Mark Ridley argues that since life started on the Earth almost as soon as it possibly could, after the planet cooled down sufficiently and the rate of bombardment from space slowed to a tolerable rate, it must be easy for life to get start. So it isn't life itself that requires explanation, but complex life, which took a long time to evolve after the first simple, single celled life began. He considers the possible reasons that life on Earth is not still made up entirely of bacteria and viruses. What would drive life to become complex instead of staying simple and what conditions would allow life to become more complex? According to Ridley, it's all down to sex and fidelity (faithful copying of genetic information). He describes a series of ceilings that limit the increase in complexity from one level of evolution to another. The high rate of copying errors in RNA based life was an early ceiling. New complexity was enabled by the evolution of DNA. DNA based life could have more genes than RNA based life. More complexity requires more genes and more genes to copy means an increase in the copying error rate. As the number of genes increased the danger of 'mutational meltdown' increased. DNA errors were reduced by the evolution of proofreading enzymes to prevent errors and then other enzymes to repair DNA errors, then more enzymes to correct the expression of errors when they slipped past the proofreading and repair enzymes. And each higher level of copying fidelity allowed an increase in the number of genes and, therefore, the level of complexity. The reason life takes the opportunity to become more complex, is that niches are available for new life-forms to evolve and fill. It was the evolution of sexual reproduction that made it possible for life to reach its current high level of complexity. Initially it generated almost as many problems as it solved, with gene in-fighting and dirty tricks. After long and arduous teething troubles, resulting in such familiar solutions as gender (with females supplying eggs, complete with mitochondria and males provided sperm and keeping their mitochondria to themselves), a fiendishly clever system of random gene shuffling, that Ridley attributes to "Mendel's Demon", evolved, to concentrate copying errors in some offspring whilst enabling some error-free offspring to be produced and also, to hide information from 'law-breaking' genes that natural selection would favour if it was possible for them to 'know' which reproductive cells contained which genes. He wonders whether humanity is currently banging it's head on a ceiling or further complexity is still possible without another advance on the gene copying front. As an example of higher complexity, he considers angels and speculates about their mating habits and how many genes there might be in the angelic genome. It's an interesting subject (pontificating on sex between supernatural entities aside) and Mark Ridley's book is full of intriguing notions - some more credible than others. I also found Matt Ridley's (that's MATT, the other Ridley, not MARK) ideas - set out in his book, "The Red Queen" - very compelling. His emphasis is on the tremendous advantage sexual reproduction gives in the arms race against parasites and disease, rather than the boost it gives to the evolution complexity. Both the Ridleys' arguments seemed convincing. But then, neither excludes the other and I can see no reason why sex shouldn't serve more than one purpose.