When it comes to making a lasting impression in geological history, the medium makes all the difference, especially in the Earth's paleo-oceans. Here, during the Archean Eon (4,000-2,500 million years ago) and at times during the Proterozoic (2,500-541 million years ago), when oxygen in the atmosphere and oceans was much lower than today, sedimentary minerals preserved signatures of biological activity in the form of fine textures created by microbial communities. The environmental conditions under which rocks like these form dictate how the crystal structure develops—the more orderly and fine-grained, the better the preservation.
