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The idea behind the theory of evolution through the process of natural selection is that all species of living things have evolved from simple life forms over a period of time. The Earth is about 4.5 billion years old and there is scientific evidence to suggest that life on Earth began more than three billion years ago.

This slideshow shows key events in evolution, from the first bacteria to humans.

Natural selection

The accepted theory of evolution explains that it happens by natural selection. The key points are:

• Individuals in a species show a wide range of variation and this variation is because of differences in their genes.
• Individuals with characteristics most suited to their environment are more likely to survive and reproduce. This is commonly known as 'survival of the fittest'. The genes that allow these individuals to be successful within their environment are passed on to their offspring, which results in these specific genes becoming more common.
• Those that are poorly adapted to their environment are less likely to survive and reproduce. Their genes are less likely to be passed on to the next generation.
• Over a period of time, a species will gradually evolve.
• Both genes and the environment can cause variation, but only genetic variation can be passed on to the next generation.
• If two populations of one species become increasingly different in phenotype that they can no longer interbreed to form fertile offspring, this can result in the formation of two species.

A simple example can be seen in peacocks:

• females choose a mate based on their colourful tail feathers
• the more colourful the tail of a peacock, the more likely they are to mate and pass on these genes
• over time, the tails of peacocks have become more colourful