For the first billion years of life, the air was almost unbreathable. Early Earth's atmosphere was dominated by nitrogen, carbon dioxide, and methane; free oxygen was essentially absent. Then, around 3.5 billion years ago, tiny prokaryotes called cyanobacteria accomplished something extraordinary: they captured solar photons, split water molecules, and converted carbon dioxide into sugar — releasing free oxygen as a byproduct.

This oxygenic photosynthesis proceeded invisibly slowly at first. The oxygen produced initially reacted with dissolved iron in the oceans, precipitating out as the banded iron formations that are mined as ore today. But as cyanobacteria proliferated, oxygen kept accumulating, and around 2.4 billion years ago — the Great Oxidation Event — it began flooding the atmosphere.

Photosynthesis bequeathed far more than an altered atmosphere. It installed a universal engine that runs the biosphere on solar energy. Every complex food chain today — from trees to whales — ultimately traces back to a photosynthetic organism capturing light. Billions of years later, humans would burn the compressed ancestral remains of these bacteria as coal and oil; fossil fuels are, in essence, frozen sunlight.

The stromatolites of Western Australia's Pilbara craton — layered biogenic structures built by cyanobacterial communities — are the most tangible physical witnesses to this process. Living stromatolite communities that still grow in shallow Australian bays today continue the same metabolic miracle that began 3.5 billion years ago.