New research could explain why it took around a billion years for animal species to flourish on Earth after oxygen levels in the atmosphere began to increase.

Animal life on Earth boomed around 800 million years ago at the end of the Proterozoic period, but scientists have long believed that there was sufficient oxygen in the atmosphere for this increase in animal diversity to occur much earlier. However, new findings published in the journal Science show that oxygen levels were only 0.1% of those we see today. The researchers theorize that oxygen levels were too low to support diverse species, and this delayed the rise of animals.

The study was supported in part by the Exobiology & Evolutionary Biology element of NASA’s Astrobiology Program.