Scientists combing Western Australia's "outback" for fossil evidence of early life on Earth say they have found a reef-like structure built by marine microbes some 3.4 billion years ago.
The team of scientists acknowledges that no one has yet found a "smoking gun" establishing microbes as the architects of the ancient, layered rocks. But absent such proof, microbial activity seems to be the most straightforward explanation for the variety of traits the formations display, they say.
The work is part of a 26-year effort by scientists worldwide to determine whether these structures, known as stromatolites, are in fact fossil evidence of early life on Earth, or whether they result merely from mineral "fallout" from evaporating sea water or hydrothermal activity.
It's a debate with space-age implications, one that may serve to sharpen scientists' tools for assessing rock samples gathered from Mars or other potential habitats in the solar system.
"The stakes are really high in terms of trying to recognize the signals of life elsewhere in the solar system," says Stanley Awramik, a geology professor at the University of California at Santa Barbara, who focuses on stromatolites and ancient microbial fossils. The test: "Let's ... see how good we are at identifying ancient life on Earth."
The findings from Australia, published in the current issue of the journal Nature, represent "a major contribution" to resolving the debate over stromatolites, Dr. Awramik adds. The research team "has put together a suite of evidence consistent with a biological interpretation" for the genesis of the rock-like formation. "This looks like a normal marine environment."
Stromatolites are built as colonies of marine microoganisms trap and bind grains of sand and other sediment into larger formations. New ones are being formed today, but some, like the ones that make up the "reef" in Western Australia, are ancient.
The group of Australian and Canadian scientists, led by Abigail Allwood from Macquarie University's Australian Center for Astrobiology, examined a six-mile stretch of stromatolites found in a formation known as the Strelly Pool Chert. Others have looked at portions of the chert's stromatolite "reef." But this appears to represent the first systemwide look.
The team discovered at least seven broad types of layered structures. Some look like egg cartons. Others look like mini dunes. Still others look like layered sheaths encasing jumbled collections of rocks. Some formations are cone-like, but with sides too steep to have been formed by geochemical processes, the group holds. These features appear in other, younger stromatolite formations widely accepted as having biological origins, the team argues. This diversity of form suggests the "ecologically controlled growth of a microbial reef," it says.
The region hosting the formations has provided plenty of fodder for scientists trying to find evidence for the earliest life, which many researchers say began about 3.8 billion years ago.
The case for microbes as sculptors in the outback is far from closed.
This diversity the Allwood team cites undermines its case, suggests Oxford University's Martin Brasier. A microbial origin for these rocks "requires the demonstration of near constancy" in stromatolites' form. "That is how scientists can recognize a species of mushroom or oak tree from one place to another."
He and others attribute the wide range of forms to self-organizing structures that arise through a range of physical and chemical processes, "although they may have had microbes living in them," he notes. He and his colleagues reported that they have found potential fossil candidates for some of those microbes in the same broad chert formation. It's best to assume nonbiological sources until that "hypothesis" is falsified, they wrote in an analysis published last month in the Philosophical Transactions of the Royal Society B.
Ms. Allwood and her team reply that perhaps a nonbiological explanation "is the 'extraordinary claim' that requires extraordinary proof" and that instead biology "offers an ordinary, plausible explanation" for the features.