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Kerr_StoryOfO2_Science2005

Kerr_StoryOfO2_Science2005 - N e w s Fo c u s Gaseous...

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In the beginning, Earth was devoid of oxygen, and then life arose from nonlife. As that first life evolved over a billion years, it began to produce oxygen, but not enough for the life- energizing gas to appear in the atmosphere. Was green scum all there was to life, all there ever would be? Apparently, yes, unless life and nonlife could somehow work together to oxy- genate the planet from the atmosphere to the deep sea. Earth scientists are flocking to the emerg- ing field of astrobiology to tease out the history of oxygen on Earth from a maddeningly subtle and fragmented rock record. The rise of atmos- pheric oxygen from nothing to abundance, they are finding, came in two big steps about 2 billion years apart. Relatively simple life probably facilitated the first step up and possi- bly the second, much to its own detriment but to the benefit of more complex life. “The rise of oxygen changed the course of evolution,” says astrobiologist David Catling of the University of Bristol, U.K. “Atmos- pheric oxygen was a precursor to advanced life on Earth, and, I would argue, to life else- where.” With the new interest in 3 billion years of oxygen history, “there’s been a great deal of progress,” says geochemist Donald Canfield of the University of Southern Denmark in Odense. “The field has matured; it used to be a hobby area for most people. I credit NASA’s [astrobiology funding] for much of that.” An invigorated field is attacking a host of big questions: When did free oxygen first appear in Earth’s atmosphere? What made it appear in the first place? What held it back for so long? And what caused the second, delayed surge of oxygen that allowed advanced animals to appear? A certain beginning Historians of oxygen have always agreed on one thing: Earth started out with no free oxygen—that is, diatomic oxygen, or O 2 . It was all tied up in rock and water. For half a century, researchers have vacil- lated over whether the gases that were there favored the formation of life’s starting materials (see side- bar, p. 1732). Without free oxygen, in any case, the first life that did appear by perhaps 3.5 billion years ago had to “breathe” elements such as iron, processing them to gain a mere pittance of energy. For decades, scientists have argued about just how long the planet remained anoxic, and thus home to nothing but tiny, simple, slow- living microorganisms. Until recently, the idea that early Earth was anoxic for more than 2 billion years—as advanced primarily by geo- chemist Heinrich Holland of Harvard Univer- sity—dominated the field but had not won the day. Its proponents pointed to diverse evi- dence. Minerals older than about 2.2 billion to 2.4 billion years found in ancient soils, streambeds, and other sediments seemed to show no sign of ever having been exposed to oxygen. There were no “red beds” of sediment stained with rusted iron minerals, for example. But a small but vocal opposition, headed by Holland’s former student Hiroshi Ohmoto of Pennsylva- nia State University (PSU), University Park,
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