The First Animals May Have Oxygenated The Oceans As They Evolved

By Josh Lieberman on March 9, 2014 2:08 PM EDT

fish oxygen
The first animals oxygenated the ocean as they evolved, suggests a new study. The conventional thinking is that increases in oxygen led to complex animals--not the other way around, as the new study suggests. (Photo: Pixabay: tpsdave)

The first animals played a key role in bringing oxygen to oceans half a billion years ago, new research suggests. The traditional scientific view is that a significant rise in the ocean's oxygen levels led to the first complex animals. But this new study, led by scientists as the University of Exeter, in England, suggests that the process happened in reverse: the evolution of the first animals brought oxygen to the deep ocean, leading to the development of more complex life forms.

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"There had been enough oxygen in ocean surface waters for over 1.5 billion years before the first animals evolved, but the dark depths of the ocean remained devoid of oxygen," said study author Tim Lenton of the University of Exeter. "We argue that the evolution of the first animals could have played a key role in the widespread oxygenation of the deep oceans. This in turn may have facilitated the evolution of more complex, mobile animals." 

The study builds on research from Denmark from February 2012 which showed that sponges--the earliest animals to evolve--can survive with almost no oxygen. The study found that sponges could live in as little as 0.5 percent of the oxygen in today's atmosphere, and suggested that complex life could evolve before significant ocean oxygen levels were present.

The researchers considered ways which the deep ocean could have been oxygenated 1,000 to 542 million years ago, during the Neoproterozoic Era, without requiring an increase in atmospheric oxygen. They argue that sponges, which feed by filtering water through their bodies, reduced the amount of dead organic material that fell down into the deep ocean. In separating dead matter from water, sponges reduced oceans' phosphorous levels, suppressing the demand for oxygen and thus oxygenating the ocean. The more ocean oxygen there is, the more opportunity there is for complex life forms to evolve.   

"The effects we predict suggest that the first animals, far from being a passive response to rising atmospheric oxygen, were the active agents that oxygenated the ocean around 600 million years ago," said Lenton. "They created a world in which more complex animals could evolve, including our very distant ancestors."

Study co-author Simon Poulton said that the research only suggests such a theory of ocean oxygenation. In order to further test the theory, geochemists would have to come with better ways to determine early Earth's oxygen levels, he said.

"This study provides a plausible mechanism for ocean oxygenation without the requirement for a rise in atmospheric oxygen," said Poulton. "It therefore questions whether the long-standing belief that there was a major rise in atmospheric oxygen at this time is correct."

The study, "Co-evolution of Eukaryotes and Ocean Oxygenation in the Neoproterozoic," was published in Nature Geoscience.

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