Early Earth was a hot, gaseous, dusty and dynamic planet with an atmosphere and an ocean. Then its surface cooled and stabilized enough for clouds, landmasses and early life to form about four billion years ago, during what’s called the isotopic age of rocks, or the Archean Period. Atmospheric chemical byproducts from that time traveled through the air and deposited inside the planet’s oldest rock, recording life’s earliest activities like photosynthesis and oxygen production.