New look at old rock questions evidence of earliest life
Washington (CNN) -- It was a rock that rocked the scientific world six years ago. Scientists using sophisticated geo-chemical dating techniques said they had evidence that life on Earth emerged at least 3.86 billion years ago.
But a new look at those rocks found on a remote Greenland island now suggests a non-biological origin.
In this week's edition of Science magazine, researchers from The George Washington University and the Swedish Museum of Natural History say there is more to establishing signs of life in rocks than just interpreting the behavior of the carbon within them.
In 1996, scientists from the Scripps Institution of Oceanography and other universities reported a rock formation on the Greenland island of Akilia showed carbon residues with a chemical fingerprint believed to come only from a living organism. Carbon is the principal element of life, and it was thought these fingerprints showed evidence of simple organisms such as bacteria.
"My scientific intuition told me the story was not completely solved yet," said Christopher Fedo, a George Washington University professor and co-author of the Science paper.
Fedo, who spent several weeks on the remote, uninhabited island of Akilia, does not challenge the carbon analysis done by geo-chemists in that 1996 study published in Nature magazine. That study focused on a green and white layered rock known as a banded iron formation, thought to be layers of sediment that were deposited on a long lost sea floor.
But Fedo has a different explanation of the rock's makeup, which he says has been influenced by a lot of complex events over billions of years that have caused the rocks to be stressed, stretched and deformed.
"We interpret the rock as having formed from molten rock, something like the rock that would form from lava in a volcano. Temperatures that would form that type of rock are far, far hotter than anything that could sustain a life form as we know it," said Fedo. A chemical study of the rocks shows their composition to be similar to a primitive type of rock called komatiite, related to basalt, which forms when molten rock solidifies.
Basalt-like rocks can interact with water to form carbon compounds that resemble those left by life forms, which could explain the "life-like" carbon fingerprints detailed in the 1996 studies.
Fedo says there may never be definitive evidence of when and where life on Earth began. That's because the early history of the planet is filled with violent asteroid impacts. Some of those could have resulted in boiling off all the water on the planet, including the oceans, and wiping out any forms of life that may have existed. So life may have originated, then been obliterated, several times before the last major asteroid impact.
"When looking at rocks this old, we have to remember that lots of events, meltings and collisions, are superimposed on top, and we'll never know all the events that took place," said Fedo.
The oldest actual fossils yet identified are bacteria and algae, a sort of pond slime discovered in Northwest Australia and thought to be about 3.5 billion years old.
Greenland, Australia, Canada and parts of Southern Africa have the best known examples of the oldest rock formations on Earth, according to scientists. Research in all those areas is "quite robust," says Fedo, and all could provide clues about how the Earth evolved over time.
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