A tiny sea sponge may have helped researchers to a significant realization.

A new study published in Proceedings of the National Academy of Sciences reveals that the popular notion that the Earth's current oxygen levels are needed to sustain life may not be true. The findings show that the most primitive animals survived in water that contained almost no oxygen.

Researchers came to their conclusion after studying a common sea sponge from Kerteminde Fjord in Denmark. The findings showed the sponge could live and grow with as little as 0.5 percent of the oxygen found in the atmosphere today, suggesting that complex life forms like animals, plants and fungi did not depend on oxygen to develop.

"Our studies suggest that the origin of animals was not prevented by low oxygen levels,” Daniel Mills of the Nordic Center for Earth Evolution at the University of Southern Denmark said in a statement.

The first microbes evolved 3.6 billion years ago, but complex life only came around 3 billion years later. Many scientists have said the rise of atmospheric levels of oxygen levels about 630 to 635 million years ago made that possible.

"But nobody has ever tested how much oxygen animals need – at least not to my knowledge.  Therefore we decided to find out,” Mills said.

But the sea sponge, which is one of the world’s most primitive animals, is physical proof that this isn’t the case. Sponges are multicellular organisms that live on the ocean floor, where they attach themselves permanently to a location. Mills and his colleague, CarriAyne Jones, a postdoctoral researcher at the University of British Columbia, grew sea sponges in tanks with low oxygen and observed their behavior.

"I expected them to maybe not feed as much or respire as well," Jones told the Canadian Broadcasting Corporation. "They did just fine. They grew. They respired. They fed."

When oxygen levels dropped to zero, some died. The researchers found as long as the levels remained above 0.5 percent, the sponges could survive. Now, researchers are confronted with finding out what prevented early life forms from evolving.

"There must have been other ecological and evolutionary mechanisms at play. Maybe life remained microbial for so long because it took a while to develop the biological machinery required to construct an animal. Perhaps the ancient Earth lacked animals because complex, many-celled bodies are simply hard to evolve,” Mills said.