The air, which has been preserved, undisturbed, in tiny pockets in the crystals for about 815 million years, appears to contain 10.9 per cent oxygen — just half the amount in the atmosphere today.

But it's about five times more than scientists expected for that time period, which is about 200 million years before the first known multicellular fossils.

"I'm surprised and excited," said Nigel Blamey, a professor of earth sciences at Brock University in St. Catharines, Ont., who co-led the study with fellow Brock geochemist Uwe Brand.

Brand says the discovery answers a key question about the evolution of complex life — did animals arise before or after the oxygen needed to support larger, more complex organisms?

"And now with our research and our result, we know there was sufficient oxygen before they arose," he said, adding that the higher oxygen levels would have allowed animals to diversify and become more complex.

That means it may be possible to find multicellular fossils older than the oldest that are currently known.

"Now paleobiologists will have reason to go looking for rocks with original traces of these first evolutionary steps," Brand said.

The team's results are published in the journal Geology.