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The oceans became oxygen-rich as they are today about 600 million years ago. Biogeochemists have found evidence that the oceans went back to being oxygen-poor around 499 million years ago. The oceans remained anoxic for two to four million years. This finding is important at many levels, from the steady growth of atmospheric oxygen in the last 600 million years, to the potential impact of oxygen level fluctuations on early evolution and diversification of life. [Source: National Science Foundation}
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