Fossils of ancient life on Earth are rare. In the 3.5 to 4 billion years since life evolved, sedimentary rocks in which biological remains are preserved have been subject to intense erosion by the elements, or extreme heat and pressure in deep Earth as tectonic activity swallows them up. However, despite being scarce, these fossils are sometimes found, and in a new paper published in Astrobiology in November 2013, DCO’s Robert Hazen (Carnegie Institution of Washington, DC, USA) and colleagues describe the fossilized evidence of a 3.5 billion year old microbial ecosystem .
The Pilbara region of Western Australia is comprised of some of the oldest rock on Earth, and has long been a destination of choice for fossil hunters. Stromatolites and other microfossils of ancient bacteria have been found here, and have provided major insights into the very beginnings of life on Earth. In this current publication, however, researchers describe their discovery of a “microbially induced sedimentary structure”, or MISS, the first of it’s kind to be found in the Pilbara region’s Dresser formation.
“This work extends the geological record of MISS by almost 300 million years,” said Nora Noffke (Old Dominion University, Virginia, USA), lead author of the study. “Complex mat-forming microbial communities likely existed almost 3.5 billion years ago.”
The fossils represent some of the oldest ever found on our planet, and have important implications for understanding how early life survived. In order to maximize the ability of photosynthetic organisms to harvest the Sun’s energy, similar MISS, or biofilms, form today in shallow regions of lakes, rivers, and lagoons. Bacteria relying on chemical energy produced from rocks would have no need to form these structures. The existence of 3.5 billion year old fossilized MISS, therefore, suggest that life used photosynthetic machinery earlier than previously thought.
“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.
Not only does this work change how we look at early life on Earth, but also the traces life might leave behind on other planets. Indeed, rovers currently investigating the surface of Mars are primed to look for MISS.
“It’s amazing,” says Hazen. “Because of the distinctive mat structures, you can study these ancient microbial ecosystems at the scale of a rocky outcrop.”
This work was supported by the NSF Paleobiology and Sedimentary Geology program, NASA Astrobiology Institute, NASA Exobiology and Evolutionary Biology Programs, the Deep Carbon Observatory, and the Carnegie Institution for Science.
Photo: A rock surface displaying ‘polygonal oscillation cracks’ in the 3.48 billion years old Dresser Formation, Pilbara region, Western Australia. This kind of distinctive sedimentary structures point to a biological origin. They document ancient microorganisms that formed carpet-like microbial mats on the former sediment surface. The Dresser Formation records an ancient playa-like setting – similar environments are occurring on Mars as well. The MISS constitute a novel approach to detect and to understand Earth’s earliest life. Photo courtesy of Nora Noffke.