The natural world constantly changes as a result of evolution. Normally this happens because a food source has changed or a new predator has come on the scene – there are many reasons. Changes in behaviour, in armour or its habitat may have an effect on the shape, size or patterns we can see in their remains.

Fossils are the remains of creatures and plants mostly found in sedimentary rocks. These are rocks made up of layers of small grains eroded from older rocks that have been slowly laid down in water and compressed over millions of years.

Palaeontologists – scientists who study fossils have found that a record of the history of life is preserved in layer after layer of sedimentary rock all over the world.

The oldest fossils are found at the bottom, and the younger ones towards the top. Imagine a pile of newspapers that you have built by putting today’s paper on top of yesterday’s – after several weeks you would have a huge pile where the oldest papers were on the bottom and the most recent on the top. – it’s the same with rocks and fossils.

Some creatures and plants have evolved very slowly over millions of years so we can find very similar fossils throughout the layers. Others have evolved very quickly and in each layer there is a new version. By putting all these rapidly changing fossils in order we can get a master sequence that can help us date life itself