Theory of Origins of Life Challenged

Most theories explaining the origins of life on Earth revolve around oxygen. Complex life was thought to have evolved around 630 to 635 million years ago, the same time that atmospheric levels of oxygen began to rise. However, a new study of a common sea sponge from Kerteminde Fjord in Denmark challenges this view.

The study showed that certain animals can live and grow with very limited oxygen supplies. "In fact animals can live and grow when the atmosphere contains only 0.5 percent of the oxygen levels in today's atmosphere," according to a press release announcing the results.

"Our studies suggest that the origin of animals was not prevented by low oxygen levels," said Daniel Mills of the Nordic Center for Earth Evolution at the University of Southern Denmark. The research paper appeared in the Proceedings of the National Academy of the Sciences.

A billion years ago, life is thought to have consisted exclusively of single-celled life form. The jump to complex life, such as animals, happened a little over half a billion years ago and coincided with increased levels of oxygen. This led scientists to "conclude that the increased oxygen levels had led to the evolution of animals," the news release said.

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

Of living animals, sea sponges most closely resemble the first animals on Earth. For their study, the researchers used the species Halichondria panicea, a sea sponge living near the research center.

"When we placed the sponges in our lab, they continued to breathe and grow even when the oxygen levels reached 0.5 per cent of present day atmospheric levels," said Mills.

By surviving at such low oxygen levels, the sponges have scientists questioning currently held beliefs about the origins of animals. If not oxygen, what prevented the explosion in growth of complex life for half a billion years?

"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.