The true story of oxygen in the Earth's atmosphere has yet to be told. Most researchers have been brought up to believe the Miller-Urey model of abiogenesis, which required the Earth to have a reducing atmosphere to facilitate the spontaneous generation of life. The atmosphere was then considered to be neutral for over a billion years. The evolution of organisms capable of photosynthesis was the next important step, with rises of oxygen levels triggering the flowering of eukaryotes, the rise of the Ediacaran fauna and then the Cambrian Explosion. The first significant rise is understood to be abrupt and sufficient of a milestone in Earth history to warrant a name of its own. Richard Kerr's comments below lead us to the new research by Ohmoto and colleagues: [quote]The first living things did not require oxygen to "breathe," but early life on Earth never would have gotten much beyond pond scum without free oxygen in the atmosphere. Conventional thinking has oxygen produced by photosynthesis gaining the upper hand 2.4 billion years ago, nearly halfway into Earth history. But new laboratory results reported in tomorrow's issue of Science challenge the late arrival of this "Great Oxidation Event."Read more here.
Ohmoto's hypothesis is that significant quantities of free oxygen were present in Earth's atmosphere prior to the GOE. He represents a minority position, but he continues to provide leadership in this area and a regular stream of relevant papers. One of the issues concerns patterns found in sulphur isotopes. Here is Kerr again: "Then in 2000, geochemist James Farquhar of the University of Maryland, College Park, came up with a nifty technique involving sulfur isotopes. The proportion of one isotope to another of the same element can change during a chemical reaction. Normally, the change depends on the masses of the isotopes. But Farquhar found isotopic shifts among three sulfur isotopes before 2.4 billion years ago that hadn't depended on isotope mass. As far as anyone knew, such "mass-independent fractionation" (MIF) could have happened only under solar ultraviolet radiation in an oxygen-free atmosphere - and MIF sulfur disappeared 2.4 billion years ago."
The new paper, with Farquar as one of the co-authors, proposes an alternative origin for these isotopic signatures that keep the door open for discussion of an early, oxygen-rich atmosphere. "The significance of this finding is that an abnormal isotope fractionation (of sulfur) may not be linked to the atmosphere at all," says Yumiko Watanabe, research associate [and co-author], Penn State. "The strongest evidence for an oxygen poor atmosphere 2.4 billion years ago is now brought into question."
Sunday, June 7, 2009
Origin of life: The misnaming of the Great Oxidation Event - which maybe never happened
British physicist David Tyler writes,