Earth's Second Wind

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Science  10 Dec 2010:
Vol. 330, Issue 6010, pp. 1490-1491
DOI: 10.1126/science.1199919

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One of the great mysteries of Earth history is why, after 3 billion years of evolution, about 600 million years ago (Ma), organisms emerged from the microscopic world and shortly thereafter developed skeletons and shells (1). Two prevalent hypotheses include climate cooling (2) and an increase in the oxygen content of the ocean (3). To date, the best evidence in support of the oxygen theory has been the size of animals themselves: Larger animals have higher oxygen demands that can only be met by oxygen-rich waters. Now Dahl et al. (4) present independent proxy evidence for an increase of oceanic (and by inference, atmospheric) oxygen concentrations— from levels below the lower limit for large animals, to levels above—at about 550 Ma. This second rise of oxygen follows nearly 2 billion years after the well-documented first “Great Oxidation Event” (GOE) at about 2400 Ma.