717px-Fictional_Snowball_Earth_1_NeethisIt took 100 million years for oxygen levels in the oceans and atmosphere to increase to the level that allowed the explosion of animal life on Earth about 600 million years ago, according to a study co-authored by two University of Washington scientists and led by the University College London.

Before now, it was not known how quickly Earth’s oceans and atmosphere became oxygenated, and whether animal life expanded before or after the rise in oxygen. The new study, published Dec. 18 in Nature Communications, shows the increase in oxygen began significantly earlier than previously thought, and occurred in fits and starts spread over about 100 million years.

It’s therefore likely that increasing oxygen kick-started early animal evolution, rather than the appearance of animals being a random evolutionary event.

“Oxygen was like a slow fuse to the explosion of animal life,” said co-author David Catling, a professor in the UW’s Department of Earth and Space Sciences. “Around 635 million years ago, enough oxygen probably existed to support tiny sponges. Then, after 580 million years ago, strange creatures as thin as crêpes lived on a lightly oxygenated seafloor. Fifty million years later, [our] vertebrate ancestors were gliding through oxygen-rich seawater.”

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