A lack of oxygen may explain why it took so long for animal life to develop on Earth, according to a new study.

For years, scientists have wondered why it wasn’t until 800 million years ago that animal species started to flourish on the planet at the end of the Proterozoic period. Yale University research Noah Planavsky and his colleagues discovered that oxygen levels stood at just 0.1 percent of today’s levels then, which means the Earth’s atmosphere would have been inhospitable to most creatures, according to the Business Standard.

Although innovation in genetics and ecology is the key driving force behind animal evolution, low oxygen levels can stunt that growth, said Planavsky.

In order to make this claim, the scientists looked for evidence by analyzing chromium isotopes from sediments in the U.S., Canada, Australia, and China. Earth’s continental crust contains chromium, and because chromium oxidation is directly linked to the availability of oxygen, the research team was able to study samples from ancient sediment and compare them to younger sediment with higher levels of oxygen.

Planavsky said it was chromium that allowed scientists to finally crack the code. Previously, scientists estimated that oxygen stood at 40 percent of today’s levels, which would have been more than enough to support animal life. However, the new findings cast that figure into doubt, suggesting oxygen was practically nonexistent in pre-animal times.

The journal Science published the study.