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How did photosynthesis change Earth’s atmosphere?

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Wispy cirrus clouds stretch across a bright blue sky above trees and a river at Pinawa Dam Provincial Park in Manitoba, Canada, with sunlight illuminating the scene. — Atmosphere The atmospheres of planets in the solar system are composed of various gases, particulates, and liquids. They are also dynamic places that redistribute heat and other forms of energy. On Earth, the atmosphere provides critical ingredients for living things. Here, feathery cirrus clouds drift across the deep blue sky over Pinawa Dam Provincial Park, near Pinawa, Manitoba, Canada.

The evolution of oxygenic photosynthesis fundamentally transformed Earth’s atmosphere from a reducing environment to an oxidizing one.1

Early photosynthetic organisms, such as cyanobacteria, began producing molecular oxygen ( $O_{2}$ ) as a metabolic waste product. Initially, this oxygen did not accumulate in the atmosphere because it was rapidly consumed by "sinks"—abundant oxidizable materials like dissolved ferrous iron, sulfides, and organic compounds. For millions of years, this oxygen reacted with dissolved iron in the oceans, leading to the large-scale deposition of banded-iron formations.213

Once these geological sinks were largely saturated, oxygen began to accumulate in the oceans and eventually outgassed into the atmosphere. This transition allowed for the development of aerobic metabolism and the formation of an ozone layer in the stratosphere, which provides a critical shield against solar ultraviolet radiation. Over billions of years, atmospheric oxygen levels rose significantly, eventually reaching concentrations that supported the emergence of complex animal life.21