Not because of a change in animal behaviour?
From ScienceDaily:
Before now it was not known how quickly Earth’s oceans and atmosphere became oxygenated and if animal life expanded before or after oxygen levels rose. The new study, published in Nature Communications, shows the increase began significantly earlier than previously thought and occurred in fits and starts spread over a vast period. It is therefore likely that early animal evolution was kick-started by increased amounts of oxygen, rather than a change in animal behaviour leading to oxygenation.
Co-author Prof. David Catling (University of Washington Earth and Space Sciences), added: “Oxygen was like a slow fuse to the explosion of animal life. Around 635 Ma, enough oxygen probably existed to support tiny sponges. Then, after 580 Ma, strange creatures shaped like pizzas lived on a lightly oxygenated seafloor. Fifty million years later, vertebrate ancestors were gliding through oxygen-rich seawater. Tracking how oxygen increased is the first step towards understanding why it took so long. Ultimately, a grasp of geologic controls on oxygen levels can help us understand whether animal-like life might exist or not on Earth-like planets elsewhere.”
More.
See also: Ocean toxicity slowed rapid evolution of complex life? Didn’t slow it much according to these folks above
and
What we know and don’t, know about the origin of life
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