There have been five great extinctions in the long history of life on earth, the most recent 65 million years ago, when all dinosaur species perished in an astonishingly brief period of time. Each of these great extinctions was unimaginably catastrophic - at least 65 percent of all species living vanished in a geological instant; in the Permian extinction, nearly 95 percent of all species were obliterated. The agency for these extinctions, the why, is hotly debated - sudden climate change, asteroids, evolutionary inadequacy - but the patterns are remarkably consistent. Now, as Leakey and Lewin show with inarguable logic based on irrefutable scientific evidence, the sixth great extinction is underway. And this time the cause is beyond dispute: By the lowest estimate, thirty thousand species are wiped out by human agency every year - a rate that matches the patterns of the other five great extinctions with frightening exactitude.