Oxygen currently makes up 21 percent of our atmosphere. But for the first half of Earth's existence, the air had almost no oxygen—if any humans could travel back to that ancient environment, they'd need spacesuits to survive.

Mysteriously, bacteria such as blue-green algae—which produce oxygen through photosynthesis—had existed for several hundred million years before oxygen finally managed to enrich the air during a period called the Great Oxidation Event.

(Also see "Nickel 'Famine' Led to Oxygen-Breathing Life?")

Now, a new study of 70,000 rock samples from around the world may have solved the mystery of oxygen's long delay.

The rocks show a dramatic change in Earth's magma composition at the end of the Archean, which lasted from 4 to 2.5 billion years