The fossils in question are stromatolites, layered rocks formed by the exudation of photosynthetic microbes. Scientists agree that the oldest stromatolites were made by living organisms 3.43 billion years ago, but there are also older specimens. In the Dresser Formation of Western Australia, 3.48 billion-year-old stromatolites have been found.
However, billions of years have erased traces of organic matter in these old stromatolites, raising questions about whether they were truly formed by microbes or by other geological processes.
New research verdict: It was ancient life.
“We were able to find specific microstructures within certain layers of these rocks that strongly indicate biological processes,” said Kieran Hickman-Lewis, a paleontologist at the Natural History Museum in London, who led the research.
The findings could have implications for the search for life on Mars, Hickman-Lewis told Live Science. The stromatolites of the Dresser Formation are bound to iron oxides from the reaction of iron with oxygen in the atmosphere. The surface of Mars is similarly oxidized — thus the rusty orange color — but its rocks may contain similar structures left over from ancient Martian life, Hickman-Lewis said.
Hickman-Lewis and her team examined West Australian stromatolites first discovered in 2000 by study co-author Frances Vestal of France’s National Center for Scientific Research (CNRS). They used a variety of high-resolution 2D and 3D imaging techniques to peer into the layers of stromatolites at a finer scale.
What they saw indicated biological growth in all its messy glory. The researchers observed uneven layers, including small dome shapes that indicate photosynthesis, since microbes with the most access to the sun would grow more vigorously than those not as high in structure. They also saw columnar structures that are common in modern stromatolites, which are still found in a few places around the world.
“Microbial mats give you layers that are uneven in their thickness and curl or curl or move up and down on a very small spatial scale,” said Linda Kah
Dresser Formation stromatolites evidence of signs of ancient life do not make them the oldest life on the planet. That (probable) honor could go to stromatolites found in 3.7 billion-year-old rocks in Greenland, or perhaps to microfossils in Canada that could be 4.29 billion years old. It is very difficult to distinguish biological life from non-biological processes in these very old rocks, however, so these and others of the same time period are controversial.
Based on minerals in stromatolites, the Western Australia microbial mats likely formed in a shallow lagoon fed by hydrothermal vents that were also connected to the ocean, the researchers reported in the Nov. 4 Geology journal.
Techniques used to study West Australian stromatolites could be useful for finding life on Mars, Hickman-Lewis says, especially if Martian samples can be returned to Earth.
Scientists should “consider some of the analysis here as a trial run of analysis that we will have to do in about a decade when we have material from Mars.”