Sixth Century B.C.: The idea that species could evolve into other species existed before Darwin and Wallace's theories. The ancient Greek philosopher Anaximander put forward early evolutionary ideas, for example. However it was not until the end of the 18th century, with the development of the sciences of botany and geology, that the idea of evolution was debated seriously. The problem for naturalists was simple, however. If God did not create every type of living creature, how did one type of animal or plant change into another? What process drove evolution?

1800: One of the first proposed mechanisms was put forward by the French naturalist Jean-Baptiste Lamarck. He argued that characteristics acquired by an animal during its life were passed on to future generations. An animal that developed muscles or a long neck would pass these on to its offspring. The idea was the first decent shot at deriving a theory of evolution. Unfortunately for Lamarck, it has not survived the scrutiny of science. Acquired characteristics are not inherited, though the idea persisted as a serious scientific concept into the 20th century.

1830-1833: Another key event in the development of a mechanism that could explain the evolution of species was the publication of the three volumes of Charles Lyell's "Principles of Geology" between 1830 and 1833. Lyell argued that the history of the Earth was not one of short-term violent transformations or catastrophes but one of gradual changes that took place over extremely long periods. This vision of a planet shaped by tiny alterations — caused by erosion, sediment formation, the impact of wind and other factors — operating over eons had a profound impact on naturalists. 1858: The ideas of Wallace and Darwin were read at the Linnaean Society in London. Both men had been deeply influenced by their observations of wildlife across the globe. On his round-the-world journey on the Beagle, Darwin had also carried a copy of Lyell's "Principles of Geology," which provided a background to his studies of animals and plants in the Galapagos and other parts of the world. Wallace, for his part, had made his observations in the Amazon and Malaysia. 1865: "The Origin of Species," published in 1859, lacked one key feature: an understanding of genetics. That knowledge was provided by Gregor Mendel in 1865 when his studies of plants led him to develop the laws of genetics. The basic unit of this process is the gene, which is the focus of the forces of natural selection. Mendel's laws were overlooked by mainstream science until the start of the 20th century, however. Only then was it possible to understand the genetic mechanisms that underpin natural selection. 1953: Francis Crick and James Watson unravel the structure — a double helix — of deoxyribonucleic acid (DNA), the material from which the genes of all living creatures is constructed. The discovery allows scientists to begin detailed studies of the impact of natural selection at a molecular level.