Take a breath and the oxygen feels like the most natural substance on Earth.
It is anything but.
For roughly the first half of Earth’s history, there was almost no free oxygen in the atmosphere. Life had already existed for more than a billion years, but it inhabited a planet whose air would be lethal to us and whose oceans were rich in dissolved iron.
Then a form of photosynthesis began changing the chemistry of the world. Cyanobacteria used sunlight to take electrons from water, built organic matter from carbon dioxide, and released molecular oxygen as waste.
At first, almost all of that oxygen disappeared into rocks, seawater and volcanic gases. Around 2.4 billion years ago, the balance shifted. Oxygen began accumulating in the atmosphere during what geologists call the Great Oxidation Event.
The consequences reached far beyond the sky. Oxygen attacked the chemistry on which anaerobic organisms depended. It altered iron, sulfur and nutrient cycles. It also helped remove methane, a powerful greenhouse gas that may have kept the young Earth warm under a fainter Sun. Severe glaciations followed, including ice deposited at what appears to have been tropical latitude.
It is tempting to call this the planet’s first pollution catastrophe. That captures something real, but it compresses several uncertainties. Oxygenic photosynthesis probably evolved well before 2.4 billion years ago. We cannot count how many microbial lineages died. And the link from oxygen to methane loss to a frozen planet remains a strong hypothesis rather than a fully recovered sequence of events.
Even the claim that every oxygen atom in the air comes from this catastrophe needs care. The atoms in oxygen molecules are continuously recycled through water, organisms and rock. What traces back to the Great Oxidation Event is the existence of a persistent oxygen-bearing atmosphere, not the uninterrupted history of each atom you inhale.
Cyanobacteria were making oxygen before the atmosphere kept it
Oxygenic photosynthesis is a remarkable chemical trick. Earlier forms of photosynthesis drew electrons from substances such as hydrogen sulfide or dissolved iron. The ancestors of cyanobacteria learned to use water, a far more abundant source.
Splitting water requires energy from sunlight and specialised molecular machinery. The cell uses the electrons and hydrogen to help turn carbon dioxide into organic matter. Two oxygen atoms left over from separate water molecules can join to form O2 and escape.
The date of that evolutionary invention remains uncertain. Molecular clocks and geochemical evidence generally place it before the Great Oxidation Event, perhaps hundreds of millions of years earlier. A review of Earth’s early biosphere describes evidence for local oxygen in surface environments long before the atmosphere became oxygenated.
These were oxygen oases rather than an oxygen world. A cyanobacterial mat could release O2 in sunlit shallow water while the atmosphere above and deep ocean below remained effectively anoxic.
The young planet contained an enormous appetite for oxygen. It reacted with dissolved ferrous iron, reduced minerals in fresh rock, hydrogen, methane and gases emerging from volcanoes. Respiration and decay consumed it too. Production could continue while the atmospheric concentration barely moved.
For oxygen to accumulate over geological time, some photosynthetic organic carbon had to be buried before other organisms could consume it. The corresponding oxygen was then left behind. At the same time, the strength of the planet’s oxygen sinks had to fall. Modern models can reproduce broad oxygenation by increasing the source, weakening the sinks, or combining both, which is why there is no single agreed trigger.
The event at 2.4 billion years ago was therefore not necessarily the birth of oxygen production. It was the point at which oxygen stopped being entirely erased.
The change is recorded in sulfur, iron and ancient soil
No sample of Paleoproterozoic air survives in a bottle. Geologists reconstruct it from reactions that the atmosphere allowed or prevented.
One of the clearest records comes from sulfur isotopes. In an oxygen-poor atmosphere without an ozone layer, ultraviolet light broke apart sulfur gases and sorted their isotopes in a distinctive, mass-independent pattern. Those products fell to the surface and entered sedimentary rocks.
Once oxygen rose and an ozone shield developed, the atmospheric pathway changed. The unusual sulfur signal largely disappeared from the rock record. A 2026 review of the Great Oxidation Event places the first persistent atmospheric rise between about 2.50 and 2.43 billion years ago, while emphasising that both the timing and the amount of oxygen remain uncertain.
Other rocks tell related stories. Minerals such as pyrite and uraninite can survive transport in oxygen-free rivers but weather readily in oxygenated conditions. They become scarce as loose grains in younger sediments. Red beds, coloured by oxidised iron, become common. Sulfate minerals expand. The chemistry of ancient soils changes.
Banded iron formations are part of the longer transition, although they are not a simple oxygen gauge. Iron dissolved in an anoxic ocean can become insoluble when oxidised and settle as iron oxide. Vast deposits record changing interactions among iron, microbes and ocean chemistry, but not every band was produced in the same way or at the same time.
The important point is that independent chemical systems move across the same broad interval. The sky itself is gone, but its reactions remain embedded in stone.
Oxygen was toxic, but “most life died” is not a fossil count
Oxygen is useful to us because it is reactive. That same quality makes it dangerous.
Many anaerobic metabolisms depend on low-potential metal centres and radical chemistry. Oxygen can directly disable those enzymes. Partly reduced forms of oxygen, including superoxide and hydrogen peroxide, can damage proteins, membranes and genetic material.
