Darwin's theory of evolution

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 three 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 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.

Natural selection

The theory of evolution states that evolution happens by natural selection. Here are the key points:

• Individuals in a species show a wide range of variation.
• This variation is because of differences in genesgenes: The basic units of genetic material inherited from our parents. A gene is a section of DNA which controls part of a cell's chemistry - particularly protein production..
• Individuals with characteristics most suited to the environment are more likely to survive and reproduce.
• The genes that allowed the individuals to be successful are passed to the offspring in the next generation.

Individuals that are poorly adapted to their environment are less likely to survive and reproduce. This means that their genes are less likely to be passed to the next generation. Given enough time, a species will gradually evolve.

The evolutionary connections between biological groups can be illustrated in a tree of life diagram.