Earth's Oceans Oxygenated By First Sea Animals
March 11, 2014

Earth’s Oceans Oxygenated By First Sea Animals

Lee Rannals for - Your Universe Online

Researchers at the University of Exeter say that the evolution of the first animals helped oxygenate the oceans.

A long held belief says that oxygenation of the atmosphere and oceans were a pre-requisite for the evolution of complex life forms. However, the latest study published in the journal Nature Geoscience contradicts this belief.

The latest study builds on previous research by scientists in Denmark who found that sponges require only small amounts of oxygen.

"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,” Professor Tim Lenton of the University of Exeter, who led the new study, said in a statement. “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 team says the mechanisms by which the ocean could have been oxygenated 1 billion to 542 million years ago did not require an increase in atmospheric oxygen.

The balance of oxygen supply and demand plays a big role in determining the oxygen levels in the deep ocean. Demand for oxygen is created by the dead organic material that sinks down into the deep ocean. The latest study says the first animals reduced this supply of organic matter directly and indirectly.

Sponges feed by pumping water through their bodies, which filters out tiny particles of organic matter from the water and helps oxygenate the shelf seas they live in. This means that larger phytoplankton are naturally selected because they sink faster, which reduces oxygen demand in the water.

The scientists said that by oxygenating more of the bottom waters of shelf seas, the first filter-feeding animals increased the removal of the essential nutrient phosphorus in the ocean. This reduced the productivity of the whole ocean ecosystem and suppressed oxygen demand, which helped oxygenate the deep ocean.

The oxygenated ocean helped create ideal conditions for more mobile animals to evolve, because they have a higher requirement for oxygen.

"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. They created a world in which more complex animals could evolve, including our very distant ancestors,” Lenton said.

Professor Simon Poulton of the University of Leeds, who is a co-author of the study, said the research provides a plausible mechanism for ocean oxygenation without requiring a rise in atmospheric oxygen.

“It therefore questions whether the long-standing belief that there was a major rise in atmospheric oxygen at this time is correct,” Poulton said. “We simply don't know the answer to this at present, which is ultimately key to understanding how our planet evolved to its current habitable state. Geochemists need to come up with new ways to decipher oxygen levels on the early Earth.″