Geologists tell a pretty broad-brush narrative of Earth’s 4.5 billion-year history. For its first half-billion years, the newly formed planet was a seething ball of lava constantly pelted by giant space rocks, including a Mars-sized object that sheared off a chunk that became the moon. Things calmed down when the Late Heavy Bombardment tapered off some 3.8 billion years ago, but volcanoes ensured Earth’s atmosphere remained a toxic stew of gases with almost no oxygen to speak of. It stayed that way for another billion years, when single-celled bacteria filled the oceans. Around 2.5 billion years ago, at the end of the Archean era, algae figured out how to make energy from sunlight, and the Great Oxygenation Event gave Earth its lungs. Complex life took its time, finally exploding in the Cambrian era some 500 million years ago. Evolution moved a lot faster after that, resulting in dinosaurs, then mammals, then us.
It’s a great story, and scientists have been telling it for decades, but tiny fossilized space pebbles from Australia may upend it entirely, giving us a new narrative about Earth’s adolescence. These pebbles rained down on our planet’s surface 2.7 billion years ago. As they passed through the upper atmosphere, they melted and rusted, making new crystal shapes and minerals that only form where there is plenty of oxygen. A new paper describing the space pebbles will be published today in the journal Nature. It suggests the atmosphere’s upper reaches were surprisingly rich in oxygen during the Archean, when Earth’s surface had practically none.