The Proterozoic EON is the second of three eons in Earth's history, lasting from 2.5 billion to 541 million years ago. It is characterized by the emergence of oxygen-producing photosynthetic organisms and the first multicellular life forms. During this time, the continents began to form and the atmosphere changed significantly. The Proterozoic EON was followed by the Phanerozoic EON, which is the current EON.