How cyanobacteria poisoned the planet, froze the Earth for 300 million years, and accidentally made multicellular life possible.
For the first ~2 billion years of life on Earth, there was no free oxygen. The atmosphere was a weakly reducing haze of nitrogen, carbon dioxide, and methane. Life was anaerobic — archaea, microbial mats, using retinal-based photosynthesis. Methanogens pumped CH₄ into the air, keeping the planet warm under a Sun that was 30% dimmer.
Then some cyanobacteria figured out a new trick: splitting water.
Oxygen was waste. The first pollution.
At first, it didn't matter. The planet had enormous chemical sinks — dissolved ferrous iron, sulfur, hydrogen sulfide, atmospheric methane. The oxygen reacted with everything it touched. Banded iron formations (BIFs) precipitated out of the ocean as Fe²⁺ oxidized to Fe³⁺ and sank. Today, those BIFs are the world's iron ore. They're the geological record of oxygen's first victims.
For maybe 500 million years, the sinks held. Then they filled.
Around 2.4 billion years ago, oxygen started accumulating in the atmosphere. The effect was immediate and catastrophic.
The atmosphere went from 0.001% O₂ to roughly 10% of modern levels. To anaerobic life, oxygen is a corrosive poison — it oxidizes DNA, proteins, cell membranes. The microbial mats that covered the planet started dying. The isotopic record suggests a biosphere collapse of >80%.
Methane (a powerful greenhouse gas) reacted with oxygen to form CO₂ and water — both much weaker greenhouse gases. The planet started to cool. Fast.
It's not usually counted among the "Big Five" extinctions because those are Phanerozoic. It was bigger than all of them.
300 million years of ice. At least three separate glacial advances. The longest glacial period in Earth's history.
The ice-albedo feedback locked it in: more ice → more reflection → colder → more ice. Evidence for this comes from glacial diamictites in the Huronian Supergroup north of Lake Huron, Ontario, first identified by Arthur Coleman in 1907, plus correlative deposits on every continent.
How do you melt a planet? Volcanoes. While the planet is frozen, silicate weathering stops. Normally weathering pulls CO₂ out of the atmosphere. Without it, volcanic CO₂ accumulates for millions of years until the greenhouse effect overwhelms the ice-albedo feedback. Then the ice melts, and the cycle resets.
Earth's Middle Ages. A billion years of tectonic stasis, climatic stability, and biological stagnation. No glaciations. No carbon isotope excursions. The planet was in a metabolic coma.
The oceans were euxinic — oxygen-poor, sulfidic, nutrient-starved. The dominant photosynthetic organisms were purple sulfur bacteria (using H₂S instead of H₂O, producing sulfur instead of oxygen). The ocean may have been black-and-milky-turquoise rather than blue or green.
But during this billion years of nothing, eukaryotes evolved. Symbiogenesis: an archaeon swallowed a proteobacterium, didn't digest it, and that bacterium became the mitochondrion. Sexual reproduction appeared. The first algae. The first fungi. The ancestors of animals.
The dullness was the incubator. Low oxygen, low predation, low competition — a billion years of quiet experimentation while the planet did nothing.
After a billion years of nothing, the planet decided to make up for lost time.
The supercontinent Rodinia broke up around 750 Ma. The fragments drifted into tropical latitudes. Tropical continents get more rain. Rain weathers silicate rock. Weathering pulls CO₂ out of the atmosphere. CO₂ dropped. The ice-albedo feedback took over. The planet iced over. Twice.
The Sturtian glaciation lasted ~57 million years (717–660 Ma). The Marinoan followed shortly after, lasting ~15 million years (650–635 Ma). Two Snowball Earths back to back.
During a Snowball Earth, the oceans are covered in ice ~1 km thick. No sunlight reaches the water. Photosynthesis slows to a crawl. Most life survives around hydrothermal vents. The only life on the surface is microbial colonies in cryoconite holes — dust-filled melt pockets on the ice surface.
Each time, volcanoes saved the planet again. CO₂ built up to extreme levels (~1,300 ppm, 5x pre-industrial). When the ice broke, it broke fast. The oceans acidified. The rock record shows these transitions as "cap carbonates" — distinctive carbonate layers that sit directly on top of glacial deposits, found on every continent.
Within 50 million years of the Marinoan thaw, the first widespread complex multicellular organisms appear in the fossil record. The Ediacaran biota. Frond-shaped, disc-shaped, quilted mattresses. No mouths, no guts, no legs, no eyes. Unlike anything alive today.
Dickinsonia, the most famous Ediacaran fossil, was an oval, segmented organism up to 1.4 metres long. It moved — trackways show it could crawl. It had no mouth, no anus, no organs. In 2018, scientists found cholesterol in its fossils. It was an animal. The oldest known.
Cloudina was the first animal to build a mineralised skeleton. Kimberella was a bilaterian that grazed on microbial mats.
Within ~20 million years, nearly all modern animal phyla appear. The Burgess Shale (508 Ma, British Columbia) preserves the most famous examples: Opabinia (five eyes, frontal nozzle with a claw), Anomalocaris (apex predator, 1m, circular mouth), Hallucigenia (walked on its spines), Pikaia (the earliest known chordate).
Multiple factors, probably all at once: oxygen levels had risen to near-modern levels (the Neoproterozoic Oxygenation Event), making large active bodies possible. The evolution of predators drove the evolution of armour, speed, burrowing. The Hox gene cluster was already in place.
The entire story — from the GOE to the Cambrian — spans ~1.9 billion years. The first 2/3 of Earth's history of life was single-celled. Then came the oxygen crisis, the billion-year stasis, the Snowball Earths, the Ediacaran rehearsal, and finally the Cambrian explosion.
The oxygen holocaust was the worst thing that happened to life on Earth. It was also the best thing.
Without it, there are no mitochondria, no animals, no Cambrian, no us. The cyanobacteria nearly killed the planet. Then they made it.