By Jonathan Fildes
Science and technology reporter, BBC News
Some zircon crystals have been dated to 4.4bn years ago
Tiny slivers of diamond forged on an infant Earth may contain the earliest traces of life, a study has shown.
Analysis of the crystals showed they contain a form of carbon often associated with plants and bacteria.
The rare gems were found inside zircon crystals, formed a few hundred million years after the Earth came into being.
Writing in the journal Nature, the researchers caution that their results are not definitive proof of early life but do "not exclude" the possibility.
"We're all a little sceptical," said Dr Martin Whitehouse of the Swedish Museum of Natural History and one of the authors of the paper.
If the carbon was derived from primitive organisms, it would push back the date for life appearing on Earth by around 500 million years, to beyond 4.25 billion years ago. The Earth itself is just 4.6 billion years old.
"When you look at the carbon isotopes, they could be interpreted as biogenic because we know that biologic processes do generate light carbon isotopes. But of course there are other processes that can do that," Dr Whitehouse told BBC News.
Other possibilities include chemical reactions involving carbon oxides or even the material being delivered from space by meteorites.
However, some observers have raised the possibility that the diamonds may be contamination, introduced during polishing of the zircons.
"If you look at the photos that they present, you always see these diamonds sat in cracks and fissures and cavities," Professor Minik Rosing of the University of Copenhagen told BBC News.
If they were original features, he said, you would expect at least some to be embedded within the structure of the crystals.
"There is always fear that they might actually not be primary."
However, Dr Rosing explained, the possibility that the signatures were from early Life was "tantalising".
The tiny zircon crystals - just 0.3mm across - were found in the Jack Hills of Western Australia. They are the tough remnants of ancient rocks that have long since disappeared.
"We don't have the rocks. These zircons are just little fragments of something that was broken up, weathered and redeposited as sediments," explained Dr Whitehouse.
Radioactive dating has suggested that some of the crystals formed as far back as 4.4 billion years ago.
Scientists describe this phase in Earth history as the Hadean, and it has long been thought that it would be impossible for life to begin at this time because of the inhospitable conditions on the young planet.
But the Jack Hills zircons have begun to cast doubt on this theory.
Earlier work raised the intriguing possibility that the infant Earth would have been cooler and wetter than previously thought as the crystals show evidence of growing out of a low-temperature magma that had been in contact with water.
The new analysis of the diamond and graphite inclusions in the crystals could lend further weight to this theory.
"I think there is an interesting possibility here," said Dr Whitehouse.
The scientists analysed 22 graphite and diamond inclusions in 18 zircon crystals.
The results showed that the capsules had unusual levels of a light form, or isotope, of carbon, known as carbon 12.
"The most common way to form light carbon on the modern Earth is photosynthesis," explained Dr Alexander Nemchin of the Curtin University of Technology, Australia, and another author of the paper.
During this process organisms preferentially extract light carbon, leaving heavier forms in the atmosphere.
"When they die, they preserve that signature," he said.
The results of the team's experiments show that the carbon inclusions have a range of isotopes, which suggested, they said, that the carbon reservoir was "heterogeneous".
This would then have had to be buried deep inside the Earth to generate the extreme pressures required to turn it into diamond.
"If this stuff was life - which then would have presumably formed on the surface - you do then need a process to take it down to something like 150km or 200km," said Dr Whitehouse.
On the modern Earth, crust is recycled at depth in so-called subduction zones, such as those found along the edge of the Pacific Ocean. Here, cool, dense oceanic crust plunges under the buoyant and long-lived continental crust.
Previous work on the diamonds supports the notion that similar processes were occurring on the Hadean Earth.
But not all scientists agree; instead they suggest that the early crust was relatively stable. Either way, Dr Whitehouse does not believe that this rules out a biogenic origin for the carbon.
"All this tells you is that there could have been a process that put things down to 200km," he said. "There may have been other things happening that we don't know about."
However, the team readily admit that the conclusion is not definitive.
Currently, what is thought to be the oldest signature of life by some - dated at around 3.7bn years old - was discovered by Professor Rosing in an area of intensely deformed rocks in West Greenland known as the Isua Belt.
Here, chemical traces again suggest the presence of photosynthetic life forms. But crucially the signature is seen in a complete sequence of rocks rather than isolated crystals.
This gives geologists clues about the environment in which the rocks were laid down and whether or not they could feasibly have contained life.
"The problem with the Jack Hills is that we don't have the rock," admits Dr Whitehouse "The carbon isotopes alone are not a distinct biosignature."
As a result they have suggested other possibilities for the origin of the carbon including inorganic chemical reactions, similar to those that take place in the catalytic converter of a car.
Professor Rosing believes that this is the most likely explanation. He points to the range of carbon values that were found in the inclusions.
"That to me is completely the opposite of a biological signature," he said. "That's the signature of some chemistry - a fractionation process or something."
Photosynthesis, he explained, would produce constant values for the carbon isotope ratios. Another possibility, the team suggests, is that the carbon came from so-called "chondritic" meteorites, which also have a similar chemical signature.
This theory is appealing as the Hadean is thought to have ended roughly 3.8bn years ago with a period of intense bombardment believed by some to have initiated the emergence of life on Earth.
However, according to Professor Rosing, if the diamonds and zircons are extraterrestrial it undermines every other theory related to the zircons, including the possibility of a cooler, more habitable early Earth.
"If that is the case, then every other argument about these zircons falls apart, he said. "Then we don't know anything."