A sufficient amount of oxygen accumulated in the atmosphere to start forming a protective ozone layer.
The rise in oxygen levels in the seas and atmosphere that enabled the explosion of animal life on Earth around 635 million years ago required 100 million years. According to co-author and professor at the University of Washington's Department of Earth and Space Sciences David Catling, oxygen was like a delayed fuse to the explosion of animal life.
There was definitely enough oxygen on Earth 635 million years ago to support small sponges. Then, after 580 million years ago, strange, crêpe-like creatures inhabited a seafloor with a low oxygen content. Ancestors of vertebrates were gliding through oxygen-rich seawater 50 million years later.
Here is another question with an answer similar to this about multicellular life: https://brainly.com/question/23394502
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