Where did Earth’s oxygen come from?

Three billion years ago on Earth, something was happening.

Microbes that had evolved the ability to carry out photosynthesis floated on the surface of the ocean, using energy from sunlight to grow on carbon dioxide and water. When the microbes died, oxygen reacted with their carbon. But a tiny bit of the organic matter from the microbes sank to the sea floor, where oxygen couldn't react with it. And so some oxygen remained.

These were among the earliest days of oxygen on Earth. The atmosphere then contained only 0.03 percent as much oxygen as today's atmosphere, but that was enough to set up a new dynamic.

I've cribbed this from Carl Zimmer's very nice column last week in The New York Times, "The Mystery of Earth's Oxygen." See Zimmer for more on oxygen's four-billion-year history.

-Paul Raeburn