Precambrian

The Precambrian (Somali: Kambriyaan ka hor) is the collective term for all geological time before the beginning of the Cambrian Period. It extends from Earth's formation until 541 million years ago, when abundant hard-bodied life forms appear in the fossil record of the Cambrian.

This span comprises more than three-quarters of Earth's history. The Precambrian is not a single formal division of the geological time scale, but a convenient term encompassing three eons: the Hadean, Archean, and Proterozoic.

Divisions

The Hadean is the earliest division, beginning with Earth's formation. During this period the planet's surface was extremely hot, and preserved rock fragments are very scarce. The Archean is defined by the oldest known rocks and signs of bacterial life. The Proterozoic is marked by the rise of atmospheric oxygen and the emergence of eukaryotes (organisms with nucleated cells).

Atmospheric change

One of the most significant changes during the Precambrian was the accumulation of atmospheric oxygen, linked to the work of cyanobacteria performing photosynthesis. This shift altered the chemistry of the oceans and atmosphere, and opened pathways for life dependent on oxygen.

Fossils

Precambrian fossils are rare and difficult to find because most organisms of that time lacked hard parts. They include stromatolites (layered rock structures built by bacterial communities) and, in the later period, soft-bodied life known as Ediacaran fauna.