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 life possible.
Significant geological changes also took place during the late Archean Era and early Proterozoic Era. The continents first began to form wide, stable continental shelves at this time and to be moved about by plate-tectonic processes. On the continents, which were still devoid of plant life, erosion and deposition proceeded rapidly. Numerous extremely thick beds of pure quartz sandstone formed—some, kilometers thick. In contrast, more recently formed beds of this type are, usually, at most, 109 yards (100 meters) thick.
Throughout both the Archean Era and the Proterozoic Era, beds of the banded iron formation were formed. This type of banded formation consists of alternating thin layers of quartz and iron oxide and were not formed during any later period. Today they are the world's major source of iron ore.
For decades, some geologists have disputed the usefulness of the term Proterozoic (from the Greek protero, earlier, and zoic, life). The Archean-Proterozoic distinction was first devised to describe the striking unconformity (change in rock type with depth) that runs horizontally through the Canadian shield, a vast area of Precambrian rock that rings Hudson Bay and includes Greenland. However, this dramatic division has not been found globally in Precambrian rocks. Furthermore, some geologists argue that it is misleading to lump 4.5 billion years of various geological history into just two compartments. Therefore, vaguer terms—early, middle, and late (or lower, middle, and upper) Precambrian—are often used.
See also Cenozoic Era; Cretaceous Period; Dating methods; Devonian Period; Eocene Epoch; Evolution, evidence of; Fossil record; Fossils and fossilization; Geologic time; Historical geology; Holocene Epoch; Jurassic Period; Mesozoic Era; Miocene Epoch; Mississippian Period; Oligocene Epoch; Ordovician Period; Origin of life; Paleocene Epoch; Paleozoic Era; Pennsylvanian Period; Phanerozoic Eon; Pleistocene Epoch; Pliocene Epoch; Quaternary Period; Silurian Period; Tertiary Period