Traditional theory has it that the origin of complex life on earth was catalysed by oxygenation of the atmospheric and ocean. Scientists now counter this argument to suggest that in fact they were the first life forms, such as sponges, which oxygenated the deep oceans and aided in the evolution of complex life forms.

The ocean contained sufficient levels of oxygen to support life for over 1.5 billion years before the first animals evolved. Yet, the oxygen so found was restricted to the upper few metres of the ocean. The ocean depths, including the bottom of the water body, continued to remain anoxic till half a billion years ago.

Central to determining the oxygen levels in deep oceans is the delicate balance between oxygen supply and demand. The demand shoots up when organic matter in the form of dead organic material settles to the bottom. Sponges that were found in the continental shelf of the ocean played a crucial role in reducing the oxygen demand by the dead organic matter.

This was achieved by the sponges first “ventilating” the continental shelf by clearing water of organic material such as phytoplankton and dissolved organic carbon. This helped in reducing oxygen consumption and creating conditions suited for the evolution of more complex, multi-cellular life with higher oxygen needs, the researchers say in a paper published in the latest edition of Nature Geoscience.

By increasing the oxygen level in the shelf, the sponges inadvertently helped in also “removing” phosphorus found in the ocean. Phosphorus is a nutrient and its removal affected the ecosystem and, in turn, reduced the oxygen demand. This led to a rise in oxygen supply in the deep ocean for the animals to evolve. Recently, for instance, researchers at Denmark’s Nordic Center for Earth Evolution found that sponges in Danish Fjords needed extremely little oxygen (0.5 per cent of today’s atmospheric oxygen levels) to survive.

“The first animals, were the active agents that oxygenated the ocean around 600 million years ago. They created a world in which more complex animals could evolve, ” lead author Prof. Tim Lenton of the University of Exeter was quoted as saying in Nature Geoscience.

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