by CARL ZIMMER
A mat of blue-green algae in China. Cyanobacteria like this were some of the first organisms on Earth to produce oxygen as a waste product of photosynthesis. PHOTO/Chang W. Lee/The New York Times
To Donald E. Canfield, there’s something astonishing in every breath we take. “People take oxygen for granted because it’s just there and we breathe it all the time,” said Dr. Canfield, a geochemist at the University of Southern Denmark. “But we have the only planet we know of anywhere that has oxygen on it.”
What’s even more astonishing is that Earth started out with an oxygen-free atmosphere. It took billions of years before there was enough of the element to keep animals like us alive.
Although scientists have been struggling for decades to reconstruct the rise of oxygen, they’re still making fundamental discoveries. In just the past two weeks, for example, Dr. Canfield and his colleagues have published a pair of studies that provide significant clues about some of the most important chapters in oxygen’s history. They’re finding that our weirdly oxygen-rich atmosphere is the result of a complicated dance of geology and biology.
To study the ancient atmosphere, geochemists examine the chemical fingerprints left behind on rocks. Some rocks contain molecules that could have only formed in the presence of oxygen. The more of those molecules geochemists find in a rock, the more oxygen must have been in the atmosphere at the time.
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