Researchers using a highly sensitive laser say they have detected microscopic remnants of life dating back 3.5 billion years - a find that could push back by 1 billion years the earliest definitive evidence of life on Earth.
Other scientists dispute the claim, however.
The apparent "microfossils" of bacteria were found in shale-like formations in western Australia in the early 1990s by a team that included J. William Schopf. The evidence consists of microscopic, filament-like strands in the rock.
Shortly after the find, Schopf, a paleobiologist at the University of California at Los Angeles, said that the material appeared to be one of the earliest recorded bits of life on Earth.
In an article in Thursday's issue of the journal Nature, Schopf reported that he and colleagues at the University of Alabama-Birmingham further analyzed the samples with a tool known as a laser-Raman imaging.
"This pushes back by a billion years the prior definitive evidence of the earliest life on Earth," said University of Alabama astrophysicist Thomas Wdowiak, a member of the research team.
Wdowiak said the laser technology can measure the molecular vibrations of very tiny particles and map the chemical makeup. Tests found the material to be made up of organic carbon - evidence that the substance was once alive, the researchers said.
"The organic matter that we detected is to very ancient microbes as coal is to less ancient plants," Wdowiak said.
In a separate paper in Nature, however, a team of researchers in England and Australia cast serious doubt on the claim. They said other tests show no evidence that the formations were once ancient life.
They said the filament-like strands might instead be just traces of the mineral graphite.
The structures "should not be accepted as being of biological origin until all possibilities of their non-biological origin have been exhausted," wrote the English-Australian team, led by Dr. Martin D. Brasier of the University of Oxford.
Even if proven to be remnants of life, at 3.5 billion years, the microfossils would not necessarily be the earliest life detected on Earth.
Scientists have found earlier signs of possible life in chemical signatures hidden in microscopic mineral grains more than 3.85 billion years old - a time when Earth might have been pummeled by asteroids.
Some scientists believe that asteroids delivered life - or the ingredients necessary for life - to Earth.