Summary: By Richard I. Gibson Listen to the podcast: We know there was some free oxygen in the atmosphere by 2.3 or 2.4 billion years ago, but it took until around 2 billion years ago, after 700,000,000 years of work by the cyanobacteria, for there to be enough oxygen in the atmosphere to think of it as relatively oxygen rich. For a while, a few hundred million years, the highly reactive oxygen given off by photosynthetic organisms probably combined with iron dissolved in the early oceans, so oxygen didn’t accumulate in the atmosphere. It produced thick iron oxide deposits like those in Minnesota, which we’ll talk more about tomorrow. Was the air breathable, if we went back in a time machine 2 billon years? No one knows for sure—there is no good way to reliably estimate the percentage of oxygen in the early atmosphere. But probably not. The concentration of oxygen may not have been great enough for another billion years or more for oxygen dependent animals to evolve, but it did happen eventually. Obviously! What was a boon for oxygen-based life was a crisis for the original anaerobic life that didn’t need oxygen. Today such life is limited to a few small niches such as the reducing environments in swamps and deep oceans and near volcanic vents. Free oxygen is poisonous to the bacteria that got things started for life on earth, but the crisis for them probably was the impetus that allowed for multicellular plants and animals to develop toward the end of the Proterozoic era. Atmosphere photo from NASA (public domain).
