For roughly the first two billion years of life on Earth, the atmosphere contained essentially no free oxygen, and the microorganisms that dominated the planet were adapted to that oxygen-free chemistry, many of them actively poisoned by oxygen in anything more than trace quantities. Then, starting around 2.4 billion years ago, a lineage of bacteria called cyanobacteria, which had evolved a new form of photosynthesis capable of splitting water molecules and releasing oxygen as a by-product, began altering the atmosphere on a planetary scale. What followed is often called the Great Oxidation Event — and to most of the life forms alive at the time, it functioned less like an environmental shift and more like a slow-motion poisoning of the entire planet.
A Waste Product That Changed Everything
Cyanobacteria did not evolve oxygenic photosynthesis in order to reshape the atmosphere; oxygen was simply a chemical by-product of a more efficient way to capture energy from sunlight, one that used the abundant hydrogen available in water molecules rather than relying on scarcer chemical sources earlier photosynthetic bacteria depended on. Because water was so much more available than those earlier energy sources, oxygenic photosynthesis allowed cyanobacteria to proliferate far more successfully, and the oxygen they released as waste began accumulating faster than existing chemical processes on the young Earth could absorb it.
Rust Written Into the Rock Record
For a long stretch of time, newly produced oxygen was chemically absorbed almost as fast as it formed, reacting with dissolved iron in the ancient oceans and precipitating out as iron oxide, which settled onto the seafloor. This process left behind a distinctive geological signature called banded iron formations — alternating layers of iron-rich and iron-poor rock, some of which today are the primary source of the world's iron ore. These bands effectively record oxygen production outpacing the ocean's capacity to chemically neutralise it in real time, layer by layer, before oxygen levels eventually rose high enough to begin escaping into the atmosphere itself in significant quantities.
An Extinction Event With No Bodies
Because oxygen was directly toxic to the anaerobic organisms that dominated early Earth — interfering with essential cellular chemistry evolved in its total absence — the rising oxygen levels are believed to have caused a mass extinction among these microorganisms, sometimes referred to informally as the "oxygen catastrophe." This extinction left no visible fossil record in the way later mass extinctions of complex animals did, since the organisms involved were microscopic and largely soft-bodied, but its chemical and geological signature in ancient rock is considered strong indirect evidence of a biosphere-scale die-off, arguably the first mass extinction event in the history of life on Earth, triggered entirely by another organism's waste product.
Ice, Then Complexity
Rising oxygen also had an unexpected atmospheric consequence: it reacted with and depleted methane, a potent greenhouse gas that had been helping keep the young Earth warm despite a fainter early sun, and its removal is believed to have contributed to an extended period of extreme global glaciation known as the Huronian glaciation, one of the most severe ice ages in Earth's history. Once oxygen levels stabilised at higher concentrations, they eventually enabled aerobic respiration — a far more energy-efficient way of extracting usable energy from food than any anaerobic process — laying essential biochemical groundwork for the complex, energy-hungry multicellular life that would not appear for well over a billion years afterward. Research into this period, an active area of interest for NASA's astrobiology programme because of its relevance to detecting oxygen as a potential biosignature on other planets, treats the Great Oxidation Event as one of the clearest examples in Earth's history of life fundamentally remaking the planet it depends on, for better and for worse simultaneously.