There have been two great mass extinctions in world history. The first took place 250 million years ago, and its causes, relating to the drift of the continents, are largely unresolved. The second, at 65 million years ago, brought to an end the Age of Dinosaurs. Again, the book argues, the causes were complex, but the culminating factor was one or more asteroids crasing into the Earth, unleashing fires, tidal waves and poisonous gases. Now it seems that a third mass extinction is under way, which will bring to an end the third great era of life on Earth, the Age of Mammals. The global ecology was already under stress from the rapid climate changes of the Ice Ages, when another great asteroid hit the Earth: that asteroid is Homo sapiens. The book maintains that we could now be witnessing the end of evolution. Worse still, it argues, a fair amount of the damage is now being caused by well-intentioned scientific intervention. However, Ward does offer some suggestions that could make the outcome slightly less bad than it otherwise might be.