When Earth's Air First Turned Poisonous

For roughly the first two billion years of Earth's history, the atmosphere had almost no free oxygen. The oceans were full of dissolved iron. The sky was probably a hazy orange. Anaerobic microbes — the only kind there was — went about their lives quite happily.

Then cyanobacteria figured out a new trick. They learned to split water molecules using sunlight, harvest the hydrogen for energy, and dump the oxygen as waste. For the bacteria themselves, oxygen was poison; for everything else alive at the time, it was worse. But the bacteria multiplied, the oxygen accumulated, and around 2.4 billion years ago the atmosphere tipped.

Geologists read the tipping point in the rocks. Before the event, sedimentary rocks contain reduced minerals like detrital pyrite that cannot survive in oxygenated water. After it, you find banded iron formations — alternating layers of rust-red iron oxide and dark chert — laid down as the dissolved iron in the oceans precipitated out, then ran out. The iron rusted out of solution, then the air finally started to keep oxygen in it.

The consequences were not minor. Most existing microbial lineages went extinct. Methane in the atmosphere — a strong greenhouse gas — got oxidized to CO2 and water, weakening the greenhouse effect, and the planet plunged into the Huronian glaciation, possibly the longest ice age in its history. Earth froze nearly to the equator for some 300 million years.

Everything that breathes now — plants, animals, fungi — descends from organisms that learned to use the new poison instead of dying from it. The most consequential pollution event in the planet's history was committed by single-celled organisms with no idea what they were doing.