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Wednesday, 6 March, 2002, 21:50 GMT
Life's early 'footprint'
University of California
Ancient fossil or flaw in the rock?
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By Dr David Whitehouse
BBC News Online science editor
Is this one of the oldest fossils known to science - a 3.5-billion-year-old microbe - or just a flaw in a rock? It all depends on which paper in the journal Nature you read.

New evidence, based on detailed chemical analysis, for it being some of the oldest life on Earth is presented alongside claims that it could be merely an imperfection in a piece of Western Australian rock that just looks a bit like a bacterium.

One report says this minute structure, like others found in the rocks, not only takes the shape of a microbe but also has high carbon content consistent with it once being a living organism.

The second report, on the other hand, says that many of the so-called fossils look nothing like microbes and are probably just blobs of the mineral graphite. Take your pick.

Scientists know life existed on Earth when the rocks were formed; they have ample chemical evidence from which the presence of the organisms can be inferred. The issue is whether this image represents something more physical - an actual fossilised bacterium from that ancient time.

Old rocks, new science

The case for it being the oldest fossil known to science is put by William Schopf of the University of California, US, and colleagues.

In the early 1990s, Schopf caused a sensation when he claimed to have evidence of ancient colonies of bacteria in 1,465-million-year-old rocks in Western Australia. He based his conclusion on the shapes of microfossils he found in the rocks.

Other scientists pointed out that relying on shape alone was unreliable, not least because bacteria have little definite shape anyway. They said it was difficult to tell the difference between a bacterium - especially a fossil bacterium - and a flaw in a rock.

To counter these criticisms, Schopf obtained other evidence by looking at the regions around the purported fossil bacteria, pointing to suggestions of widespread bacterial activity in the rocks.

Another explanation

And since then, Schopf and colleagues have sought to back up the morphology claims using laser-Raman imaging, a technique in which the chemical composition, as well as the structure of the fossils, can be mapped in two dimensions.

It is this technique he uses to justify his claims about these even older structures reported this week in Nature.

But Martin Brasier of the University of Oxford, UK, and colleagues are sceptical of Schopf's latest work. They said the rocks in which the so-called fossils were found came from a vein that might have been produced hydrothermally, by the action of heated water on minerals.

Unsurprisingly, no point of view about the question of ancient bacteria in these rocks is clear cut. While the type of carbon atoms in the rocks is consistent with life, other studies suggest that the same carbon could have been produced through volcanic action.

Martian life

Brasier and colleagues are not even impressed by the shape of the supposed fossils, suggesting that they seem to have a haphazard orientation that is not found in bacteria today. They said the fossils were just collections of quartz and graphite sheets.

Schopf may feel somewhat awkward at having his belief that the ancient fossils are genuine thrown back at him.

He was one of the first to cast doubt on the claims made in 1996 that unusual shapes found in a meteorite from Mars (meteorite ALH 84001) were evidence of past life on the Red Planet.

With two opposing papers, both with considerable merits, published in the same issue of the journal Nature, scientists know that no one team has won the argument and the mystery about life's origins on our planet deepens.

See also:

09 Aug 01 | Sci/Tech
Earth story: Plants arrived early
12 Feb 99 | Sci/Tech
Martian 'bacteria' matched to Earth
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