Reconstructing the rise of life during the period of Earth's history when it first evolved is challenging. Earth's oldest sedimentary rocks（沉积岩） are not only rare, but also almost always altered by hydrothermal（热源的） and tectonic activity. A new study from a team including Carnegie's Nora Noffke, a visiting investigator¹, and Robert Hazen revealed the well-preserved remnants of a complex ecosystem² in a nearly 3.5 billion-year-old sedimentary rock sequence in Australia. Their work is published in Astrobiology.

 

The Pilbara district of Western Australia constitutes one of the famous geological regions that allow insight into the early evolution of life. Mound-like deposits created by ancient photosynthetic³（光合作用的） bacteria, called stromatolites（叠层石）, and microfossils of bacteria have been described by scientists in detail. However, a phenomenon called microbially induced sedimentary structures, or MISS, had not previously⁴ been seen in this region. These structures are formed from mats of microbial material, much like mats seen today on stagnant⁵ waters or in coastal⁶ flats.

 

The team included Noffke, Hazen, Daniel Christian⁷ of Old Dominion⁸ University, and David Wacey of the University of Western Australia. They described various MISS preserved in the region's Dresser Formation. Advanced chemical analyses point toward a biological origin of the material. The Dresser MISS fossils resemble strongly in form and preservation⁹ 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.

 

"This work extends the geological record of MISS by almost 300 million years," said Noffke, who is also a professor at ODU. "Complex mat-forming microbial communities likely existed almost 3.5 billion years ago."

 

The team proposes that the sedimentary structures arose from the interactions of bacterial¹⁰ films with shoreline sediments¹¹ 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," Noffke said.

 

MISS are among the targets of Mars rovers, which search for similar formations on that planet's surface. Thus, the team's findings could have relevance¹² for studies of our larger Solar System as well.

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