Earth had enough oxygen to fuel the respiration needs of animals 1.4 billion years ago. That’s millions of years before the first animals even evolved. The findings, published in Proceedings of the National Academy of Sciences this week, suggest that the appearance of the first animals – which occurred around 600 million years ago – didn’t immediately follow the increase of oxygen on the planet as expected. 

Before 2.3 billion years ago, oxygen in the atmosphere was less than 0.001 percent of present atmospheric levels. Then, at about 550 million years ago, oxygen levels became greater than 20 percent of today’s levels, and that’s sufficient to sustain the respiration of large animals. But what happened in between is a bit of a mystery. The oxygen levels required for the respiration of early animals (which likely resembled sponges in the ocean) would have been less than or equal to 1 percent of present-day levels.

Some researchers think that the amount of oxygen on Earth first increased to sufficient levels for animal respiration during the Neoproterozoic Eon some 1 billion to 542 million years ago. And that could explain why animals first evolved during this time. 

To investigate, a team led by Shuichang Zhang of China National Petroleum Corporation and the University of Southern Denmark’s Donald Canfield conducted geochemical analyses of trace metals in 1.4 billion-year-old sediments from northern China’s Xiamaling Formation, which was deposited in an oxygen minimum zone. They then used an ocean model to reproduce oxygen cycling for these ancient bottom waters. 

The team revealed that the atmospheric oxygen level 1.4 billion years ago was at least 3.8 percent of present-day levels. That means oxygen levels likely didn’t inhibit the emergence of animals. So if that’s the case, why did animals show up so late in the planet’s history? “The sudden diversification of animals probably was a result of many factors,” study coauthor Emma Hammarlund from the University of Southern Denmark says in a statement. “Maybe the oxygen rise had less to do with the animal revolution than we previously assumed.”