AP BIO !!! That is the hardest AP class.
The Proterozoic division of the geological time scale is an era of the Phanerozoic eon. The Proterozoic division began 2.5 billion years ago and ended 542.0 million years ago.
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Without caffeine, electronics, climate-controlled interiors, pharmaceuticals, artificial light, late-night TV, and other features of modern society that help us override our body's natural signals, hunter-gatherers tend to go “to bed” within a couple of hours of sunset and wake up just before sunriseAnswer:
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