GEOLOGIC TIME
The earth is approximately 4.6 billion years old and has undergone large changes throughout its lifetime.  The earliest forms of life may have appeared as early as about 4 billion years ago, with bacteria and algae-like creatures appearing about 2.5 billion years ago.  Evidence has emerged recently that about 600-700 million years ago, the earth was probably completely ice-covered more than once (snowball earth).  Then, about 540 million years ago, the Cambrian Explosion occurred.  This marks the first appearance of creatures with hard skeletons and is associated with an striking increase in biological diversity.