Darwin's theory of evolution explains how species of living things have changed over geological time. Scientists believe this is the reason why all living things on Earth exist today. Many species have become extinct in the past and the extinction of species continues to happen.
The basic idea behind the theory of evolution is that all the different species have developed over time from simple life forms. These simple life forms first developed more than three billion years ago - the Earth is about 4.5 billion years old.
This timeline below shows some of the key events in the evolution of life forms on Earth, from the first bacteria to the first modern humans
You can see a more detailed history of life timeline on BBC Nature.
When environments change, some species survive or evolve. However, some species become extinct. The fossil record shows that many species have become extinct since life on Earth began.
Download and print Tree of Life poster (pdf file, 2.6Mb)