The earth has existed for approximately 4.6 billion years, which is very difficult for us to grasp since our view of time is much shorter. Here are a couple of examples to help you understand how long 4.6 billion years is relative to human history:

If a 24 hr clock is equivalent to 4.6 billion years:

First life appears at about 8 a.m.
Humans appear less than a minute before midnight
Human civilization less than 30 seconds before midnight.

John Mc Phee's time exercise
"With your arms spread wide to represent all time on earth, look at one hand with its line of life. The Cambrian begins in the wrist, and the Permian extinction is at the outer end of the palm. All of the Cenozoic is in a fingerprint, and in a single stroke with a medium-grained nail file you could eradicate human history.."

We use the concept of Geologic Time to break up the periods of earth history into manageable chunks.

	The Cenozeric Era from 65 mya to the present is the most recent geologic era when the world as we know it developed and when human history begins.
	The Mezozoic Era from 65 mya to 250 mya this was the time of the dinosaurs.
	The Paleozoic Era from 250 mya to 600 mya is the time period where complex life first existed, animals mostly in marine environments and the first land plants.
	80% of earth history is lumped into the Precambrian Era, which is the time period before complex life appeared.
© 1995 Royal Tyrrell Museum

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