There has been two eons in the Earth's geologic history so far, the Precambrian eon, and the Phanerozoic eon. The eons are the basic and by far longest units on the geological time scale, spreading over hundreds of millions of years.
The Precambrian eon is the eon that starts with the formation of the Earth, up until the development of multi-cellular organisms. A very important event in this eon can be considered the formation of the Earth's oceans, which later enabled the conditions for the development of life. The organisms that dominated this eon are the one cellular organisms.
The Phanerozoic is the eon in which the multi-cellular organisms started to develop and evolve. This eon started with the end of the Precambrian eon, 543 million years ago up until the present. A very important event in this eon is the change of the composition of the atmosphere, which enabled the terrestrial life, as well as the better living conditions for the marine life. It is very hard to choose a dominant form of life in this eon, but worthy of mentioning are the plants, the sinapsids, dinosaurs, mammals, fish.
