Lack of Oxygen Delayed Animal Life on Earth During One Billion Years

Scientists may have found out a little more about when animal life first evolved on Earth--and what may have delayed it. They've discovered that a lack of oxygen may have prevented animal species from flourishing sooner on our primordial planet.

Animals began to prosper at the end of the Proterozoic period, which is about 800 million years ago. Yet during the billion stretch before this time period, animal life didn't exist-a fact that has long puzzled researchers.

Now, scientists have an explanation for this "boring" stretch of a billion years. It turns out that oxygen levels during this time period were only about .1 percent of what they are today. In other words, the atmosphere couldn't support the diversity of creatures that are present today.

"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," said Noah Planavsky, one of the researchers, in a news release. "We're providing the first evidence that oxygen levels were low enough during this period to potentially prevent the rise of animals."

The scientists analyzed chromium isotopes in ancient sediment from China, Australia, Canada and the United States. Chromium oxidation is directly linked to the presence of free oxygen in the atmosphere, which allowed the scientists to detect what oxygen concentrations were present at different periods in the Earth's history.

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

The findings reveal a bit more about the early history of our Earth. Not only that, but it explains how life first emerged and shows why it took so long for animal life in particular to appear.

The findings are published in the journal Science.