Almost all living things need oxygen to survive. The Earth’s atmosphere is currently around 21 percent oxygen, but it didn’t always provide that big of a breath of fresh air to its inhabitants. Millions and billions of years ago, oxygen levels were much lower, but exactly how low was unclear.

Now, a remarkable discovery has been made by a team at Brock University in Canada by studying 815-million-year-old air bubbles, preserved in rock salt. They’ve found that oxygen levels were five times what was originally thought at that time. Their results show that oxygen existed in abundance well before the appearance of complex animals, and it calls into question the evolution of these life forms, long believed to be linked to oxygen increase.

To get their air samples, the group went to southwestern Australia. Some 815 million years ago, a body of water evaporated there, in what is now called the Officer Basin. A mineral called halite, or rock salt, began to form on its surface. As it grew, it captured the air around it in small air pockets, creating tiny time capsules of the Earth’s ancient atmosphere.

The research team, led by geochemist Nigel Blamey, smashed these rocks in a vacuum chamber to release the air and measured 31 air bubbles with a quadrupole mass spectrometer. It’s the first time that air this old has been directly studied.

“We came out of left field, and I think some people are going to embrace it, and other people are going to be very skeptical,” Blamey said. “But the data is what the data is.”

Their results, published in Geology, suggest that 815 million years ago, the Earth’s atmosphere was 10.9 percent oxygen (half our current level of 21 percent). Oxygen at this amount wasn’t estimated to be present until up to 200 million years later. They compared the ancient air bubbles with modern samples of halite, to confirm their findings.

So what’s the big deal? Knowing the Earth’s atmospheric makeup throughout time can help scientists understand a lot of things: climate change, tectonic events, mass extinctions but also the evolution of life on Earth.

Around 2.3 billion years ago, cyanobacteria converted sunlight into energy, and made a waste product that would become critical for our survival: oxygen. The primitive microbes that came before survived without it, but once the cyanobacteria began to photosynthesize, oxygen became a constant part of the Earth’s atmosphere, and life forms that thrived on oxygen began to appear.

Before having an air sample to measure, scientists had to estimate ancient levels of oxygen through indirect means, often by looking for traces of chemical reactions that required oxygen. Those methods generally indicated that there was a major oxygen increase around 500-600 million years ago, right alongside the appearance of many new, complex life forms around 542 million years ago.

The oxygen increase and that explosion of new life were long thought to be related. What was unclear, was exactly how the atmosphere and the animals co-evolved, but biologists have argued for decades that oxygen was the spark that led to the myriad new creatures. It made sense that along with a burst of oxygen, there would come an influx of oxygen-dependent animals.

But now, the new study puts millions of years between the two events, and that timeline is getting more complicated.

“Our finding answers one big question, did life come first or atmospheric oxygen; it is the latter,” Uwe Brand, a co-author on the paper, said in an article released by Brock University. “Now the next thing is for paleobiologists to look for the organism that gave us this kick in oxygen.”

A paleontologist at the University of Cambridge, Nicholas Butterfield, told Science News that the findings disrupt a long-held belief that complex animals didn’t evolve sooner because there wasn’t enough oxygen available. Now he’s not certain why it took it took so long, but it wasn’t the lack of oxygen.

“The delayed evolutionary appearance of animals had nothing to do with limiting levels of atmospheric oxygen,” he said.

Armed with this new knowledge of oxygen levels, scientists can start to better understand how life evolved on Earth, and even how it might on other planets. Blamey said in an interview with Brock University that their findings could change the way scientists are looking for extraterrestrial beings.

Instead of assuming that finding oxygen is associated with complex life, that might not be the case. They have showed that high oxygen levels can persist for a long time without it.

“Scientists have long debated which came first: higher life or O2,” Brand told Gizmodo. “With our finding, we can put that debate to rest.”