The evolutionary timeline taught in science classes around America supports the theory of natural selection: Animals evolved over time, and the ones that were best adapted for survival flourished while lesser-equipped species died out.
Scientists believe that when Earth formed 4.6 billion years ago, life didn't exist. The oldest known fossils are about 3.4 billion years old. More than 2 billion years after life began, single-celled bacteria were the only living things on Earth. Sometimes they formed colonies in shallow water along the shore.
The information and images provided are all courtesy of the Denver Museum of Nature and Science.
(Courtesy Denver Museum of Nature & Science)