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Fossilized structures suggest that mat-forming microbes have been around for almost 3.5 billion years.
March 1, 2014|
COURTESY NORA NOFFKE
N. Noffke et al., “Microbially induced sedimentary structures recording an ancient ecosystem in the ca. 3.48 billion-year-old Dresser Formation, Pilbara, Western Australia,” Astrobiology, 13:1103-24, 2013.
Modern microorganisms leave traces on substrates called microbially induced sedimentary structures (MISS)—textures that arise from a biofilm or microbial mat interacting with the dynamics of the sediments upon which it forms. Until recently, the oldest fossilized MISS, located in South Africa, dated back to 3.2 billion years ago. However, evidence from microfossils and stromatolites, another rock structure shaped by bacteria, suggests that microbes existed at least 200 million years earlier.
In the Dresser Formation in Western Australia—one of the only places in the world with well-preserved 3.48-billion-year-old rocks—Nora Noffke of Old Dominion University in Norfolk, Virginia, and colleagues recorded microtextures characteristic of biofilms and microbial mats and uncovered geochemical signals consistent with a biological origin. The morphology and distribution of the fossils in this ancient coastal salt flat strongly resembled modern MISS.
The finding “supports interpretations that life had evolved before 3.4 billion years [ago], as indicated by the presence of both stromatolites and microfossils,” Kath Grey, who is the former chief paleontologist of the Geological Survey of Western Australia and was not involved in the research, wrote in an e-mail to The Scientist. According to Noffke, the fossilized MISS’s similarities to contemporary MISS suggest that ancient biofilms behaved in the same way that modern microbes do.
The Mars rover is currently hunting for MISS as a sign of life. Here on Earth, the origin of life predates the fossils from the Dresser Formation. Even 3.5 billion years ago, “life is already so complex,” Noffke says. “Its evolution must have taken a lot of time.”