The oxygen we breathe began as poison for Earth's early microbes
Around 2.4 billion years ago, cyanobacteria were releasing a waste gas that most life on Earth could not tolerate. As free oxygen built up in the air and shallow seas, it probably wiped out many anaerobic microbes. Yet the same upheaval may have opened the path to complex cells, and eventually to us.
Earth's early atmosphere was mostly nitrogen and carbon dioxide, with oxygen at roughly a thousandth of a percent of today's level. Cyanobacteria, which may date back 3.5 billion years, split water during photosynthesis and gave off oxygen. For ages that gas was mopped up by dissolved iron, sulfur and methane. Only once those sinks were exhausted could oxygen accumulate, beginning roughly 2.46 to 2.426 billion years ago and continuing until about 2.06 billion. By the end, oxygen may have reached a tenth of modern levels.
The change was brutal for a biosphere built without oxygen. Free oxygen attacks organic molecules, including genetic material, and a global glaciation compounded the damage to microbial mats. Sulfate minerals hint that the biosphere shrank by more than 80 percent near the end of the event. It rarely appears on lists of great extinctions, partly because tallying microscopic life that old is so difficult. Survivors may have adapted by absorbing oxygen-using bacteria that became mitochondria, a partnership that could have given rise to eukaryotes and later multicellular life.
Reconstructing this story began with the American geologist Preston Cloud in the 1970s. He noticed that sediments older than about 2 billion years hold grains of pyrite, uraninite and siderite, minerals that oxidise quickly in oxygen-rich air, while rust-coloured red beds stained by hematite appear around then. Banded iron formations, striped layers of chert and iron oxide, peak about 2.5 billion years ago and largely vanish by 1.85 billion, which is read as the deep ocean finally oxygenating. Heinrich Holland refined the timeline through the 1980s.
Pinning down the start remains stubborn. Papers from 2016 to 2022 disagree by about 500 million years, with estimates from 2.7 billion down to 2.3 billion years. Perhaps the strongest clue is an unusual sulfur isotope signature that forms only without atmospheric oxygen; it appears in rocks older than about 2.4 to 2.3 billion years and then disappears.
Source: Great Oxidation Event
