A theory first proposed in the nineteenth century by Charles Darwin, according to which the Earth's species have changed and diversified through time under the influence of natural selection. Life on Earth is thought to have evolved in three stages. First came chemical evolution, in which organic molecules were formed. This was followed by the development of single cells capable of reproducing themselves. This stage led to the development of complex organisms capable of sexual reproduction. Evolution is generally accepted as fact by scientists today, although debates continue over the precise mechanisms involved in the process. (See mutation, punctuated equilibrium, and creation science.)
Note: The first cell is thought to have been formed when the Earth was less than a billion years old.