According to the researchers, each type of formation has distinct attributes and geometries that are inconsistent with mechanical or chemical process, such as the way tides rake over rock or sediments are spewed from hydrothermal vents.
In addition, each stromatolite contains features that can be explained by biological processes that are present in the geologic record.
Features best explained by known mechanical processes exist in the spaces between the stromatolites.
"Every single line of evidence you have requires something unusual [and] implausible to explain for it to be anything other than biologicit becomes ridiculous," Allwood said.
Writing in Nature, she and her colleagues conclude that the "diversity, complexity, and environmental associations of the stromatolites describe patterns thatin similar settings throughout Earth's historyreflect the presence of organisms."
UC Davis' Sumner said the analysis is convincing and is "really good science."
"What strikes me is this was a controversial subject, and the authors chose to address the controversy by doing a very thorough study, addressing as many issues as they could. That's just an outstanding way to do science," she said.
Several other structures dated between 3.3 billion and 3.8 billion years may have biological origins, but they are all controversial, Sumner says.
"In the suite of uncertain structures, [Allwood] has very strongly documented the presence of [microbial] mats at 3.5 billion [years]," she said. "Whether you accept that as the oldest [life form] is a little matter of opinion."
According to Allwood, if the Strelley Pool Chert stromatolites are the result of biological activity, they suggest that some of Earth's earliest life flourished far away from hydrothermal vents.
Hydrothermal vents are places found deep on the ocean floor where chemical-rich hot water flows up through cracks in the Earth's crust (see a hydrothermal vent photo gallery).
Many scientists believe the chemistry surrounding these vents allowed life to first evolve. (Read "Heat-Loving Microbes Offer Clues to Life's Origins.")
Stanley Awramick, an earth scientist at the University of California, Santa Barbara, wrote a commentary on the new analysis that also appears in Nature.
"The setting for the stromatolites of the Strelley Pool Chert indicates that microorganisms were adapted to and thriving in shallow marine environments," he wrote.
"It is not necessary to invoke the presence of hydrothermal activity."
Sumner says that the microbes responsible for the stromatolites were not the first life on Earth, but they suggest that life rapidly evolved and adapted to the shoreline environment.
"The ecological diversity of these stromatolites and their ability to withstand radiation in shallow water means they had evolved significantly," she said.
For Allwood, such musing "is the real significance of this kind of research. It tells us about the origins of life on our own planet, and we can use the information for understanding how and where we look for signs of life on other planets."
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