Three and a half billion years ago, Earth hosted life, but was it barely surviving, or thriving? A new study carried out by a multi-institutional team with leadership including the Earth-Life Science Institute (ELSI) of Tokyo Institute of Technology (Tokyo Tech) provides new answers to this question. Microbial metabolism is recorded in billions of years of sulfur isotope ratios that agree with this study’s predictions, suggesting that life flourished in the ancient oceans. Using this data, scientists can more deeply link the geochemical record with cellular states and ecology.