Earliest Signs Of Microbial Life Yet Confirmed In 3.5 Billion-Year-Old Rocks

Photomicrograph of a stromatolite from the 3.5 billion-year-old Dresser Formation in Western Australia. Some of the original organic material has been protected by the pyrites, preserving the oldest remnants of life that we know, as well as the flowing shapes of long gone microbe colonies. Raphael Baumgartner

Organic material found inside 3.5 billion-year-old stromatolites has strengthened the case for these being the earliest remnants of life we have found. The discovery could influence where we target when searching for life on Mars.

Finding the oldest evidence of life is crucial to understanding our origins, as big a question as science tackles. However, most potential candidates are very degraded and could be the product of non-living geologic processes.

A particularly controversial case has been stromatolites from Western Australia's Pilbara. Although similar-looking objects have been found in Greenland that are 200 million years older, the Pilbara specimens have been considered strong contenders for the oldest traces of life since they were discovered in 1980, but many scientists have not been convinced. Dr Raphael Baumgartner of the University of New South Wales thinks that's about to change.

“This is an exciting discovery – for the first time, we’re able to show the world that these stromatolites are definitive evidence for the earliest life on Earth,” Baumgartner said in a statement

The Pilbara stromatolites look like fossilized versions of the colonies of cyanobacteria found living in nearby hypersaline pools today. However, most specimens were found at the surface of the encasing rock and had been highly weathered. Biological-looking general shape and texture hasn't been enough for everyone.

Baumgartner drilled deeper into the rocks and found samples protected from weathering for billions of years. In Geology he reports multiple lines of evidence all pointing to a living origin for the stromatolites. These include apparent surviving organic matter, with isotopic ratios consistent with a biological origin, and elements like nitrogen that would be expected to be left behind by living things.

Moreover, “We’re looking at exceptionally preserved coherent filaments and strands that are typically remains of microbial biofilms,” Baumgartner said, adding he didn't expect to find something so clear. “I think it was around 11 pm when I had this ‘eureka’ moment, and I stayed until 3 or 4 o'clock in the morning, just imaging and imaging because I was so excited. I totally lost track of time.”

The discovery could influence Martian missions, Baumgartner told IFLScience, because the bulk of the fossils have turned to pyrite (FeS2), also known as fool's gold. The fact the surviving organic material was protected by the pyrite was also unexpected. Mars is rich in both iron and sulfur, and future missions could seek deposits hoping similar signs of life may have been preserved from the era when the planet was wet.

Two years ago Baumgartner's colleague Tara Djokic demonstrated some Pilbara stromatolites came from hot volcanic springs, rather than ocean inlets. Baumgartner didn't look at Djokic's specimens, but told IFLScience confirmation that some of the stromatolites were of biological origin strengthens the case those were too. Such a conclusion would increase the chances life on Earth began in terrestrial pools, rather than around vents on the ocean floor, with major implications for where we should seek life on other worlds.

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