PREHISTORY
5.5 BILLION YEARS B.C.:

Formation of the Earth. The oldest rocks so far studied by science are over 3.5 billion years old. It has therefore been theorized that Earth is between 4 and 6 billion years old. Since the maximum time required for the formation of all the elements of the Earth's crust has been determined by the latest scientific evidence to be about 5.5 billion years, this is the date that we shall adopt here.
The first four-fifths of th
e estimated five billion years of Earth's history is recorded in rocks which contain no fossils. Adequate fossil records exist only for the past 600 million years, probably due to the fact that the earliest lifeforms were soft-bodied, with the hard body parts necessary for preservation not developing until the Cambrian period.
This has conveniently enabled scienc
e to separate Earth's span of existence into two major time divisions: the Cryptozoic (meaning "hidden life") or Pre-Cambrian Age, and the Phanerozoic (meaning "obvious life") or Cambrian Age.

THE CRYPTOZOIC (OR PRE-CAMBRIAN)
(5.5 BILLION TO 570 MILLION
YEARS B.C.):

THE PALEOZOIC
(570 TO 225 MILLION YEARS B.C.):

The Paleozoic is divided into six sub-divisions:

THE MESOZOIC
(225 TO 65 MILLION YEARS B.C.):

The Mesozoic i
s divided into three sub-divisions:

THE CENOZOIC
(65 MILLION YEARS B.C. TO PRESENT):

The Cenozo
ic is divided into the following periods:

ANTIQUITY  

MODERN HISTORY

Present to far future history
EARTH