"Geologists are letting the air out of a nagging mystery about the development of animal life on Earth.

Scientists have long speculated as to why animal species didn’t flourish sooner, once sufficient oxygen covered the Earth’s surface. Animals first appeared and began to prosper at the end of the Proterozoic period, about 600 to 700 million years ago — but what about the billion-year stretch before that, when most researchers think there was also plenty of oxygen?"

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