The diversity of life on Earth today is the result of evolution. Life began on Earth at least 3.5 to 4 billion years ago, and it has been evolving ever since. At first, all living things on Earth were simple, single-celled organisms. Much later, the first multicellular organisms evolved, and after that, Earth's biodiversity greatly increased. Figure below shows a timeline of the history of life on Earth. The idea of evolution has been around for centuries. In fact, it goes all the way back to the ancient Greek philosopher Aristotle. However, evolution is most often associated with Charles Darwin. Darwin published a book on evolution in 1859 titled On the Origin of Species. In the book, Darwin stated the theory of evolution by natural selection. He also presented a great deal of evidence that evolution occurs. Evolution is a change in the characteristics of living things over time. As described by Darwin, evolution occurs by a process called natural selection.