The 'Great Oxidation Event' (GOE), nearly 2.3 billion years ago, marks the point when oxygen started to make an impact on the atmosphere, stimulating the evolution of air-breathing organisms.
But new research from MIT suggests that oxygen may actually have been around hundreds of millions of years earlier, in 'oxygen oases' in the oceans. And the team says there's good evidence that tiny aerobic organisms may have evolved to survive on extremely low levels of the gas in these spots.
Yeast — an organism that can survive with or without oxygen — is able to produce key oxygen-dependent compounds, even with only miniscule puffs of the gas, they've discovered.
And this suggests that early ancestors of yeast could have been similarly frugal, working with whatever small amounts of O2 may have been available in the oceans.
"The time at which oxygen became an integral factor in cellular metabolism was a pivotal point in Earth history," says Professor of Geobiology Roger Summons.
"The fact that you could have oxygen-dependent biosynthesis very early on in the Earth's history has significant implications."
The group's results may help explain how sedimentary rocks from 300 million years before the GOE can come to to contain fossil steroids, an essential component of some organisms' cell membranes that requires at least 10 molecules of O2.
The MIT team suggests that perhaps O2 was in fact present, and enough to feed aerobic, sterol-producing organisms — just at extremely low concentrations that wouldn't have left much of a trace in the rock record.
To find the lowest level of O2 yeast can consume, the team set up an experiment to identify the point at which yeast switches from anaerobic to aerobic activity.
And they found that yeast are able to make steroids using vanishingly small, nanomolar concentrations of O2, supporting the theory that oxygen may have indeed been around long before it the GOE.
Instead of building up in the oceans and then seeping into the atmosphere, it could have been rapidly consumed by early aerobic organisms.
"We know all kinds of biology happens without any O2 at all," says former graduate student JacobWaldbauer. "But it's quite possible there was a vigorous cycle of O2 happening in some places, and other places it might have been completely absent."