For the first 2 billion or so years of Earth's history, the atmosphere would have been suffocating. Then, mysteriously, the atmosphere began to brim with oxygen. Now a team of atmospheric scientists claims to know why: The evolution of bacteria shifted the balance of gases in Earth's atmosphere, resulting in a loss of methane and an ultimate elevation of oxygen levels.
Oxygen levels in the atmosphere did not rise until at least 400 million years after the origin of oxygen-producing bacteria. Theories about the origin of Earth's oxygen have to account for this gap. Until recently, the leading theory held that volcanic gases changed their composition during this time, ultimately consuming less oxygen in atmospheric reactions. But recent evidence from the rock record has put a damper on that idea.
Now comes a different scenario. The key player is bacteria that emit methane as they decompose dead organisms. Planetary scientist David Catling and his colleagues at the NASA Ames Research Center in Mountain View, California, propose that the methane rose to the upper atmosphere, where sunlight broke it down into carbon and hydrogen. The hydrogen, the lighter of these elements, then floated off into space. The loss of hydrogen, in turn, upset the balance of elements back on Earth and 400 million years later produced an oxygen glut.
Why the delay? Catling and co-workers argue that at first the continental crust absorbed the extra oxygen. Then Earth reached a critical point 2.3 billion years ago when the fumes released when crustal rocks are squeezed and deformed could no longer absorb the oxygen produced by bacteria. Only then, the team reports in the 3 August issue of Science, did the extra oxygen begin to accumulate, leading to the modern atmosphere.
The researchers present a "good case" for the new theory, says atmospheric scientist James Kasting of Pennsylvania State University, University Park. But he says the argument would be more persuasive if it explained why the crust would have absorbed the excess oxygen produced by hydrogen escape.