A common sulfate-reducing bacterium (shown above) co-evolves with another microbe to create a synergistic situation where both microbes thrive in the subsurface without oxygen, which does not penetrate below the top few centimeters of sediment. In 21 separate experiments, the synergistic relationship evolved between the two organisms 13 times. Photo is a color-enhanced digital micrograph of a black and white scanning electron microscope image.