That’s why Noffke and her colleagues corroborated their story by measuring the carbon that makes up the textured rocks. About 99 percent of carbon in non-living stuff is carbon-12, a lighter version of the element than the carbon-13 that accounts for most of the remaining 1 percent. Microbes that use photosynthesis to make their food contain even more carbon-12 and less carbon-13. That bias, a signature of “organic” carbon that comes from a living being, showed up in the Australian rock.
“It’s always nice to have a number of different lines of evidence, and you definitely want to see organic carbon,” says geomicrobiologist John Stolz of Duquesne University in Pittsburgh.
What wasn’t preserved: any proteins or fats or body fossils that would clinch the case for life and identify what types of bacteria left behind this organic carbon. Most microbial mats today contain lots of photosynthetic cyanobacteria, which make the food that sustains the other bacteria. Named after the blue-green pigment they use for this process, called phycocyanin, cyanobacteria also make oxygen and are given the credit for creating Earth’s atmosphere about 2.4 billion years ago.
Cyanobacteria living in microbial mats nearly 3.5 billion years ago could shake up the history of the air we all breathe.
“Studying this kind of past life is really about learning how the Earth got to be the way it is today,” says Michael Tice, a geobiologist at Texas A&M University.
Ultimately, the fossils found on Earth could help those looking for the building blocks of life on Mars, where NASA’s Curiosity rover has recently found evidence for ancient waterways. Remnants of life on the Red Planet might even be better preserved than they are here on Earth, says Harvard University paleontologist Andrew Knoll. That’s because old terrestrial rocks tend to get banged around by the movement of tectonic plates and cooked by the extreme heat of the planet’s depths. Mars, a planet that’s nearly dead geologically, lacks such tectonic activity.
Though no signs of ancient Martian microbes have been found, fossil hunters may now have something new to start looking for.
Powell is a Washington-based freelance science reporter.