A trilobite, one of the earliest anthropods. (AP Photo/Mary Altaffer)

Why did complex life evolve precisely when it did? According to a new study, simpler life forms may have been waiting on a proper oxygen supply -- for as long as a billion years.

The first metazoans (or organisms in the animal kingdom) only emerged around 500 million years ago. For a billion years before hand, life flourished on Earth without making this jump in complexity. The cause of the long delay has been a source of scientific debate.

In a study published Thursday in Science, researchers present new data on the oxygen levels of Earth during that time. Based on their findings, they believe that the planet may have had too little oxygen for complex life to evolve.

"Of course we already knew that all animals needed high oxygen levels in order to evolve," said lead author Noah Planavsky, 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."

And as soon as the oxygen levels went up, the data suggests, animal diversification began.

If Planavsky's estimation of these oxygen levels are correct, it could mean that simple organisms were ready to evolve into complexity for ages before they actually got the chance. But while his methods -- analyzing the chemical components of rocks to trace when they may have been exposed to certain levels of oxygen in their development -- are advanced, he realizes he'll need more data to show that oxygen was the magic ingredient to jump-start Earth's animal boom.

"It's exciting, but only when we develop it further and have multiple lines pointing to the same story will it become something that's textbook knowledge," Planavsky said.