While genetic and ecological innovation are attributed with the rise of animals, there is also evidence that animal life would not have flourished if a sufficient level of oxygen had not been present.

Animals first appeared around 600 to 700 million years ago. Scientists learned that for those billions of years before this period, the oxygen levels measured at only 0.1 percent of today's level.

"In other words, Earth's atmosphere couldn't have supported a diversity of creatures, no matter what genetic advancement were in place," said Timothy W. Lyons, co-author of the study and a distinguished professor of biogeochemistry in the University of California's department of earth sciences.

Scientists have long been intrigued by the role of oxygen in the first appearance of animals.

"Of course we already knew that all animals needed high oxygen levels in order to evolve," said Noah Planavsky, lead author of the study and assistant professor of geology at Yale. "The really significant thing we found is that during this waiting period, oxygen levels were really low -- lower than traditionally thought, and low enough that animals couldn't have evolved and diversified."

The study further suggests that animal diversification began as soon as the levels of oxygen went up.

The team also analyzed the chromium levels of rock sediments from China, Canada, Australia and the U.S. This method allowed them to gain more understanding of the amount of oxygen that could have been present.

Scientists learned that around 800 million years ago, there was a change in the rocks' chemical components. During that same period, oxygen levels started to increase rapidly.

"The authors have been very careful about which samples they have chosen to analyze -- focusing on the same type of samples from similar settings across Earth history," says Simon Poulton, a geochemist at the University of Leeds. "If correct, the very low levels of oxygen found in this study would have provided a major restriction on early animal evolution, thus suggesting that it was indeed a rise in oxygen that ultimately stimulated the evolution of our earliest animal ancestors."

The team also went on to study samples deposited in shallow ocean areas near an ancient shoreline believed to be rich in iron. They compared the data from ther samples that were taken from younger places in similar settings but are known to have higher oxygen levels.

While this method of analyzing chemical components of rocks, in order to trace the period of time when they were exposed to certain oxygen levels, is advanced, Planavsky admits that more data is still needed to show that oxygen is key to the Earth's animal boom.

The analysis of oxygen levels as a stimulus to animal life and evolution is published in the journal Science.