By Richard I. Gibson
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.
Update, September 2014. In the episode above we talked about the origin of atmospheric oxygen, a development tied to the expansion of oxygen-excreting, photosynthesizing cyanobacteria. The point at which oxygen began to become plentiful in the atmosphere has been pegged at about 2.96 billion years ago, with levels becoming noticeably high by about 2.4 billion years ago or later. A new study by researchers at Trinity College Dublin, Ireland, and the Presidency University in Kolkata, India, pushes the onset of oxygenation back about 60 million years, to about 3.02 billion years ago. Here’s a link to a report on that research.
Atmosphere photo from NASA (public domain).