A review of how modern anaerobes respond to oxygen shows that the old picture is not quite as simple as defenceless cells meeting instant death. Many possess repair systems and can tolerate some exposure. Even so, oxygen blocks the growth of obligate anaerobes and makes an oxygenated habitat unavailable to them.
When O2 spread through the atmosphere and surface ocean, some communities must have lost territory. Others retreated into mud, deep water, rock pores and other places where oxygen could not reach. Their descendants still occupy anoxic sediments, hydrothermal environments, animal intestines and waterlogged soils.
What we cannot do is turn that ecological upheaval into a percentage. Life 2.4 billion years ago was microbial, and microbes leave a sparse, difficult fossil record. There is no global catalogue of species immediately before and after the event. Calling it the largest mass extinction in history, or saying it killed most life, goes beyond what the rocks can demonstrate.
The most recent synthesis reaches a more complicated conclusion: the biosphere was forced to reorganise and oxygen-sensitive organisms faced a new hazard, but total biological productivity may have grown as oxygen-based metabolism expanded. The poison for one community became an energy source for another.
Removing methane may have pushed Earth towards ice
The Sun 2.4 billion years ago was substantially fainter than it is today. Liquid oceans therefore required a stronger greenhouse than modern sunlight would need.
Carbon dioxide supplied part of it. Methane produced by microbes may have supplied another important part. In an oxygen-poor atmosphere, methane could persist much longer than it does today. As oxygen rose, atmospheric reactions shortened that lifetime and converted methane into carbon dioxide and water.
Carbon dioxide is also a greenhouse gas, but molecule for molecule methane is much more effective. If methane concentrations were high enough, their decline could have removed a large part of Earth’s warming blanket.
The geological timing makes the idea compelling. Paleoproterozoic rocks preserve several glacial intervals around the Great Oxidation Event. A 2020 sulfur-isotope study found that persistent atmospheric oxygen appeared before a severe glaciation recorded in north-western Russia and correlated with the Makganyene deposits of South Africa.
Palaeomagnetic measurements indicate that the Makganyene glacial sediments formed at low latitude. That is the basis for saying ice reached close to the equator, not a surviving map of one continuous global ice sheet. The low-latitude evidence is strong enough to place the event within “Snowball Earth” territory, although the exact reach and continuity of the ice remain debated.
The methane chain is plausible, but it is not the only possible explanation. Methane abundance cannot be read directly from those rocks. Carbon dioxide, volcanic activity, weathering, biological productivity and the reflectivity of growing ice all affected climate. A study of Proterozoic methane cycling also showed that sulfate-driven methane consumption in the ocean complicates the idea of one large atmospheric methane reservoir.
It is safest to say that oxygenation could have weakened a methane greenhouse and helped push an already vulnerable planet into extreme glaciation. Saying cyanobacteria single-handedly froze Earth is neater than the evidence allows.
The first oxygenated atmosphere was nothing like ours
The Great Oxidation Event did not take Earth from zero oxygen to the modern 21 per cent in one jump.
Estimates of Paleoproterozoic oxygen differ by orders of magnitude. The rise may have included advances, reversals and an early overshoot. Much of the deep ocean remained without oxygen, and atmospheric concentrations later settled far below modern levels for an immense stretch of the Proterozoic.
That long delay matters beyond geology. In my earlier article on whether a future telescope could miss life on an early Earth, I explored why an inhabited planet need not advertise itself with a strong oxygen signal. Photosynthesis can exist while rocks, water and gases consume nearly all of its product.
Earth required later oxygenation episodes before animals and large active bodies became possible. Oxygen also helped create stratospheric ozone, reducing the ultraviolet radiation reaching the surface. It changed the availability of nutrients and multiplied the number of oxidised minerals that could form.
None of this means evolution was aiming at animals or humans. Cyanobacteria altered their environment because of the chemistry of their metabolism. The consequences opened some ecological possibilities and closed others.
The oxygen in a breath is ancient in origin, not in identity
The atmosphere now contains a vast reservoir of O2, but its molecules are not museum pieces left untouched since the Paleoproterozoic.
Photosynthesis makes oxygen by splitting water. Respiration, decay, fire and weathering consume it. Oxygen atoms move through water, carbon dioxide, organic matter and minerals before returning to the air in new combinations.
I recently wrote about how roughly half of present-day oxygen production is carried out by oceanic phytoplankton, including modern cyanobacteria. Nearly all of that gross production is balanced by consumption. The atmospheric reservoir persists because a small fraction of organic carbon escapes oxidation and is buried over geological time.
So the oxygen molecule entering your lungs this second probably did not float continuously through the atmosphere for 2.4 billion years. Its existence in the atmosphere is nevertheless part of a planetary state established by oxygenic photosynthesis and made persistent during the Great Oxidation Event.
That event was catastrophic from the perspective of organisms whose chemistry oxygen disrupted. It may have contributed to ice reaching the tropics by weakening a methane greenhouse. It also enabled ozone, aerobic metabolism and eventually the oxygen-hungry biosphere we know.
The same substance can be waste, poison, climate agent and fuel. Oxygen did not simply make Earth habitable. Life made oxygen, oxygen remade habitability, and every organism that followed inherited the altered planet.
