Precambrian

The Precambrian is the earliest part of Earth’s history, encompassing 88% of geological time and consisting of three eons: the Hadean, Archean, and Proterozoic. Our knowledge of the Precambrian is limited because many Precambrian rocks have undergone intense metamorphism, obscuring their origins, or have been eroded away, or are buried beneath Phanerozoic sediments.
The beginning of the Precambrian is called the Hadean, a time that started with the formation of the Earth. During this eon, the earliest igneous rocks of the Earth's crust formed. It is likely that no living organisms existed on Earth during the Hadean.

The Archean marks the time of continental formation. During the Archean, the first living cells probably developed in the oceans. By the late Proterozoic, multicellular organisms such as sponges, jellyfish, and hydra inhabited the oceans.

Stromatolites are examples of microfossils that are relatively abundant in Precambrian sediments. These organisms are considered the main builders of Precambrian sedimentary rocks; therefore, stromatolite fossils are used as key index fossils in Precambrian stratigraphic studies. The oldest stromatolitic evidence, 3.3 to 3.5 billion years old, has been reported from western Australia. Cyanobacteria were the primary origin and builders of stromatolites.

The existence of atmospheric oxygen is largely owed to stromatolite-forming organisms, and stromatolite life has persisted to the present day in some regions, particularly along the coasts of Australia.