Their work has revealed spherical and rod-shaped bacteria dining on the cylindrical outer shells of another, larger bacterium known as Gunflintia. To digest those Gunflintia sheaths, the feeding bacteria would have had to use oxygen atoms taken from salts, or "sulfates," in seawater. In the process, the microbes formed gaseous carbon dioxide, which would have been released into the atmosphere.

Another byproduct of this biochemical process is hydrogen sulfide, which produces a stench commonly known as "the rotten egg smell," explained Martin Brasier, a paleobiologist at Oxford University in London.

"The whole world didn't smell of rotten eggs," said Brasier, "but if you had a sensitive nose, it would have been very widespread indeed."

There is chemical evidence that organisms were eating