Roughly 2.4 billion years ago, the atmosphere of the Earth underwent the most consequential chemical change in the planet’s history. Free oxygen, a gas that had been present only in trace amounts for the previous two billion years, began accumulating in the air. The shift took several hundred million years to play out, but the inflection point that geochemists call the Great Oxidation Event left a distinct signature in the rock record and reshaped Earth’s biology, climate and chemistry. The signature is unambiguous. Banded iron formations, vast layered deposits of iron oxide laid down on ancient seafloors, peak globally in deposition just before and through this period, marking the first time dissolved iron in seawater was being mass-precipitated by reaction with free oxygen. The cause of the change was biological. The most consequential by-product the change produced was a mass extinction.
The cause was a group of microbes called cyanobacteria. According to the American Society for Microbiology’s account of the event, cyanobacteria evolved the ability to perform oxygenic photosynthesis somewhere around 2.7 to 3.5 billion years ago, perhaps significantly earlier than the GOE itself. They split water molecules using sunlight, used the hydrogen to build sugars, and released the oxygen as waste. For most of the time cyanobacteria existed before the GOE, the oxygen they produced was immediately absorbed by dissolved iron, volcanic gases, and exposed rocks. Only when those sinks began to saturate did free oxygen start accumulating in the atmosphere. The gap between the evolution of oxygenic photosynthesis and the actual oxygenation of the air ran to hundreds of millions of years.
Why the delay, and why the breakthrough
A 2025 study led by Dilan M. Ratnayake of Okayama University, published in Communications Earth & Environment, proposes that the delay was caused in part by the chemistry of the early ocean. High concentrations of dissolved nickel and urea appear to have favoured methane-producing archaea over cyanobacteria, suppressing cyanobacterial growth and limiting the rate at which oxygen could be released. As volcanic activity declined over billions of years and the chemistry of the oceans shifted, the nickel concentration dropped, the methane producers lost their advantage, and the cyanobacteria proliferated. Oxygen production began outpacing the planet’s ability to absorb it.
A separate 2021 paper in Nature Communications by Olejarz and colleagues, available in PMC, models the GOE as an ecological tipping point triggered when the supply of available chemical reductants in the environment fell below a critical threshold relative to phosphate. Both explanations describe the same general pattern: cyanobacteria had been producing oxygen for a very long time, but the environmental conditions that would allow that oxygen to accumulate as free atmospheric gas took most of the Archean eon to arrive.
What oxygen did to the existing biosphere
The biology of Earth before the GOE was predominantly anaerobic. The dominant lifeforms were single-celled organisms, mostly bacteria and archaea, that lived in oceans, hot springs, and sediments, and that derived their energy from chemical reactions that did not involve oxygen. For these organisms, oxygen was not just unnecessary. It was actively toxic. The molecule is highly reactive and oxidises cellular components, damaging proteins, lipids, and DNA in any organism that has not evolved specific defenses against it. Modern aerobic life depends on a complex set of enzymes and antioxidant molecules that neutralise these effects. The anaerobic life of the Archean had no such defenses, because there had been no reason to develop them.
When atmospheric oxygen began rising during the GOE, this entire pre-existing biosphere came under sudden chemical attack. Anaerobic microbes that had dominated Earth’s oceans and shallow waters for nearly two billion years were now living in an environment that was systematically poisoning them. The extinction that followed is sometimes called the Oxygen Catastrophe or the Oxygen Crisis. It is the earliest mass extinction event the rock record clearly preserves, and on a planet whose biosphere was almost entirely microbial, it is plausibly the most lethal in absolute terms. Quantifying it precisely is impossible. There are no fossil species counts to draw on; microbial diversity in the Archean is reconstructed primarily from chemical biomarkers and from molecular clocks calibrated against living descendants. What is clear from those reconstructions is that anaerobic life retreated from most of Earth’s surface and survived only in places where oxygen could not reach.
Those places still exist, and the descendants of the anaerobic survivors still live there. Anaerobic microbes thrive today in deep ocean sediments, in waterlogged soils, in animal digestive tracts, in deep underground rock formations, and around hydrothermal vents. Methanogenic archaea, sulfate-reducing bacteria, and other oxygen-intolerant lineages are direct evolutionary survivors of the GOE, occupying the niches where the planet’s old chemistry still applies. They are no longer dominant. Before the GOE, they were everything.
The other consequences
The Great Oxidation Event did not only kill things. It changed the planet’s climate, atmosphere, and geology in ways that shaped the next two billion years of evolution. The newly oxygen-rich atmosphere broke down the methane that had been a major greenhouse gas in the Archean, and Earth’s surface temperature dropped sharply. The result was the Huronian Glaciation, one of the most severe ice ages in the planet’s history, which lasted from roughly 2.4 to 2.1 billion years ago and may have produced a “Snowball Earth” event in which the planet’s surface froze nearly from pole to pole.
Oxygen also enabled new biochemistries. Aerobic respiration, which uses oxygen to extract energy from food, generates roughly 18 times more ATP per glucose molecule than fermentation does. This dramatic energy advantage opened the door to organisms that could afford the metabolic costs of being larger and more complex. Most biologists consider the GOE a precondition for the eventual evolution of eukaryotic cells, the type of cell that all plants, animals, fungi, and protists are built from. Without the energy availability that oxygen-based metabolism provided, the cellular complexity that culminated in multicellular life would probably not have been chemically feasible.
The oxygen level after the GOE remained low by modern standards, perhaps 1 to 5 percent of present-day atmospheric concentrations, and it stayed that way for over a billion years. The interval is sometimes called the “boring billion” because relatively little evolutionary change is visible in the fossil record. A second major rise, called the Neoproterozoic Oxygenation Event, occurred roughly 600 million years ago and brought oxygen up to something approaching modern levels. That second event coincides closely with the appearance of the first animals.
What this changes about the standard story
The popular framing of Earth’s history tends to treat oxygen as an unambiguous good, the gas that makes life possible. The longer view is that oxygen is the gas that makes a particular kind of life possible, and that the kind of life it enabled, our kind, required the elimination or marginalisation of the kind of life that came before it. The most successful pollution event in the planet’s history was caused not by industry but by photosynthesis, and the organisms that triggered it were the ancestors of the green plants and algae that now produce the oxygen humans breathe. The same chemistry that nearly wiped out life two and a half billion years ago is now the chemistry that almost every animal on Earth depends on. The transition was not gentle. It took several hundred million years to settle, and it left behind a biosphere structurally different from anything that had existed before.
The Great Oxidation Event is one of the more useful reminders that the relationship between organisms and their environment is not a one-way street. Life on Earth has, on at least one occasion, reshaped the chemistry of its entire planet, killed off most of what was living at the time, and inadvertently created the conditions for everything that followed. The lineage that survived to write about it is still doing the same chemistry, on a slightly more controlled scale, now.