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

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