The evolution of the first animals may have oxygenated Earth's oceans. New research led by the University of Exeter challenges the long held belief that oxygenation of the atmosphere and oceans was a precondition for the evolution of complex life forms. The study, published in the journal Nature Geoscience, builds on the recent work of scientists in Denmark who found that sponges the first animals to evolverequire only small amounts of oxygen. Professor Tim Lenton of the University of Exeter, who led the new study, said: "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 the evolution of more complex, mobile animals." Critical to determining oxygen levels in the deep ocean is the balance of oxygen supply and demand. Demand for oxygen is created by the sinking of dead organic material into the deep ocean. The new study argues that the first animals reduced this supply of organic matterboth directly and indirectly. Sponges feed by pumping water through their bodies, "digesting" the tiny particles of organic matter, and thus helping oxygenate the shelf seas that they live in. By oxygenating more of the bottom waters, the first animals actually increased the removal of the essential nutrient phosphorus in the ocean. This in turn reduced the productivity of the whole ocean ecosystem, lowering oxygen demand and thus oxygenating the deep ocean. A more oxygen-rich ocean created ideal conditions for more complex mobile animals to evolve, because they have a higher requirement for oxygen. These included the first predatory animals with guts that started to eat one another, marking the beginning of the type of food webs we are familiar with today. Professor Simon Poulton of the University of Leeds, who is a co-author of the study, added: "This study provides a possible 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 explain oxygen levels on the early Earth." The main purpose of the passage is to   _   .
present the view that the first animals played an active role in oxygenating oceans introduce two opposite opinions on how the first animals oxygenated oceans analyze how atmospheric oxygen stepped up the evolution of oceanic animals explain how oceanic oxygen determines the amount of atmospheric oxygen