3.5 billion-year-old bacterial ecosystem found in WA

A team of researchers has discovered the well-preserved remnants of a complex microbial ecosystem in a nearly 3.5 billion-year-old sedimentary rock sequence in Western Australia. The discovery is a particularly lucky one - not only are Earth’s oldest sedimentary rocks very rare, but they are almost always altered by hydrothermal and tectonic activity.

The research team comprised UWA Research Assistant Professor David Wacey and his US colleagues - Associate Professor Nora Noffke and Daniel Christian of Old Dominion University, and Bob Hazen of the Carnegie Institute for Science. Reporting in the journal Astrobiology, the researchers described a phenomenon called microbially induced sedimentary structures (MISS) formed from mats of microbial material, much like mats seen today on stagnant waters or in coastal flats.

“The structures record highly diverse communities of microbial mats and have been reported from numerous intervals in the geological record up to 3.2 billion years old,” the researchers said.

But while WA’s Pilbara district is “one of the rare geological regions that provides insight into the early evolution of life on Earth,” said Professor Wacey, MISS had not previously been found in the region. Furthermore, the researchers discovered the MISS in the region’s Dresser Formation, a rock sequence which is 3.48 billion years old.

A rock surface displaying ‘polygonal oscillation cracks’ in the 3.48 billion-year-old Dresser Formation. Such and similar sedimentary structures are of biological origin. They document ancient microorganisms that formed carpet-like microbial mats on the former sediment surface. The MISS constitute a novel approach to detect and to understand Earth’s earliest life. Photo courtesy of Nora Noffke.

The fossils have been found to resemble in form and preservation the MISS from several other younger rock samples, such as a 2.9 billion-year-old ecosystem Noffke and her colleagues found in South Africa. Indeed, the researchers note, “Identical suites of MISS occur in equivalent environmental settings through the entire subsequent history of Earth including the present time. This work extends the geological record of MISS by almost 300 million years.”

Professor Wacey said MISS were created as microbial communities responded to changes in physical sediment dynamics - for example, “the binding together of sediment grains by microbes to prevent their erosion by water currents. The significance of MISS is that they not only demonstrate the presence of life, but also the presence of whole microbial ecosystems that could coordinate with one another to respond to changes in their environment.”

Thus, the team proposes that the sedimentary structures arose from the interactions of bacterial films with shoreline sediments from the region.

“The structures give a very clear signal on what the ancient conditions were, and what the bacteria composing the biofilms were able to do,” said Noffke. She concluded, “Complex mat-forming microbial communities likely existed almost 3.5 billion years ago.”

The findings could also be relevant to studies of life elsewhere in the solar system, as the Mars rovers search for biological signals similar to MISS on the planet’s surface.