largest time span on earth
Eons and Eras. The first principal subdivision is called the eon. An eon, the largest division of the geologic time scale, spans hundreds to thousands of millions of years. Geologists generally agree that there are two major eons: the Precambrian eon and the Phanerozoic eon. The Precambrian goes from the formation of the earth to the time when multicellular organisms first appeared - that's a really long time - from 4,500 million years ago to just about 543 million years ago.