Scientists have just found the oldest meteorites ever on our planet — 2.7 billion years old, to be exact. And it’s an astonishing find that could change scientists’ understanding of the early history of our Earth’s atmosphere.

In particular, the discovery shows that the Earth had a surprisingly oxygen-rich atmosphere at a time when scientists thought it to be mostly devoid of the life-giving gas, according to a Monash University statement.

Scientists found 60 meteorites fossilized in limestone in the Pilbara region of Western Australia. They were first discovered in 2014 and 2015, but scientists didn’t understand what they had at the time. They were just dust-sized grains, smaller than the width of a human hair.

In a paper published in the journal Nature, scientists detailed the chemical composition of the meteorities, claiming that it’s the first evidence of the makeup of the Earth’s upper atmosphere so long ago — and it completely contradicts scientists’ understanding that the Earth’s atmosphere lacked oxygen back then.

The researchers were actually looking for these meteorites, knowing that such tiny ones can be quite commonly found and could contain some of the secrets of the early Earth. They surmised that they could tell the chemical makeup of the atmosphere by then because it would have affected how the meteors melted and what molecules they formed when they cooled.

“Using cutting-edge microscopes we found that most of the micrometeorites had once been particles of metallic iron – common in meteorites – that had been turned into iron oxide minerals in the upper atmosphere, indicating higher concentrations of oxygen than expected,” team leader Dr. Andrew Tomkins said in the statement. “This was an exciting result because it is the first time anyone has found a way to sample the chemistry of the ancient Earth’s upper atmosphere.”