Most of Earth history elapsed before complex animals appeared. The Precambrian encompasses the time from the formation of Earth about 4.54 billion years ago until the start of the Cambrian Period 541 million years ago. It includes the Hadean, Archean, and Proterozoic eons.

During the Precambrian the planet cooled, oceans formed, the atmosphere gained oxygen through photosynthesis, and the first continents assembled. Life existed as single-celled organisms for most of this interval; soft-bodied multicellular life appeared only near its end. Rock records are incomplete because of extensive metamorphism and erosion.

Precambrian shields form the stable cores of continents and host major mineral deposits. Studying these ancient rocks reveals the conditions under which life originated and the early tectonic regimes that shaped the planet. Absolute dating is essential because fossils are scarce.

• Time before the Cambrian, 4.54 Ga to 541 Ma
• Includes Hadean, Archean, and Proterozoic
• Records origin of life and rise of oxygen
• Forms continental shield areas