Evolution is the process by which life on Earth developed from simple cells into the many complex creatures that exist on our planet today.

This process has taken over three billion years and is still going on today.

Evolution explains why some animals have similar physical structures with different functions - such as a bird's wing and a human being's arm.

By looking at fossils, we can see how these structures originated. In this case, both structures started out as fins in early fish.

Natural Selection

The theory of natural selection, which was first proposed by Charles Darwin in his book, On the Origin of Species, in 1859, explains how evolution occurred.

In order to survive, animals and plants must adapt to changes in their environment, such as changes in climate or changes in the availability of food sources.

Living things compete with one another for resources. Those living things that are better able to adapt to these changes are better able to obtain resources. Therefore, they  are more likely to survive and produce young.

The ability of plants and animals to adapt depends upon genetic variation.

When parents reproduce, they pass on their genes, which contain instructions about the characteristics of their young.

With sexual reproduction, the young receive a mixture of genes from both parents.

Sometimes, when the genes are being mixed up, a mutation - the altering of a gene - takes place.

A mutation can have a negative effect, in which case, the new plant or animal dies before it can produce young.

On the other hand, it can have a positive effect, increasing the creature's chance of surviving to produce more offspring.

These offspring continue to produce more offspring, with the same beneficial gene.

Thus, natural selection occurs. It is natural because it is caused by mutations which occur naturally during the process of sexual reproduction.