The Precambrian, a broad term for all geologic time before the Phanerozoic Eon, spans from the Earth’s dramatic birth at about 4.6 billion years ago to roughly 539 million years BP when macroscopic life burst onto the scene and left its indelible mark in the fossil record. While researchers have made some strides in uncovering the composition and characteristics of Earth’s early atmosphere during the Precambrian, the ancient oceans remain largely shrouded in mystery. Real data are scanty, and geological evidence is confined to a small number of sites. Existing models, built on numerous assumptions, frequently yield inconclusive insights. Despite these obstacles, ongoing investigations continue to find pieces of the puzzle of our planet’s first 4 billion formative years, and even from such sparse evidence, we can draw some broad insights and credible conclusions.