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.