Lack of oxygen delayed the rise of animals on Earth

Scientists have long speculated as to why animal species didn't flourish sooner, once sufficient oxygen covered the Earth's surface. Animals began to prosper at the end of the Proterozoic period -- but what about the billion-year stretch before that, when most researchers think there also was plenty of oxygen? Yale University researcher Noah Planavsky and his colleagues found that oxygen levels during the 'boring billion' period were only 0.1 percent of what they are today.

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