Younger of the two divisions of Precambrian time , from 2.
5 billion to 543 million years ago. Proterozoic rocks have been identified on all the continents and often constitute important sources of metallic ores, notably of iron, gold, copper, uranium, and nickel. The many small protocontinents that had formed during early Precambrian time had coalesced into one or several large landmasses by the beginning of the Proterozoic. Rocks of the Proterozoic contain many traces of primitive life forms, such as the fossil remains of bacteria and blue-green algae (see cyanobacteria ).