Rise of Animals Delayed by Oxygen Deficiency

The rise of the animals millions of years ago didn't happen sooner due to an oxygen deficiency, according to a new study, letting the air out of one of Earth's nagging mysteries.

Scientists have long wondered why animals didn't roam the planet sooner than they did - about 800 million years ago during the end of the Proterozoic period. There was supposedly an abundance of oxygen before then, but new research shows that the quality of that air wasn't so great.

In a study published in the journal Science, Yale researcher Noah Planavsky and his colleagues found that oxygen levels during the "boring billion" period, as they call it, were only 0.1 percent of what they are today. So while life-supporting oxygen may have covered Earth a billion years ago, it was not up to par with what animals need to survive.

"There is no question that genetic and ecological innovation must ultimately be behind the rise of animals, but it is equally unavoidable that animals need a certain level of oxygen," Planavsky, a study co-author, said in a statement. "We're providing the first evidence that oxygen levels were low enough during this period to potentially prevent the rise of animals."

Evidence of these findings came from an analysis of chromium isotopes found in ancient sediments from China, Australia, Canada, and the United States. Chromium, a component of Earth's continental crust, is directly linked to the presence of free oxygen in the atmosphere.

Previous estimates had made scientists believe that pre-animals, oxygen levels were at 40 percent of today's conditions, leaving room to support life. But the latest research indicates, after comparing data with other samples taken from younger sites known to have higher levels of oxygen, that this is a very generous estimate.

The researchers do note, however, that back then oxygen levels were "highly dynamic" in the atmosphere, with the potential of occasional spikes.

But, "if we are right, our results will really change how people view the origins of animals and other complex life, and their relationships to the co-evolving environment," said co-author Tim Lyons. "This could be a game changer."