Scientists have found that oxygen, and the life that generates it, might have enriched the Earth far earlier than currently supposed.
The discovery, sure to be controversial, also suggests life could arise earlier than now thought possible on alien planets.
Currently, scientists widely believe photosynthetic microbes helped oxygenate the planet roughly 2.4 billion years ago, and that oxygen was scant in the atmosphere before then.
They base this idea on isotopes of sulfur in ancient rocks. Isotopes of sulfur all behave virtually the same chemically, but have slightly different masses.
New signs of life
Rocks older than 2.4 billion years contain abnormal ratios of sulfur isotopes compared to younger rocks.
The only way known to generate these abnormal ratios is through reactions between sunlight and sulfurous volcanic gas in the absence of an ozone shield that would normally help screen out ultraviolet rays.
Ozone is an allotropic, or molecularly different, form of oxygen — three atoms rather than the regular two — and if the early atmosphere can be shown to have contained no ozone, it is can be assumed it had no regular oxygen either.
Geochemist Hiroshi Ohmoto, director of the Penn State Astrobiology Research Center, and his colleagues examined rocks from western Australia that were laid down as sediments on a lake bed and the ocean floor between 2.76 and 2.92 billion years ago.
These rocks displayed sulfur isotope ratios like those of more modern rocks from higher oxygen eras.
"'When did the Earth's atmosphere become oxygenated?' has been an important question for Earth scientists and biologists, because this question is closely linked to those related to the biological evolution on Earth and other planets," Ohmoto told LiveScience. "According to the currently popular theory, it took the Earth ... more than 2 billion years to develop an oxygen-rich atmosphere."
An implication of the new findings is that "an oxygen-rich atmosphere, generated by oxygen-producing organisms, may be found in young, as well as old, planets of other stars," Ohmoto said.
Earth formed about 4.5 billion years ago. Nobody knows exactly when or how life began, or how quickly it altered the planet's chemistry.
The new findings could mean oxygen levels on Earth have uniformly high since 3.8 billion years ago, Ohmoto said. They could also mean oxygen levels went through yo-yo fluctuations between highs and lows.
Alternatively, ultraviolet radiation from the sun might not be the only way that can generate the abnormal ratios of sulfur isotopes seen in older rocks.
"There is going to be a howl, even outrage," over these findings, geologist and isotope geochemist Paul Knauth at Arizona State University told LiveScience.
"They will say hot springs could have swamped the rocks Ohmoto and his colleagues looked at with normal sulfur, or that the crystals they analyzed washed in from elsewhere, or that their measurements are inaccurate," he said.
However, Knauth noted Ohmoto and his colleagues did address these points "and make good arguments."
Future experiments should look at rocks of many different ages and many different parts of the world, Ohmoto said.
Lab experiments should also see if there are other ways of creating abnormal sulfur isotope ratios, he added.
Ohmoto and his colleagues report their findings in the Aug. 23 issue of the journal Nature.
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