The earth has kept a diary of her long past and geologists can trace the changing events in the rocky crust of her skin. Recent geological events are easy to read. But tide has blurred the older records and ancient events are harder and often impossible to decipher. The eras are natural divisions, like chapters in a diary. Each records a busy period of planetary construction and ends with a pause in activity, like a quiet Amen. Four natural geological eras are completed and closed. The fifth opened with the present Cenozoic Era that began a mere 60 million years ago.
The third and fourth eras, with part of the fifth, trace the last 520 million years of the earth's geological history. But our planet is estimated to be about four billion years old. If this is so, more than three quarters of the story is buried in the first two chapters. The earliest is the Archeozoic Era, the next is the Proterozoic Era. The division between them is vague, and it is hard to say exactly how long each era lasted.
Most geologists suspect that the Proterozoic Era may have lasted for more than 1500 million years and that the preceding Axcheotoic Era was even longer. But at present we cannot be sure which of these two first chapters was the longest of the earth's geological eras.