Rocks often contain the remains of living things that died long ago. These remains are called fossils. 
The kinds of rock that contain most fossils are called sedimentary rocks.
Sedimentary rocks build up in layers very slowly.
So we know that the rocks near the surface, just under the soil, are younger than the layers of rock below them. The oldest rocks are right at the bottom.
By studying chemicals in the rocks, scientists can tell how old they are.
This fossil of a reptile was found in Switzerland. The animal probably lived about 200 million years ago.

Rocks that were made less than 300 million years ago contain fossils of all kinds of different animals and plants, including horses, tigers, birds, dinosaurs, dragonflies, and flowers.
In rocks that are between 500 million and 300 million years old, fewer animals and plants have been found, though there are plenty of fossils of fish, plants without flowers, and simple animals such as mollusks.
Older rocks, made over 500 million years ago, contain no vertebrate fossils. 
The oldest animal invertebrate fossils are about 700 million years old. These animals include very simple creatures like jellyfish and worms.
The oldest fossils are about 3.5 billion years old and these fossils are of very primitive bacteria.
Scientists believe that fossil records show that life began with very small, simple creatures.
As time went by, more and more complex creatures appeared.
The most complex animals and     plants-mammals, bird, and flowering plants-appeared last of all.