Evolution has taken place over millions of years and scientists believe this is the reason why all living things on Earth exist today. There are different views and theories about the origin of life and the evolutionary process. English naturalist Charles Darwin wrote one of the first major scientific books on this subject.
Charles Darwin was an English naturalist who studied variation in plants and animals during a five-year voyage around the world in the 19th century. He explained his ideas on evolution in a book called, 'On the Origin of Species', published in 1859. This shows how species adapt and change by:
Darwin's ideas caused a lot of controversy, and this continues today, because they can be seen as conflicting with religious views about the creation of the world and the creatures living in it.
The basic idea behind the theory of evolution is that all the different species have evolved from simple life forms. These simple life forms first developed more than 3 billion years ago (the Earth is about 4.5 billion years old). The timeline below shows some of the key events in the evolution of life on Earth, from the first bacteria [bacteria: Single-celled microorganisms, some of which are pathogenic in humans, animals and plants. Singular is bacterium. ] to the first modern humans.
You can see a more detailed history of life timeline on BBC Nature.