In DepthGeochemistry

Low oxygen stifled animals' emergence, study says

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Science  31 Oct 2014:
Vol. 346, Issue 6209, pp. 537
DOI: 10.1126/science.346.6209.537

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Between the appearance of complex cells about 2 billion years ago and the explosive diversification of multicellular animals some 800 million years ago was the so-called boring billion—a long span of time during which evolution seems to have mired. Now, a new study looking at chromium isotopes in ancient ironstones suggests that the oxygen content of the atmosphere during that time was only 0.1% of present oxygen levels—at least an order of magnitude smaller than previous lower limits for oxygen content at the time. Oxygen levels rose again about 800 million years ago, coinciding with animal diversification—and suggesting that such environmental conditions can play a powerful role in controlling biological evolution.