July 2, 2008
New Finding Hints at Earliest Life on Earth
A recent study has shown that tiny slivers of diamond forged on an infant Earth may contain the earliest traces of life.
The crystals contain a form of carbon often associated with plants and bacteria.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 study is proven to be true then 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.
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.
He said if they were original features, 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."
The Jack Hills of Western Australia is where the tiny zircon crystals were found.
They are the 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.
Some of the crystals formed as far back as 4.4 billion years ago, according to radioactive dating.
The Hadean, which is what the phase of the Earth was when these crystals were supposedly formed, was thought by scientists to not contain or form life because of the Earth being so young.
The scientists analysed 22 graphite and diamond inclusions in 18 zircon crystals.
The results showed that the capsules had unusual levels of 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 - or isotopes - in the atmosphere.
"When they die they preserve that signature," he said.
The carbon reservoir, the team suggested, 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 Earth today, 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.
Similar processes were occurring on the Hadean Earth is what this study suggest.
Not every scientists agree. Those that do not agree 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."
The team still readily admits that the conclusion is not definitive.
Currently, what is thought to be the oldest signature of life by some was discovered by Professor Rosing in an area of intensely deformed rocks in West Greenland known as the Isua Belt.
Here chemical traces 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 where the rocks were and whether or not they could have feasibly 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."
Other possibilities for the origin of the carbon have been suggested, including inorganic chemical reactions.
Professor Rosing believes that this is the most likely explanation.
"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."
Another possibility, the team suggests, is that the carbon came from so-called "chondritic" meteorites, which also have a similar chemical signature.
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."
On the Net:
Swedish Museum of Natural History
University of Copenhagen