According to Nick Lane, "mitochondria have shaped our lives, and the world we inhabit, in ways that defy belief". Mitochondria are the "tiny power-houses within cells that control our lives in surprising ways". Every cell in our bodies contains 300-400 of them, a total of 10 million billion in each of us, or 10% of our body weight. They generate our energy by burning oxygen. Unfortunately, this process also releases free radicals (reactive molecules), which attack our genes causing mutations and cell death. This in turn leads to ageing and degenerative diseases. Thus, Lane explains, the secret of prolonging life may lie with mitochondria. Centenarians often have the same variation within a mitochondrial gene, a single letter change in their genetic code that seems to limit the damaging leakage of free radicals. If you have it you are 50% more likely to live to 100 and half as likely to get an age-related disease. From the secret of longevity to why we are here at all, Lane's story is intriguing; and although his enthusiasm for his subject sometimes draws him into unnecessary detail, he argues convincingly that "mitochondria are the clandestine rulers of our world".