Analyses of 3.5-billion-year-old rocks from Australia indicate that some of the microorganisms living when those rocks formed were able to derive energy from sulfur, the first time such a metabolic feat has been chronicled in rocks of that age.
Because bacteria have no hard parts, they don't fossilize well. Nevertheless, signs of ancient life are often recorded in a rock's chemistry. For example, bacteria that extract energy by metabolizing sulfate minerals leave behind sulfides partially depleted of the heavier isotopes of sulfur, says Pascal Philippot, a geochemist at the Paris Geophysical Institute.
To discern the influence of such bacteria on ancient rocks, scientists look for a lower-than-normal concentration of the isotope sulfur-34 relative to that of sulfur-32.