The Proterozoic eon was an important time period in Earth's history, spanning from 2.5 billion to 541 million years ago. It includes the emergence of multicellular life and the continuation of geological and atmospheric changes that paved the way for the evolution of complex organisms. Synonyms for this eon include the Proterozoic era, the middle Precambrian, and the second eon. It is considered the longest eon in Earth's history, followed by the Phanerozoic, which includes the present day. The Proterozoic eon laid the foundation for the development of life on our planet and saw the rise of early forms of algae, invertebrates, and even the first vertebrates.