Evolution is the process of gradual change that occurs naturally through a combination of inherited changes and adaptation to environmental opportunities. Explored by Charles Darwin in his seminal work On the Origin of the Species by Means of Natural Selection, this process takes place in generation after generation of a species. The theory of evolution--that all forms of life on Earth can be traced back to single-celled organisms that arose in the oceans 3.5 billion years ago--is the only scientifically accepted explanation for how this happened. Evolution explains the scientific theory of evolution and its role in the development and continuation of all life.