The extraordinary role of viruses in evolution and how this is revolutionising biology and medicine.Darwin's theory of evolution is still the greatest breakthrough in biological science. His explanation of the role of natural selection in driving the evolution of life on earth depended on steady variation of living things over time – but he was unable to explain how this variation occurred. In the 150 years since publication of the Origin of Species, we have discovered three main sources for this variation – mutation, hybridisation and epigenetics. Then on Sunday, 12th February, 2001 the evidence for perhaps the most extraordinary cause of variation was simultaneously released by two organisations – the code for the entire human genome. Not only was the human genome unbelievably simple (it is only ten times more complicated than a bacteria), but embedded in the code were large fragments that were derived from viruses – fragments that were vital to evolution of all organisms and the evidence for a fourth and vital source of variation – viruses.Virolution is the product of Dr Frank Ryan's decade of research at the frontiers of this new science – now called viral symbiosis – and the amazing revolution that it has had in these few years. As scientists begin to look for evidence of viral involvement in more and more processes, they have discovered that they are vital in nearly every case. And with this understanding comes the possibility of manipulating the role of the viruses to help fight a huge range of diseases.