New research suggests animals didn't flourish until the end of the Proterozoic period (about 800 million years ago) because early oxygen levels were not as stable as we thought.
Researchers determined the oxygen levels during the "boring billion" were only 0.1 percent of what they are today, Yale University reported. This suggests the conditions on the early Earth were most likely inhospitable to most life forms.
"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 Yale researcher Noah Planavsky. "We're providing the first evidence that oxygen levels were low enough during this period to potentially prevent the rise of animals."
To mak their findings the researchers looked at chromium isotopes in ancient sediments from China, Australia, Canada and the U.S. Chromium oxidation signifies a presence of free oxygen in the atmosphere. The samples were taken from iron-rich ocean areas off the shore and were compared with those taken from other areas known to have higher oxygen levels.
"We were missing the right approach until now," Planavsky said. "Chromium gave us the proxy."
Pre-animal oxygen levels have been estimated at about 40 percent of today's condtions, suggesting the planet could have supported life. This new study finds that early oxygen levels were "highly dynamic" and prone to spikes.
"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 of the University of California-Riverside. "This could be a game changer."
"There's a lot of interest right now in a broader discussion surrounding the role that environmental stability played in the evolution of complex life, and we think our results are a significant contribution to that," concluded Christopher Reinhard of the Georgia Institute of Technology.
The findings were published Oct. 30 in the journal Science.
