Sponges may have played a critical role in the evolution of several species, including our own, according to a new study. 

These primitive animals first evolved 700 million years ago, in oxygen-poor oceans. Theories concerning the rise of oxygen claimed the atmosphere became filled with the gas before the waters. This new study turns that old idea on its head, providing evidence the waters of the Earth became oxygen-rich before the air. The concentration of any gas in water, including oxygen, is dependent on supply an demand. 

Living deep under the ocean, sponges use a filtering system to removes organic material from water. A portion of this material is rotting cell matter, which would normally consume oxygen as it decays. But, sponges remove this debris, freeing oxygen. 

As tiny oxygen-breathing organisms died, they could sink deep under the water surface, preventing oxygen depletion in shallower waters. All these effects together could have produced enough oxygen in oceans to allow for a tremendous growth in animal populations. Within a hundred million years, the dissolved gas began to fill the waters.

This allowed O2-loving organisms to thrive in the new environment. But, rather than the Earth experiencing one large oxygenation event as many paleontologists believed, this process may have taken place over the course of a chain of events. 

"The Neoproterozoic era (about 1,000 to 542 million years ago) was a time of turbulent environmental change. Large fluctuations in the carbon cycle were associated with at least two severe - possible Snowball Earth - glaciations. There were also massive changes in the redox state of the oceans, culminating in the oxygenation of much of the deep oceans. Amid this environmental change, increasingly complex life forms evolved," Researchers wrote in the article announcing the study. 

A recent separate study showed sponges could live in a low-oxygen atmosphere. This could explain how the first sponges survived early on, before causing levels to rise. 

Tim Lenton of the University of Exeter was the lead author of the study, which was published in the journal Nature Geoscience. 

"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," Lenton said

Once waters became filled with oxygen, larger animals arose, including the first predators. This started the marine eco-system as we know it today. These animals also eventually left the oceans, leading to all land animals, including humans.

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