The earliest evidence of life consists of rock samples in which organic carbon compounds show an isotope signature low in carbon-13. Both carbon-12 and carbon-13 are stable isotopes of carbon, but biological processes preferentially incorporate carbon-12, and carbon that has been processed by biology becomes depleted of carbon-13. Slightly younger (3400–3500 Mya) rocks contain several lines of evidence indicating the presence of biological carbon and sulfur cycles by this time.
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The oldest evidence of life is seen in rock samples with organic carbon compounds that have less carbon-13, as biological processes tend to use more carbon-12. Rocks from around 3400-3500 million years ago also show signs of biological carbon and sulfur cycles.