New research suggests evolving animals helped oxygenate the Earth, as opposed the popular view that developing animals appeared after the Earth was already oxygenated.
"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. 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," Professor Tim Lenton who led the study, said in a University of Exeter news release.
Oxygen levels in the deep ocean can be determined by supply and demand; a demand for oxygen is created by sinking organic dead matter. The study suggests the first creatures reduced the supply of this organic matter. These creatures may have been the first sea sponges.
Sponges eat by pumping water through their bodies and filtering out the tiny digestible pieces of organic matter, this process also oxygenates the water. This encourages larger phytoplankton, which increase aquatic oxygen demand.
When the sponges oxygenated the bottom of the ocean nutrient phosphorus was depleted; this reduced the productivity of the entire ocean ecosystem causing less energy demand. This phenomenon allowed the bottom of the ocean to be oxygenated.
The newly-oxygenated waters would have allowed more advanced life to form.
"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.
″This study provides a plausible mechanism for ocean oxygenation without the requirement for 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. 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," Study co-author Professor Simon Poulton of the University of Leeds said in the news release.
