Forces of Change
Mary Parrish Painting
Illustration by Mary Parrish © Smithsonian Institution

Oxygen at Last (2.6 billion to 400 million years ago)

Life and Earth’s atmosphere evolved together. Over time, tiny photosynthetic organisms produced enough oxygen to react with the methane in the atmosphere, transforming it forever. About two billion years ago, the methane haze cleared and the sky turned blue.

After photosynthetic bacteria had been making oxygen for about 300 million years, enough oxygen built up in the atmosphere to oxidize (combine with) the iron in rocks and soil to make reddish iron oxide and create vast expanses of redbeds. Scientists study redbeds to learn more about Earth’s early atmosphere.

This sample of a redbed from the Bar River Formation of the Huronian Supergroup in southern Ontario is 2.2 billion years old.
Specimen on loan from Alan Jay Kaufman, University of Maryland