New research delved into early Earth to solve a long-standing scientific mystery: why did it take a billion years after the first signs of simple life formed for complex animal life to spring up? Animal life started evolving on Earth roughly 600 to 700 million years ago, but scientists thought that there was enough oxygen for animals to have evolved approximately a billion years before that. The answer, it turns out, is that there was actually not enough oxygen back then.

In a new research paper published October 30 in the journal Science, researchers showed that there is evidence that oxygen levels dating back about a billion years before the rise of animals were only 0.1 percent what oxygen levels are today.

This means that, unlike what was previously thought, the Earth could not have supported animal life earlier than 600 to 700 million years ago.

Simple life began to grow on Earth almost as soon as the Earth was cool enough to support such life. However, scientists were puzzled by why it took so much longer for complex life to form. This evidence gives a compelling reason to explain the time gap.

"We're providing the first evidence that oxygen levels were low enough during this period to potentially prevent the rise of animals," said Noah J. Planavsky, co-lead author of the research paper. Planavsky worked with researchers Timothy W. Lyons and Christopher Reinhard.

Microbes eventually created the higher oxygen levels needed for complex life to grow.

The team studied chromium isotopes from ancient sediment deposits in China, Australia, Canada and the US to find out how much chromium oxidization occurred on early Earth. This, it turned out, was the key to finding out how much oxygen there was in the atmosphere 800 million years ago.

"We were missing the right approach until now. Chromium gave us the proxy," Planavsky said.

Previous estimates of oxygen levels on early Earth were estimated much less exactly. The researchers said that since early oxygen levels on Earth fluctuated dramatically, this finding has some room for doubt, but they said that if these findings prove to be correct, it could be a "game changer" for the way we think about early life.