Evolution and natural selection

The basic idea behind the theory of evolution is that different species have developed over time from other life forms. The Earth is about 4.5 billion years old and there is scientific evidence that life on Earth began more than three billion years ago.

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

Archean eon. About 3,500,000,000 years ago: the first bacteria appeared (prokaryotes).

An evolution timeline

Archean eon

You can see a more detailed history of life on Earth timeline on BBC Nature.

Natural selection

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

  • 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. The genes that allow these individuals to be successful are passed to their offspring.
  • Those 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 on to the next generation.
  • Given 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.