The evolution of large single-celled organisms and simple, sponge-like animals between 750 and 500 million years ago could have led to the oxygenation of the deep oceans, argues a review article published online in Nature Geoscience. This conclusion is at odds with the commonly held suggestion that the oxygenation of the oceans spurred animal evolution.
Until about 550 million years ago, much of the Earth’s deep oceans lacked any appreciable amounts of oxygen, despite the presence of moderate amounts of oxygen at the ocean surface. Timothy Lenton and colleagues suggest that this state arose because of high oxygen usage in the surface waters, primarily from the decomposition of tiny bacteria that rarely sank below the surface. However, once larger cells evolved - particularly amoebae with an organic shell - the dead cells sank below the surface, lowering the surface oxygen demand. The authors suggest that oxygen from the surface could then diffuse and mix downwards. The later evolution of filter-feeding sponges, some tens to hundreds of millions of years later, would have removed even more organic matter from the surface oceans, allowing further oxygen diffusion.