The Proterozoic is a geological eon that lasted from about 2.5 billion to 541 million years ago. It is the second eon of the Earth's history, following the Archean eon. During this time, the Earth's atmosphere and oceans underwent significant changes, leading to the accumulation of oxygen, which was crucial for the development of complex life. This eon is also marked by the emergence of multicellular organisms and the first known animals, such as the Ediacaran biota. The Proterozoic ended with the Cryogenian period, characterized by severe ice ages, paving the way for the Cambrian explosion of life that followed.