Early life: Oxygen enters the atmosphere

Stromatolites

Exactly when the first life on Earth - the ancestors of modern bacteria - began is a subject of debate, but evidence suggests it could be as much as 3.5 billion years ago.

Early bacterial life introduced oxygen to the atmosphere. As the first free oxygen was released through photosynthesis by cyanobacteria, it was initially soaked up by iron dissolved in the oceans and formed red coloured iron oxide, which settled to the ocean floor. Over time, distinctive sedimentary rocks called banded iron formations were created by these iron oxide deposits. Once the iron in the oceans was used up, the iron oxide stopped being deposited and oxygen was able to start building up in the atmosphere about 2.4 billion years ago.

Image: Stromatolites in Shark Bay, Western Australia. Stromatolites, which are formed by microscopic bacteria, are rare on Earth today but were much more common in the ancient Earth's seas. (credit: L Newman & A Flowers/SPL)

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Early life: Oxygen enters the atmosphere

The Great Oxygenation Event (GOE, also called the Oxygen Catastrophe, Oxygen Crisis, Oxygen Holocaust, Oxygen Revolution, or Great Oxidation) was the biologically induced appearance of dioxygen (O2) in Earth's atmosphere. Although geological, isotopic, and chemical evidence suggest that this major environmental change happened around 2.3 billion years ago (2.3 Ga), the actual causes and the exact date of the event are not clear. The current geochemical and biomarker evidence for the development of oxygenic photosynthesis before the Great Oxidation Event has been mostly inconclusive.

Oceanic cyanobacteria, which evolved into multicellular forms more than 2.3 billion years ago (approximately 200 million years before the GOE), are believed to have become the first microbes to produce oxygen by photosynthesis. Before the GOE, any free oxygen they produced was chemically captured by dissolved iron or organic matter. The GOE was the point in time when these oxygen sinks became saturated, at which point oxygen, produced by the cyanobacteria, was free to escape into the atmosphere.

The increased production of oxygen set Earth's original atmosphere off balance. Free oxygen is toxic to obligate anaerobic organisms, and the rising concentrations may have destroyed most such organisms at the time. Cyanobacteria were therefore responsible for one of the most significant extinction events in Earth's history. Besides marine cyanobacteria, there is also evidence of cyanobacteria on land.

A spike in chromium contained in ancient rock deposits formed underwater shows they had accumulated chromium washed off from continental shelves. Chromium is not easily dissolved and its release from rocks would have required the presence of a powerful acid. One such acid, sulfuric acid, might have been created through bacterial reactions with pyrite.Mats of oxygen-producing cyanobacteria can produce a thin layer, one or two millimeters thick, of oxygenated water in an otherwise anoxic environment even under thick ice, and before oxygen started accumulating in the atmosphere, these organisms would already be adapted to oxygen. Additionally, the free oxygen would have reacted with atmospheric methane, a greenhouse gas, greatly reducing its concentration and triggering the Huronian glaciation, possibly the longest episode of glaciation in Earth's history and called snowball Earth.

Eventually, the evolution of aerobic organisms that consumed oxygen established an equilibrium in its availability. Free oxygen has been an important constituent of the atmosphere ever since.

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