The Proterozoic Era, also termed the Algonkian, is the second of the two eras into which the Precambrian has traditionally been divided. The Precambrian includes over four fifths of Earth's history: the 4.5 billion years from the formation of Earth to the start of the Cambrian Period some 570 million years ago. The first half of the Precambrian is known as the Archean Era and the second half as the Proterozoic Era.

Eukaryotic cells (cells with nuclei) first appeared in the early Proterozoic, about 2.5 billion years ago. Until that time only prokaryotic cells (cells without nuclei) existed. Bacteria and marine algae also evolved during the Proterozoic, and, late in the era, the first multicellular life appeared. During the Proterozoic, photosynthetic bacteria and algae liberated enough oxygen (O₂) from carbon dioxide (CO₂) to change Earth's atmosphere from oxygen-free to oxygen-rich. This chemical transformation made the Cambrian explosion of multicellular...