Though life remains a mystery, a small sea sponge found in a Danish fjord has overturned one of science’s most basic presumptions about our genesis — the need for modern levels of oxygen in the earth’s atmosphere.

Rather, some forms of animal life on the planet may develop and grow in environments with only 0.5 percent of the oxygen found in today’s atmosphere. For many years, scientists had thought that newly gushing supplies of oxygen on a changing planet had spurred the development of complex life some 630-635 million years ago. Rather, the development of life beyond single-cell organisms might have occurred even without the terrestrial metamorphosis.

"Our studies suggest that the origin of animals was not prevented by low oxygen levels,” Daniel Mills, of University of Southern Denmark, said in a statement. "But nobody has ever tested how much oxygen animals need — at least not to my knowledge. Therefore we decided to find out.”

Sea Sponge Upends Theory On Beginning Of Life A small common sea sponge may have changed minds about the beginning of life with regard to requirements for oxygen.

Mills and a colleague from the California Institute of Technology chose to study the common sea sponge for its similarity to simpler forms of life during that time. “When we placed the sponges in our lab, they continued to breathe and grow even when the oxygen levels reached 0.5 percent of present day atmospheric levels,” Mills said.

However, the study leaves a big question now for other scientists. Assuming the development of complex life had nothing to do with rising levels of oxygen in the Earth’s past, scientists are left with a big question. Why did life on earth for billions of years consist only of single-celled bacteria and amoebae, before a sudden start?

"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.