Harvard researchers have found important clues about the Earth’s environment 1.5 billion years ago. Their results present quite a different picture from present times, in which oceans have oxygen-rich waters from top to bottom and are capable of supporting large animal life even in their depths. The findings are important not just for what they tell us about prehistoric oceans, but also for what they tell us about oxygen in the air at the time. Scientists believe oxygen in the Earth’s atmosphere rose to today’s levels in two distinct jumps, 2.3 billion years ago and at about 800 million years ago. “This strongly supports the idea that the initial rise in oxygen didn’t bring us to modern levels,” said Andrew Knoll, Fisher Professor of Natural History, who, along with postdoctoral fellow Yanan Shen, and Australian colleague Malcolm Walter, conducted the research. “It tells us that halfway through Earth’s history we still didn’t have an atmosphere we could breathe.” Knoll, Shen, and Walter have been working to illuminate the Earth’s transition from its lifeless creation to the world we know today, with oxygen-rich skies, and life teeming on land and in the sea.