When Martin Brasier discovered what looked like fossil cells in between the cemented sand grains of an ancient beach in Western Australia, he knew he had his work cut out for him. One of the biggest challenges for geologists is deciding when a fossil is really a fossil, particularly when it comes to early life. There are no bones to go by, and the mineralized spheres representing simple cells and sometimes filaments could easily be crystals or other irregularities in the sediment itself. What's more, Brasier, a geologist at the University of Oxford in the United Kingdom, has been one of the most vocal critics of many of the most ancient fossils other researchers have found.
But now Brasier and David Wacey, a geologist at the University of Western Australia in Crawley, say they have discovered 3.4-billion-year-old cells, possibly the oldest fossils ever found. Other microorganisms as old or older have been reported, including an extensive set of photosynthesizing bacteria 20 kilometers away from Brasier and Wacey's find, but Brasier and others have questioned the validity of those fossils. The two scientists say their chemical analyses of the minerals near the cells suggest the microorganisms depended on sulfur for fuel. Such a beach might have been life's first breeding ground, Brasier says.
The work represents "some of the best evidence for the nature of Earth's earliest life," says Bruce Runnegar, a geologist at the University of California, Los Angeles, who was not involved in the study.
Brasier first suspected the ancient rock formation would be a good place to look because it so closely resembled a modern beach, indicating that the sediments had not been badly heated or distorted since they were laid down. Wacey and colleagues analyzed the rock formation that included this ancient shoreline to verify the beach was about 3.4 billion years old.
The fossil cells they discovered looked promising: They were hollow, and some were clustered together in groups surrounded by what looked like a membrane. "The morphology is very cell-like," Brasier says. The cells were also patchily distributed in the sediment, just as modern bacteria tend to congregate near sources of food, the researchers report today in Nature Geoscience.
Turning to chemistry, Wacey and Brasier found that the apparent cell walls contained a different isotope, or version, of carbon from the surrounding rock. They also found tiny mineral crystals containing a different version of sulfur in and around the cells—evidence that the microorganisms were processing sulfur from the environment to extract energy.
Earth at that time was nothing like it is today. Oceans were likely a steamy 45ºC; oxygen was lacking. So it makes sense that early life depended on sulfur-containing compounds, Brasier says. Sulfur bacteria are common today in hydrothermal vents, hot springs, and low-oxygen environments. Other researchers have argued that early bacteria were photosynthetic or used hydrogen for energy. "In the end, it will probably turn out there were a wide variety of organisms using a wide range of metabolisms," says Stanford University geologist Donald Lowe.