The Proterozoic Eon is a geological time period that lasted from about 2.5 billion to 541 million years ago. It follows the Archean Eon and precedes the Phanerozoic Eon. During this time, the Earth's atmosphere and oceans underwent significant changes, including the increase of oxygen due to the process of photosynthesis by early cyanobacteria. The Proterozoic Eon is also notable for the emergence of multicellular life forms. Fossils from this period show the first complex organisms, including Ediacaran biota, which are some of the earliest known multicellular life. This eon set the stage for the rapid diversification of life that occurred in the subsequent Phanerozoic Eon.