Precambrian

Precambrian is the informal geologic time interval that spans Earth’s history from its formation about 4.6 billion years ago to the beginning of the Cambrian Period about 541 million years ago. It accounts for roughly 88 percent of Earth's history. The interval is divided into three eons: Hadean, Archean, and Proterozoic. The Hadean is the earliest interval, marked by planetary formation, intense heat, and the initial crust and oceans. Very few rocks from this time survive, so knowledge is largely derived from isotopic data and meteorites.

In the Archean, continental crust stabilizes and the first stable landmasses appear. Evidence for life—mostly microbial mats and stromatolites—emerges, with the development of photosynthesis and basic metabolism. Atmospheric and ocean chemistry were reducing for much of this time.

The Proterozoic records significant biological and geological advances. It includes the appearance of more complex single-celled eukaryotes and later multicellular life, major shifts in atmospheric oxygen such as the Great Oxygenation Event around 2.4–2.0 billion years ago, and ongoing crustal growth. The formation and breakup of supercontinents such as Rodinia and Pannotia occurred during this eon. The Proterozoic also features dramatic climate episodes, including Neoproterozoic Snowball Earth glaciations. The interval ends with the start of the Cambrian, which is characterized by a marked increase in the fossil record and the rapid diversification of multicellular life.

Precambrian fossils are relatively rare and mostly consist of microbial structures, chemical signatures, and, in a few exceptional deposits, soft-bodied organisms. The Precambrian laid the foundational conditions for later biological and geological evolution.