxygen resulted from ancient photosynthetic microorganisms, particularly cyanobacteria, releasing oxygen as a metabolic byproduct over hundreds of millions of years — effectively the largest and longest pollution event in the planet's history from the perspective of the anaerobic organisms that dominated before it.

This oxygenation, sometimes called the Great Oxidation Event, was catastrophic for many existing microbial life forms that couldn't tolerate the newly reactive gas, while simultaneously opening the door for entirely new, oxygen-dependent forms of life, including eventually all complex multicellular organisms, humans included.

Kean uses this history to point out the irony that the "toxic waste" of one era's dominant life form became the essential resource enabling the next era's — a pattern that reframes atmospheric composition as a byproduct of biological competition rather than a fixed, neutral backdrop. Takeaway: the air that makes complex life possible today was originally a toxic waste product of ancient microbial life.