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 today'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.