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Low Mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals

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Science  31 Oct 2014:
Vol. 346, Issue 6209, pp. 635-638
DOI: 10.1126/science.1258410

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Low oxygen limited the rise of animals

Oxygen levels in Earth's early atmosphere had an important influence on the evolution of complex life. Planavsky et al. analyzed the isotopic signature of chromium in sedimentary rocks from across the globe—a proxy for past oxygen levels. Oxygen levels in the mid-Proterozoic (1.6 billion to 900 million years ago) were very low: less than 0.1% of the modern atmosphere. These low levels were probably below the minimum oxygen requirements for the earliest animals, delaying their emergence and diversification.

Science, this issue p. 635