They said they had found evidence that bacteria or something equally tiny lived in what is now Greenland 3.8 billion years ago -- not very long after the planet became fit for life.
The earliest known fossils -- microscopic holes in ancient sedimentary rocks that used to be bacteria -- are 3.5 billion years old.
Anything older would have been subjected to tremendous stresses as the Earth's crust shifted, so scientists looking for clues to the origin of life have to find chemical traces inside tiny pieces of hard minerals that could resist obliteration.
Gustaf Arrhenius of the Scripps Institute of Oceanography in La Jolla, Calif., and colleagues in Britain and Australia say they found traces of carbon in apatite crystals that seem to have been formed by biological processes.
Using biochemical analysis techniques similar to those used by scientists who found evidence of life on Mars, Arrhenius's group said the crystals contained carbon of a type that, as far as scientists know, is only created by living organisms.
"We conclude that the isotopic results reported here give strong evidence for life on Earth by (3.85 billion years ago)," they wrote in a report in the science journal Nature.
"Although this finding pushed back the horizon for the emergence of life by 300 to 400 million years, it is not entirely unexpected, given also the apparently evolved nature of life-forms at (3.5 billion years ago)."
The problem is, they added, that scientists believe that just a few million years before the Earth was hit by a bombardment of meteorites so severe that it should have sterilized the planet.
"The evidence for life presented here overlaps this critical time period," they pointed out. But, if they were right, then the meteorite bombardment didn't wipe out all traces of early life and did not disturb the Greenland sediments.
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EARLY SIGNS OF LIFE STUDIED
.c The Associated Press
by Malcolm Ritter
AP Science Writer
NEW YORK (AP) -- Scientists have found the earliest signs of life on Earth, chemical signatures hidden in microscopic mineral grains more than 3.85 billion years old.
That pushes back the record of life by 300 million to 400 million years, to a time when Earth might have been pummeled by a lethal bombardment of asteroids. So if that bombardment really happened, some life might have survived, or perhaps it rose again from extinction.
The newfound traces of life were left by microorganisms, probably single-celled, that could have been "very undistinguished blobs" living at the bottom of an ocean, said researcher Gustaf Arrhenius of the Scripps Institution of Oceanography at the University of California, San Diego.
"It's not some precursor to life, it's real life," said Arrhenius, who reports the finding with graduate student Stephen Mojzsis and colleagues in Thursday's issue of the journal Nature.
Other scientists called the evidence for the ancient life strong, though not proof.
Researchers looked for evidence of life in the oldest sedimentary rocks known, from Greenland, more than 3.85 billion years old. They found tiny grains of a mineral called apatite, which is often produced by microorganisms. The calcium-containing mineral also makes up bones and teeth.
The real news is what showed up inside the grains. Living things process carbon from the environment and they prefer a kind of carbon called carbon-12, rather than the heavier carbon-13. So a lump of carbon that has been processed by a living thing has a higher ratio of carbon-12 to carbon-13 than one finds elsewhere in nature.
Inside the apatite grains, researchers found carbon ratios in the range to have come from living things.
"That looks an awful lot like a biological activity," said a scientist familiar with the work, David Des Marais of the National Aeronautics and Space Administration's Ames Research Center at Moffett Field, Calif.
He called the results strong evidence of ancient life.
Norman Pace, a microbial biologist at the University of California, Berkeley, agreed.
"I think it's wonderful," Pace said, adding that the work overcame objections to a previous, controversial hint of life of about the same age.
Still, Pace cautioned that it's not proof, because the telltale carbon ratio might have come from some non-biological process that scientists don't know about.
John M. Hayes, a senior scientist in the geology and geophysics department at the Woods Hole Oceanographic Institution in Massachusetts, said it will take multiple lines of evidence to prove the case. For example, for life at 3.5 billion years ago, scientists have not only suggestive carbon ratios but also bacteria-like fossils and other evidence, which altogether make an impressive case, he said.
While the current evidence for older life is weaker, "it's more than we had and it deserves attention," he said. And in a Nature commentary, he wrote that the new results probably do indicate life.
The idea of an asteroid bombardment of Earth more than 3.8 billion years ago, which some scientists say might have wiped out life, is based on crater evidence from the moon. "Nobody knows if the same bombardment happened on Earth," Arrhenius said.
He said his work questions whether it happened on Earth, because the finely layered sediments that contained the apatite grains cover tens of thousands of years from around that time, and they were totally undisturbed.
If it did happen, then either it didn't wipe out all life or life arose again from extinction, he said.
Original file name: CNI - Oldest Life on Earth
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