The earliest direct evidence of organisms pumping oxygen into Earth's ancient atmosphere has been found in the fossilized remnants of bacterial slime, according to research that also gives scientists a new tool in the hunt for life on Mars.
The 2' billion-year-old molecular fossils from Australia show that early forms of life began paving the evolutionary path for oxygen-breathing animals at least 700 million years earlier than previously known, researchers said in Thursday's journal Nature.
"Life wouldn't be what it is today if we didn't have oxygen in the atmosphere and in the ocean," said Roger Summons, the study's lead author and chief research scientist for the Australian Geological Survey Organization.
Scientists had long suspected that organisms called cyanobacteria first started converting sunlight and carbon dioxide into food energy and oxygen billions of years ago. But no evidence had been previously found in fossils.
Then Summons and his colleagues tested samples from a drill core in Western Australia's Hammersley Basin, one of the most geologically stable areas on Earth, and later, samples from Mexico, Germany and Israel.
They discovered a durable molecular signature -- "fingerprints" - unique to certain cyanobacteria that lived on the shores of the ancient oceans, long before plants, animals and other complex life emerged.
"This is the first good evidence of early oxygenic photosynthesis," Jere Lipps, professor of biology at the University of California, Berkeley, said. "That's important because we couldn't be certain based on microfossils or other geologic evidence."
The technique of identifying the molecular signature of ancient organisms also could be used in the hunt for life elsewhere in the universe.
David McKay, a NASA researcher who led a 1996 team that announced a meteorite from Mars showed evidence of ancient life, agreed.
"This is really potentially useful," he said Wednesday. "We think that this is the first of a number of molecular biomarkers that may be documented in the future and that can be applied directly to returned Mars samples."
The markers discovered by Summons' team survived the ravages of 2« billion years on Earth. Mars, a quieter place geologically, could preserve the record even longer, McKay said.
Forms of cyanobacteria exist today, often referred to as blue-green algae because they resemble the slimy substance. They can be found in places like the hot springs of Yellowstone National Park.