Earth's geological history is divided into major sections, called Eras.
The earth is estimated to be about 4.5 billion years old. The first 90% of this time is referred to as the Precambrian Era. This is the time of geology
"For most of its long history, Alberta lay beneath the sea. Two billion years ago, it drifted near the equator, its clear waters warmed by a tropical sun. This primeval seabed was a perfect propagator of primitive life, but not far away, the pink and gray Precambrian landscape harbored not a single living thing. ( Huck & Whiteway, p. 19)
The division of time is not arbitrary, nor is it uniform (eg. every billion years). Instead it is marked by "great events". The first of these is called the Cambrian Explosion - the first appearance of muticellular life. This occurred about 570 million years ago and heralded the beginning of the Paleozoic Era.
The next demarcation occurs about 225 million years ago and separates the Paleozoic from the Mesozoic Era. The event marking this boundary is called the Permian extinction, when 96% of all marine life died.
Finally there is the separation of the Mesozoic from the Cenozoic Era (our present era) at about 65 million years ago. This was when the dinosaurs perished.
The following web site is a rich source of more detailed information.
