Since the earth came into existence some 4600 millions of years ago, it has a history of continuous change. Its surface had been broken apart and re-formed by violent geological movements from time to time.
As continents and seas changed and climate altered, life was modified time and again, to suit the changing environment.
The geological history of the earth is divided into two great Eons:
The Cryptozoic Eon covers all of what is called the Pre-Cambrian Era and goes back to the very beginnings of the earth. The Phanerozoic Eon includes all eras beginning with the Cambrian Era and coming down to the Holocene Era which covers the modern times.
There are four major Eras:
1. Pre-Cambrian Era
This is the only era that belongs to the Cryptozoic Eon.
2. Palaeozoic Era
This is the first era of the Phanerozoic Eon, and shows old forms of life.
3. Mesozoic Era
The second era of the Phanerozoic Eon, shows middle forms of life.
4. Caenozoic Era
The last era of the Phanerozoic Eon shows the recent forms of life.